In vivo dose measurement using TLDs and MOSFET dosimeters for cardiac radiosurgery
Abstract
In vivo measurements were made of the dose delivered to animal models in an effort to develop a method for treating cardiac arrhythmia using radiation. This treatment would replace RF energy (currently used to create cardiac scar) with ionizing radiation. In the current study, the pulmonary vein ostia of animal models were irradiated with 6 MV X-rays in order to produce a scar that would block aberrant signals characteristic of atrial fibrillation. The CyberKnife radiosurgery system was used to deliver planned treatments of 20-35 Gy in a single fraction to four animals. The Synchrony system was used to track respiratory motion of the heart, while the contractile motion of the heart was untracked. The dose was measured on the epicardial surface near the right pulmonary vein and on the esophagus using surgically implanted TLD dosimeters, or in the coronary sinus using a MOSFET dosimeter placed using a catheter. The doses measured on the epicardium with TLDs averaged 5% less than predicted for those locations, while doses measured in the coronary sinus with the MOSFET sensor nearest the target averaged 6% less than the predicted dose. The measurements on the esophagus averaged 25% less than predicted. These results provide an indication of the accuracy with which the treatment planning methods accounted for the motion of the target, with its respiratory and cardiac components. This is the first report on the accuracy of CyberKnife dose delivery to cardiac targets.
9 Figures
a Corresponding author: Edward A. Gardner, CyberHeart Inc., 3282 Alpine Rd., Portola Valley, CA 94028, USA;
phone: 650-851-0091; fax: 650-851-0095 email: egardner@cyberheartinc.com
In vivo dose measurement using TLDs and MOSFET
dosimeters for cardiac radiosurgery
Edward A. Gardner,1a Thilaka S. Sumanaweera,2 Oliver Blanck,3 Alyson
K. Iwamura,4 James P. Steel,4 Sonja Dieterich,5 Patrick Maguire1
CyberHeart Inc.,1 Portola Valley, CA; Voyage Medical Inc.,2 Redwood City, CA;
Department of Radiation Oncology,3 Lübeck Campus of the University Clinic, Schleswig-
Holstein, Germany; Sutter Institute for Medical Research,4 Sacramento, CA; Department
of Radiation Oncology – Radiation Physics,5 Stanford University, Palo Alto, CA, USA
egardner@cyberheartinc.com
Received 16 August, 2011; accepted 31 January, 2012
In vivo measurements were made of the dose delivered to animal models in an effort
to develop a method for treating cardiac arrhythmia using radiation. This treatment
would replace RF energy (currently used to create cardiac scar) with ionizing
radiation. In the current study, the pulmonary vein ostia of animal models were
irradiated with 6 MV X-rays in order to produce a scar that would block aberrant
signals characteristic of atrial brillation. The CyberKnife radiosurgery system
was used to deliver planned treatments of 20–35 Gy in a single fraction to four
animals. The Synchrony system was used to track respiratory motion of the heart,
while the contractile motion of the heart was untracked. The dose was measured
on the epicardial surface near the right pulmonary vein and on the esophagus using
surgically implanted TLD dosimeters, or in the coronary sinus using a MOSFET
dosimeter placed using a catheter. The doses measured on the epicardium with
TLDs averaged 5% less than predicted for those locations, while doses measured
in the coronary sinus with the MOSFET sensor nearest the target averaged 6% less
than the predicted dose. The measurements on the esophagus averaged 25% less
than predicted. These results provide an indication of the accuracy with which the
treatment planning methods accounted for the motion of the target, with its respira-
tory and cardiac components. This is the rst report on the accuracy of CyberKnife
dose delivery to cardiac targets.
PACS numbers: 87.53.Ly, 87.53.Bn
Key words: in vivo dosimetry, radiosurgery, arrhythmia, ablation, CyberKnife
I. INTRODUCTION
Atrial brillation (AF) is the most common arrhythmia seen in medical practice. It has an es-
timated prevalence of 1%–2% in the general population and its prevalence increases with age.
It is a signicant contributor to cardiac morbidity and mortality. Atrial brillation confers a
ve-fold risk of stroke, with one in ve strokes being attributed to the arrhythmia. AF-related
strokes are twice as deadly as non-AF strokes.(1,2)
Traditionally, arrhythmia is treated by applying radiofrequency (RF) energy to specic
locations in the heart to create a scar that blocks aberrant electrical signals. A less common
arrhythmia, atrial utter, is treated by applying the RF to the cavotricuspid isthmus of the right
atrium. Paroxysmal AF is treated with a more challenging procedure by applying the RF to
the junction of the pulmonary veins and left atrium. The cardiac RF ablation procedure for
AF carries with it a signicant risk of major complications (4.5%),(3) and long-term efcacy
JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 13, NUMBER 3, 2012
190 190
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Journal of Applied Clinical Medical Physics, Vol. 13, No. 3, 2012
following catheter ablation of AF is unknown. The patient population receiving RF ablation is
predominantly younger and healthier than patients with AF at large.(4)
A small company, CyberHeart (Portola Valley, CA), was established to investigate the use of
radiosurgery as an alternative to RF ablation in the treatment of heart rhythm disorders. Sharma
et al.(5) used the CyberKnife platform (Accuray Inc., Sunnyvale, CA) to create scar tissue in
the cavotricuspid isthmus and pulmonary vein atria in normal animal models. The working
hypothesis is that radiation, like RF energy, can create localized and discrete scar tissue to
block the aberrant electrical signals that cause arrhythmia. The heart is not usually the target
of radiation treatment, but rather a critical structure that is avoided during radiation therapy. A
clear link has been established between radiation and various cardiac pathologies.(6) The evi-
dence is based on whole body (animal studies, atomic bomb survivors) and whole heart (e.g.,
thoracic CT, Hodgkin’s lymphoma treatment) irradiation. However, the CyberKnife is capable
of delivering high and precisely focused doses which can spare coronary arteries and the apex
of the heart. A radiosurgery approach may be a viable alternative for older patients where the
risks of the more invasive RF ablation procedure are greater and the risks of radiation related
complications are lower. The CyberKnife has been used on several occasions to treat tumors
in or near the heart using stereotactic body radiotherapy (SBRT).(7)
The heart is a challenging target for radiosurgical ablation for several reasons: the heart structures
are difcult to visualize in CT images without contrast agent; the heart is deforming and moving
during the treatment due to contractility and respiration; and critical structures are nearby.
In the human heart, the left atrium is immediately anterior to the esophagus and the pul-
monary veins straddle the esophagus. Delivering a dose that can create scar in the pulmonary
vein ostia without causing complications in the esophagus requires maintaining high spatial
dose gradients even during respiratory motion tracking. The amount of heart motion varies
with location in the heart, with the apex moving the most and the posterior structures moving
the least. The heart motion has two components: deformation from contraction/relaxation of
the heartbeat, and respiratory motion of the heart. To date, no commercial system is available
for tracking the contractile motion of the heart for the delivery of radiation.
Systems have been developed to track respiratory motion(8-10) based on monitoring the
surface of the chest. All tracking systems track articial (e.g., ducials) or natural (e.g., lung
tumor silhouettes) surrogates. Any untracked motion of the target relative to the surrogates will
degrade the dose delivery accuracy, particularly for plans with high spatial dose gradients.(11)
Measuring the dose during radiosurgical cardiac ablation was considered valuable due to the
substantial differences between this treatment and the normal use of radiosurgery. The heart
moves in a much more complicated manner than other radiosurgical targets. Motion measure-
ments were made in a human heart using a CARTO tracking catheter (Biosense Webster,
Diamond Bar, CA) during a RF ablation procedure. As shown in Fig. 1, cardiac structures move
in response to breathing as well as cardiac motion in a complicated pattern. At the same time,
high dose gradients were required due to the proximity of critical structures. Traditional in vivo
dosimetry methods with entrance and exit dose measurements could not be used because of the
noncoplanar beam conguration and large number of beam directions of the CyberKnife.
The implantable MOSFET DVS dosimeter(12) (Dose Verication System, Sicel Technologies,
Morrisville, NC) can be placed at or near the target organ and used to record doses throughout
the radiation treatment.(13) A variant of the DVS dosimeter has been created (DVS-HFT) for
use in hypofractionation. Scalchi et al.(14) have tested this system with phantoms and shown
that the measurements agreed with calculations to within 4%. The DVS-HFT was not used in
this study because of its size relative to the canine heart and because of limited availability at
the time of the study.
Two types of dosimeters were used to measure the dose to the heart. TLD crystals were
implanted on the surface of the heart and MOSFET detectors were threaded into the heart via
a catheter. To our knowledge, this is the rst report of an in vivo measurement of dosimetry at
a cardiac target.
192 Gardner et al.: In vivo dose measurement 192
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II. MATERIALS AND METHODS
A. Dosimeter assembly and placement
Two methods of dosimetry were used. Each system presented unique challenges. TLDs were
surgically placed in the canine model on the beating heart during a mini-thoracotomy under
general anesthesia. MOSFET sensors were delivered via catheter in both canine and porcine
models just prior to ablation and placed as close to the target volume as feasibly possible. The
catheter was placed in the coronary sinus which was posterior to the superior aspect of the
atrium and inferior to the pulmonary vein targets.
TLD dosimetry was conducted in a single canine model. The TLDs (TLD 100 crystals
LiF:MgTi, 3.2 × 3.2 × 1 mm, Landauer, Glenwood, IL) were placed in biocompatible, heat
shrink tubing (Palladium “Pebax”; Cobalt Polymers, Cloverdale, CA) and 3.2 mm diameter
Teon plugs were placed at each end. An oven was used at 150°C for 3 minutes to shrink the
tubing around the Teon and the TLDs. Twenty-seven TLDs were used to create four TLD cap-
sules. The TLD capsules were sterilized using a hydrogen peroxide plasma system (STERRAD,
Advanced Sterilization Products, Irvine, CA).
Two of the TLD capsules were surgically implanted in the canine model prior to CyberKnife
treatment, while the others were reserved for calibration. One capsule was sutured to the right
posterior epicardial surface at the pulmonary vein left atrial junction (see Fig. 2). Closer place-
ment of the TLD capsule to the target area was limited due to the small right thoracotamy. A
second capsule was placed on the esophagus (see Fig. 3). Gold ducial beads (2 mm diameter,
CIVCO, Kalona, IA.) were attached to the epicardium near the pulmonary vein ostia to guide
the CyberKnife treatment. Less invasive methods for ducial placement, such as using the spine
for rotational alignment and a temporary ducial in the heart for tracking, were considered as
alternatives. However, alignment uncertainties produced by anatomical differences between the
animal and human spine mandated the adoption of the surgical approach for this study.
Fi g . 1. Measurement made using a CARTO system of the motion (in millimeters) of a human left inferior pulmonary vein
ostium. The right–left motion is shown in the top panel, the inferior–superior motion is shown in the middle panel and
the posterior–anterior motion is shown in the lower panel. Two periodicities were present: that resulting from respiration
(period ~ 4 s) and that resulting from cardiac contraction (period < 1 s). The largest component of motion was produced
by respiration in the inferior–superior direction.
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Forty-ve days after ducial/dosimeter placement, cardiac-gated CT scans were acquired.
Radiosurgical cardiac ablation was carried out 11 days after CT scanning. It is important to
note that the TLD dosimeters were present and visible in the CT scans.
Dosimetry using MOSFET sensors was conducted in two canine models and one porcine
model. A MOSFET array dosimeter (Linear 5ive Array from Best Medical Canada, Ottawa, On-
tario) was used to measure the dose in the coronary sinus. The array consisted of ve MOSFET
radiation dosimeters attached to a exible printed circuit (ex) strand at 2 cm intervals with a
Fi g . 2. TLD capsule sutured to the epicardial heart surface of a dog. The capsule is located at the junction of the left atrium
and right superior pulmonary vein. Caudad to cephalad is the left to right orientation in the picture. The pulmonary vein
is the tubular structure emerging from behind the TLD capsule oriented toward the top of the picture. The heart appears at
the bottom center of the image, and the boundary between the right atrium (above) and the right ventricle (below) appears
as a distinct line above a lighter colored area on the right ventricle.
Fi g . 3. A TLD capsule attached to the anterior midthoracic esophagus of a dog. Cephald to caudad is the left to
right orientation.
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single ducial at its tip. To better localize the positions of the dosimeters in X-ray images, ve
additional platinum-iridium ducials were cemented to the ex midway between the sensors
and 1 cm past the proximal sensor (see Fig. 4). The array was selected over a single channel
MOSFET (microMOSFET) dosimeter because the length of the array ex (46 cm) was sufcient
for catheter placement in the heart from a jugular incision in our canine models. The length of
the single channel device (37.5 cm) was not.
A cardiac introducer sheath (6 Fr Flexor Check-Flo Introducer with Ansel modication, Cook
Inc., Bloomington, IN) was modied for placement of the MOSFET array in the coronary sinus.
The sheath was cut to a length of 46 cm to allow the MOSFET array to extend its full length,
and the tip was heat-shaped to easily nd the coronary sinus ostium in the canine or porcine
heart. The distal lumen was plugged with Loctite 5056 biocompatible, UV-curable silicone to
prevent blood from contacting the dosimeter.
Gold ducial beads (2 mm diameter) were surgically attached to the epicardium near the
pulmonary vein ostia to guide the CyberKnife treatment as before. Approximately 40 days
after ducial implantation, CT scans were acquired. The MOSFET dosimeters had not been
placed in the animal when the CTs were acquired. Approximately 14 days after CT scanning,
the treatments were delivered.
The MOSFET dosimeters were inserted into the coronary sinus of the animals just prior to
treatment delivery. A cut-down was performed at the right jugular vein, and a 12 French CSG
introducer sheath was placed. Using uoroscopic imaging, a 9 French CSG coronary sinus
sheath (SafeSheath, Pressure Products, San Pedro, CA) was advanced through the introducer
sheath and contrast was injected to nd the ostium of the coronary sinus. Once the ostium
was found, the coronary sinus sheath was advanced into the coronary sinus. The 6 Fr Flexor
catheter containing the MOSFET sensor was then advanced and seated in the lumen of the
coronary sinus sheath.
B. Planning, treatment, and dose calculation
Cardiac-gated, contrast-enhanced CT scans were acquired on a GE Lightspeed 16 slice CT
scanner, using retrospective reconstruction into 10 cardiac phases (Snapshot Burst+ mode).
Cardiac gating was required in order to clearly visualize the left atrium. Iodine contrast (Iso-
vue-300, Bracco Diagnostics, Princeton NJ) was used to enhance the density of the blood pool.
The contrast bolus was timed to preferentially enhance the left heart structures (left atrium and
pulmonary veins).
The contrast-enhanced CT was used to create a CyberKnife treatment plan using the Multi-
Plan 2.1 software, targeting the left atrium in order to isolate the pulmonary veins. To prevent
Fi g . 4. The MOSFET dosimeter array with additional ducials added midway between the MOSFET sensors and the
introducer sheath used to place it in the heart. The MOSFET sensors are dark spots (red arrows), while the platinum-iridium
ducials appear as bright bands along the array.
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dose miscalculation due to the presence of contrast agent in the planning CT, the model used for
photon/tissue interaction was modied as follows. All tissues with Hounseld numbers greater
than 1000 were modeled as Hounseld 1000. By clipping the tissue model, the effect of the
contrast was eliminated at the cost of underestimating the dose to bony structures. Because the
bones (ribs) in the path of the radiation beams were relatively far from the target, little effect
was expected. This method was chosen instead of registering the contrast CT with a noncontrast
CT to avoid registration errors that would be difcult to avoid due to the cardiac-gating used
in CT acquisition.
The Synchrony respiratory tracking system (Accuray Inc., Sunnyvale CA) was used to cre-
ate a model for the location of the heart as a function of respiratory phase. As shown in Fig. 1,
the heart motion consisted of both respiratory motion and contractile motion of the heart. The
chest light sensors of the Synchrony system were insensitive to the contractile motion, so this
motion appeared as random error between the model and the heart location. Because 15 points
were used to create each Synchrony model, the models approximated the position of the heart
at the average point in the cardiac cycle.
Table 1 shows some details from the treatment plans that were delivered. The four animals
reported on in this paper were part of a larger animal study where several treatment pa-
rameters were varied. The effect of these parameters on treatment efcacy will appear in a
subsequent publication.
Each TLD was contoured in MultiPlan and the median dose off all points within the contour
was recorded (see Figs. 5 and 6). The predicted doses for the heart TLDs ranged from 9 to 15 Gy,
while the doses predicted for the esophagus TLDs ranged from 1 to 2 Gy. The treatment plan
was delivered 56 days after the initial surgery. Five months after the radiation treatment, the
dog was sacriced and the TLDs were harvested. A relatively lengthy survival post-treatment
was required to allow for any lesions produced by the radiation to develop. Examination of the
TLD capsules showed that bodily uid had leaked onto the TLDs. The TLDs were rinsed in
distilled water and air-dried. The chips were then read by a commercial TLD reading service
(Landauer Inc., Glenwood, IL).
Since the MOSFET sensors were not present in CT scans, their location was not known
at the time of treatment planning. However, during CyberKnife treatment, the CyberKnife’s
stereotactic X-ray system provided the location of the MOSFET sensors relative to the sutured
ducials. Since the sutured ducials were visible in CT also, we determined the location of the
MOSFET sensors in CT as follows.
After the placement of the MOSFET dosimeters in the coronary sinus and the alignment of
the animal in the CyberKnife system, but before the treatment had begun, the exact positions
of the MOSFET sensors were determined using the CyberKnife’s stereotactic X-ray system.
As shown in Fig. 7, the ducials attached to the MOSFET array and the ducials sutured to the
epicardium were identied in both images and the CyberKnife imaging coordinates were deter-
mined. A rigid-body transform was created to map the CyberKnife coordinates of the sutured
ducial positions to their CT coordinates. This same rigid-body transform was then used to
determine the locations of ducials attached to the MOSFET array in CT space. The MOSFET
Ta b l e 1. Treatment plan information.
MOSFET Pig
TLD Dog MOSFET Dog #1 MOSFET Dog #2 Left Target Right Target
Plan Type Conformal Conformal Isocentric Isocentric Isocentric
Prescription dose (Gy) 20 35 35 25 25
Collimator size (mm) 15 15 20 15 20
Nodes 69 39 29 23 23
Beams 150 89 29 23 23
MU 25699 22393 7737 6597 6386
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Fi g . 5. CT scan showing the TLD capsule sutured to the heart (red arrows). The capsule contained seven square TLD
crystals bounded by two Teon plugs. The capsule was positioned parallel to the spine on the right, posterior wall of the
heart. The isodose contours show 21.4 Gy (red), 20 Gy (green), 12.6 Gy (yellow), 9.0 Gy (cyan), and 6.0 Gy (blue).
Fi g . 6. CT scan showing the TLD capsule sutured to the esophagus (red arrows). The capsule contained seven square TLD
crystals bounded by two Teon plugs. The capsule was positioned parallel to the spine on the left wall of the esophagus.
The isodose contours show 21.4 Gy (red), 20 Gy (green), 12.6 Gy (yellow), 9.0 Gy (cyan), 6.0 Gy (blue), and 2.4 Gy
(dark blue).
197 Gardner et al.: In vivo dose measurement 197
Journal of Applied Clinical Medical Physics, Vol. 13, No. 3, 2012
dosimeters were located midway between the ducials inside the exible sheath. The MOSFET
detector locations were estimated in the CT coordinate system using a spline interpolation. A
spline was used as a convenient way to account for the gradual curvature of the array.
The position of the MOSFET array was then adjusted so that one of the MOSFET detectors
was near the planned dose cloud. Additional X-ray images were acquired during CyberKnife
treatment. The location of the sensors was determined for each image pair and the average sensor
Fi g . 7. The method used to determine the dosimeter locations: (a) the locations of the sutured ducials (red, blue, and green
balls) in the CT image are projected to match the images from the CyberKnife system and show their spatial relationship
to the prescription dose cloud (green); (b) enhanced CyberKnife X-ray images showing the sutured ducials (FL, FRI, FRS)
and the ducials attached to the MOSFET ex (M0–M4); (c) positions of the MOSFET sensors (orange balls) relative to
the ducials on the MOSFET ex circuit (gray balls), the ducials sutured to the epicardium (red, green, and blue balls),
and the prescription dose cloud (green).
198 Gardner et al.: In vivo dose measurement 198
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location was used to predict the dose to the sensor. Figure 8 shows the locations determined
for the MOSFET detectors in the two dogs and one pig studied.
C. Calibration
The two TLD capsules that were not implanted in the dog were irradiated using a 6 MV linac
X-ray source (Trilogy, Varian Medical Systems Inc., Palo Alto, CA) one week after the animal
was irradiated. One capsule, containing six crystals, was irradiated to 18 Gy and the other, con-
taining seven crystals, was irradiated to 22 Gy. These capsules were retained at room temperature
until the other capsules were harvested from the animal. In order to determine the effect of
washing the in vivo crystals in distilled water, half of the calibration crystals were also washed
in the same way. The calibration TLDs were processed at the same time as the in vivo TLDs.
It was not feasible to maintain the calibration TLDs at body temperature for the ve months
between exposure and processing. The measurements could, therefore, have been affected by
temperature dependence of dose fading. For LiF: Mg, Ti TLDs, Burgkhardt et al.(15) measured
dose fading of 5% over 50 days at 25°C and 15% over the same period at 50°C.
Because the rst set of calibration TLDs received a much higher dose than the in vivo TLDs,
a second set of calibration TLDs was irradiated at 2, 3, 9, 15, 18, and 22 Gy. Twelve TLDs were
used to create six capsules in the same manner as before. Because of time constraints, these
were read only ve days after irradiation.
Figure 9 shows the TLD calibration curve. The rst set of TLDs showed that washing the
TLDs with distilled water did not affect the measured dose. The calibration factor from these
TLDs (subsequently called the extrapolated calibration) can be extrapolated to the range of the
in vivo TLDs. The second calibration set (subsequently called the fade-corrected calibration)
provided data near the predicted doses of the in vivo TLDs, but were processed much sooner
after the exposure (ve days) than were the in vivo TLDs (ve months).
The signal from TLDs is known to fade over time resulting in reduced readings when the
time between exposure and reading is large.(16) By calculating the ratio of the TLD readings for
doses (18 and 22 Gy) that were calibrated with both ve-day and ve-month fade duration, the
magnitude of the fading was estimated as 16%. A correction factor was calculated for the heart
TLDs using the 9 and 15 Gy calibration doses adjusted for fading. A second correction factor
for the esophagus TLDs was calculated based on the 2 and 3 Gy calibration doses adjusted for
the fading. Any dose dependence of the TLD fading was neglected in this calibration.
The 13 TLDs used for the extrapolated calibration (those read after ve months) can be
used to estimate the precision of the TLD measurements. The standard deviation of the TLDs
exposed to 18 Gy was 0.9% of their average value, while the standard deviation of the TLDs
exposed to 22 Gy was 5.4% of their average value. As can be seen in Fig. 9, two of the six
Fi g . 8. Reconstruction of the MOSFET sensor positions relative to the cardiac anatomy (from CT) for (a) dog 1, (b) dog
2, and (c) the pig. A single 35 Gy dose cloud was delivered to the main right pulmonary vein in each of the dogs, while
two separate treatments were delivered to right and left pulmonary veins in the pig. The positions of the MOSFET sensors
at the time of treatment are shown with orange balls labeled S0 (distal) to S3 or S4 (proximal). In the pig, a MOSFET
sensor (S3) was much closer to the right target lesion than any of the sensors were to the left target lesion. In the pig (c),
the distal sensor (S0) is hidden behind the pulmonary vein.
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TLDs exposed to 22 Gy had signicantly greater response. This variation may have appeared
in the implanted TLDs, as well.
The MOSFET dosimeters were calibrated using the CyberKnife system in a conguration
where one monitor unit delivered 0.01 Gy. The MOSFET array was sandwiched between solid
water so that the photon beam passed through 1.5 cm of water equivalent before reaching the
array and 4 cm after passing it. The X-ray source was positioned 80 cm from the array with the
beam normal to the surface of the solid water. Using a 6 cm collimator, 2.00 Gy was delivered
to each element of the array four times. An average of the voltage change on the MOSFET
was used as a calibration factor for determining the experimental dose. Because the MOSFET
detectors were of the dual-bias type, minimal temperature dependence was expected.(17) The
standard deviation of these calibration factors provided an indication of the inherent reproduc-
ibility of the MOSFET measurements. The standard deviations ranged from 1.2% to 1.8%,
with an average standard deviation of 1.5%. This variation was expected on the in vivo dose
measurements.
D. Animal care
All animals were treated humanely and in accordance with the Animal Welfare Act (7 USC
2131) and the National Institutes of Health Guide for the Care and Use of Laboratory Animals.
All activities involving animals were conducted with strict veterinary oversight and with full
Institutional Animal Care and Use Committee approval at the Sutter Memorial Hospital Research
Institute, Sacramento, CA. The research facility is certied by the United States Department of
Agriculture (USDA), and is fully accredited by the Association for Assessment and Accredita-
tion of Laboratory Animal Care International (AAALAC).
Fi g . 9. TLD Calibration. The rst set of calibration TLDs were exposed at 18 and 22 Gy (blue circles and red asterisks).
The TLDs in this rst set that were washed (red asterisks) showed no signicant difference from the unwashed TLDs (blue
circles). The correction factor extrapolated from this calibration is shown as a green line. The second set of calibration
TLDs were exposed at 2, 3, 9, 15, 18, and 22 Gy (black crosses). The correction factor calculated from this set of TLDs
is shown as a black dashed line. This correction factor was then adjusted for loss of thermoluminescence over time to
produce a fade-corrected dose calibration.
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III. RESULTS
All animals survived the procedures and demonstrated no adverse effects. Prior to sacrice,
ECG and transesophageal ultrasound evaluations were performed on each animal. These tests
showed normal cardiac function.
The results of the TLDs placed on the epicardium are shown in Table 2. The TLDs were
positioned at a small distance from the target area. Therefore, the predicted doses ranged from
46% to 73% of the prescription dose of 20 Gy. The measured doses were within 1 Gy of the
predicted doses, except for the most caudal TLD where the measurement was 2.1 Gy lower
than the predicted dose of 12.5 Gy. The relative difference between the measured and predicted
doses ranged from 6% to -16.8%, with ve of the seven measurements within 5% of the pre-
dicted value for either calibration measurement. The TLD measurements on the esophagus are
shown in Table 3. The TLDs were positioned at some distance from the target, so doses and
dose gradients were relatively small. The largest difference between measured and predicted
was 1.2 Gy where the predicted dose was 2.9 Gy (using the extrapolated dose calibration with
the most cranial TLD). The fade-corrected calibration resulted in less deviation from the pre-
dictions than the extrapolated calibration. The TLD measurements were predominantly lower
than the planned dose, but the differences were mostly within 1 Gy.
Tables 4 and 5 show comparisons between the MOSFET measurements and plan dose
in the two dogs in which this dosimeter was used. Tables 6 and 7 show comparisons for the
MOSFET measurements in the pig. Since the dosimeters were spaced apart by a large distance
(approximately 2 cm), we were able to measure a large range of locations across the planned
dose cloud. The highest doses measured were 13.9 Gy in the rst dog, 22.0 Gy in the second
dog, and 15.4 Gy in the pig. These corresponded to 40%, 63%, and 61% of the prescription
doses of 35 Gy in the dogs and 25 Gy in the pig. The relative differences between measurements
and predicted values were smaller for the higher doses (nearer the target). For points with doses
above 9 Gy, the MOSFET measurements were within 7.5% of the predicted value.
Ta b l e 2. Doses measured in vivo with TLDs in the canine heart.
Extrapolated Calibration Fade-Corrected Calibration
Heart TLDs Predicted Dose (Gy) Measured (Gy) Difference (%) Measured (Gy) Difference (%)
Most caudal 12.5 10.8 -13.6 10.4 -16.8
12.1 12.1 0.0 11.7 -3.3
13.4 14.2 6.0 13.7 2.2
14.1 13.9 -1.4 13.4 -5.0
14.6 14.9 2.1 14.4 -1.4
10.6 10.7 0.9 10.3 -2.8
Most cranial 9.2 8.9 -3.3 8.6 -6.5
Ta b l e 3. Doses measured in vivo with TLDs from the canine esophagus.
Extrapolated Calibration Fade-Corrected Calibration
Esophagus TLDs Predicted Dose (Gy) Measured (Gy) Difference (%) Measured (Gy) Difference (%)
Most caudal 2.1 1.7 -19.0 2.0 -4.8
2.2 1.6 -27.3 1.8 -18.2
2.1 1.7 -19.0 2.0 -4.8
2.0 1.5 -25.0 1.8 -10.0
2.2 1.9 -13.6 2.2 0.0
2.6 1.7 -34.6 2.0 -23.1
Most cranial 2.9 1.7 -41.4 2.0 -31.0
201 Gardner et al.: In vivo dose measurement 201
Journal of Applied Clinical Medical Physics, Vol. 13, No. 3, 2012
IV. DISCUSSION
This represents the rst published report of in vivo dose measurement during cardiac ablation
with radiosurgery.
The doses measured in vivo were very close to the planned dose. The heart doses measured
with TLDs using the extrapolated calibration averaged 1% lower than the plan dose, while using
the fade-adjusted calibration the measurements were, on average, 5% lower. The extrapolated
calibration was based on chips that were irradiated and processed with the same interval as the
test chips but at higher doses, while the fade-adjusted calibration was based on chips irradi-
ated at similar doses but processed much sooner after irradiation than the test chips. The better
agreement with the extrapolated calibration may indicate that the fade correction applied led to
more error than extrapolating the dose from chips that were aged appropriately but calibrated
Ta b l e 4. Measured vs. predicted doses for MOSFET measurement in dog 1. The treatment was isocentric with a
prescription dose of 35 Gy to 83% of the maximum dose.
Sensor ID Predicted Dose (Gy) Measured Dose (Gy) Difference (%)
S0 1.03 0.98 -4.9
S1 8.74 11.70 33.9
S2 14.59 13.90 -4.7
S3 14.21 13.60 -4.3
S4a 1.30
aThe exact location of S4 could not be determined in this experiment, so no dose could be predicted for it.
Ta b l e 5. Measured vs. predicted doses for MOSFET measurement in dog 2. The treatment was isocentric with a
prescription dose of 35 Gy to 83% of the maximum dose.
Sensor ID Predicted Dose (Gy) Measured Dose (Gy) Difference (%)
S0 9.69 9.50 -2.0
S1 23.79 22.00 -7.5
S2 1.03 1.07 3.9
S3 0.38 0.29 -23.7
S4a 0.14
aThe exact location of S4 could not be determined in this experiment so no dose could be predicted for it.
Ta b l e 6. MOSFET results from porcine experiment, right target lesion.
Sensor ID Predicted Dose (Gy) Measured Dose (Gy) Difference (%)
S0 0.33 0.32 -3.0
S1 0.31 0.31 0.0
S2 0.35 0.40 14.3
S3 1.77 2.52 42.4
S4 16.32 15.40 -5.6
Ta b l e 7. MOSFET results from the porcine experiment, left target lesion.
Sensor ID Predicted Dose (Gy) Measured Dose (Gy) Difference (%)
S0 0.66 1.46 121.2
S1 1.26 1.15 -8.7
S2 0.56 0.67 19.6
S3 3.26 1.81 -44.5
S4 0.23 0.30 30.4
202 Gardner et al.: In vivo dose measurement 202
Journal of Applied Clinical Medical Physics, Vol. 13, No. 3, 2012
at a higher dose. Despite these calibration issues, the agreement between measurement and
prediction is quite close considering TLD variation of approximately 2%(18) and reproducibility
of the TLDs estimated using the calibration chips at between 1% and 5%.
The doses measured at the esophagus did not agree with the plan dose as well as in the
heart. Using the extrapolated calibration, the measurements were on average 25% lower than
predicted. With the fade-adjusted calibration, the measurements were on average 14% lower.
The extrapolated dose calibration was based on measurements at 18 and 22 Gy, so it could be
expected to be less reliable for doses less than 3 Gy than for doses in the 9 to 15 Gy range.
There were sources of uncertainty in the TLD measurements. The contamination of the in
vivo TLDs with bodily uid could have caused an erroneous reading. Surface contamination
could have absorbed light emitted by the TLD before it was detected in the reading process,
reducing the measured dose. Alternatively, contaminants could have created additional light as
they burned in the 300°C oven used to read the TLD. This would have resulted in an errone-
ously high dose measurement.
The MOSFET measurements agreed well with the planned dose. Except for sensor S1 in dog
1, the difference between predicted and measured doses was less than 10% in locations receiving
more than 5 Gy. Consorti et al.(19) considered the overall uncertainty in the response of similar
MOSFET dosimeters to be 3.5% and measured differences from predicted values less than 5%
in most cases in intra-operative clinical treatments (the largest difference was 11.7%).
Much of the uncertainty in the MOSFET comparison came from the difculty in identifying
the position of the MOSFET in the dose cloud. Because the sensors themselves could not be
seen in the X-ray images, differences between the actual catheter shape and the cubic hermite
spline that was used to estimate the MOSFET position would result in a dose error due to the
high gradients involved. Furthermore, the mapping of MOSFET positions from the CyberKnife
X-rays to CT assumed a rigid relationship between the sutured ducials and the MOSFET
sensors. The reproducibility, as estimated during MOSFET calibration, would be expected to
contribute to between 1% and 2% of the variation seen.
There are also possible differences between the predicted and delivered doses. Errors in the
respiratory tracking or the untracked cardiac motion could have affected dose delivery. Errors
in beam targeting or the positioning of the animal could have resulted in dose delivery inac-
curacies. Errors in the dose model used to accommodate the contrast agent could also have
resulted in a difference between the predicted and delivered doses. The planning system used
a ray-tracing–based method to calculate the dose. This method has been shown to overesti-
mate dose in the thorasic region due to dose rebuildup. The dose delivered to the esophagus
could have been affected by spatial averaging caused by relative motion between the heart
and esophagus. However, the dose gradient (as shown in Fig. 6) was very small in this region,
which would minimize this effect.
V. CONCLUSIONS
Both dosimeter methodologies tested in these animal models of cardiac ablation documented
that doses delivered corresponded to predicted dose-to-volume plans for the cardiac targets to
within about 10%. This is greater than the generally accepted +5% target for therapeutic dose
delivery accuracy, but dose measurement uncertainty could easily account for the additional
deviation. This showed that target motion, with its respiratory and cardiac components, did not
strongly affect the dose delivery accuracy.
Although uncertainty in dose measurement and dose prediction could have partially offset one
another, the agreement of these methods was strongly suggestive that the actual dose delivered
was near the predicted dose. The general agreement between the in vivo measurements and the
planned dose suggests that treatment planning and CyberKnife dose delivery for pulmonary
vein ablation in the heart are accurate to within 10%.
203 Gardner et al.: In vivo dose measurement 203
Journal of Applied Clinical Medical Physics, Vol. 13, No. 3, 2012
ACKNOWLEDGMENTS
CyberHeart would like to thank Dr. Vivek Reddy for his help with the pulmonary vein motion
measurement (Fig. 1).
REFERENCES
1. Benjamin EJ, Wolf PA, D’Agostino RB, Silbershatz H, Kannel WB, Levy D. Impact of atrial brillation on the
risk of death: the Framingham Heart Study. Circulation. 1998;98(10):946–52.
2. Camm AJ, Kirchhof P, Lip GY, et al. Guidelines for the management of atrial brillation: the Task Force
for the Management of Atrial Fibrillation of the European Society of Cardiology (ESC). Eur Heart J.
2010;31(19):2369–429.
3. Cappato R, Calkins H, Chen SA, et al. Updated worldwide survey on the methods, efcacy, and safety of catheter
ablation for human atrial brillation. Circ Arrhythm Electrophysiol. 2010;3(1):32–38.
4. Weerasooriya R, Khairy P, Litalien J, et al. Catheter ablation for atrial brillation: are results maintained at 5
years of follow-up? J Am Coll Cardiol. 2011;57(2):160–66.
5. Sharma A, Wong D, Weidlich G, et al. Non-invasive stereotactic radiosurgery (CyberHeart) for the creation of
ablation lesion in the atrium. Heart Rhythm. 2010;7(6):802–10.
6. Darby SC, Cutter DJ, Boerma M, et al. Radiation-related heart disease: current knowledge and future prospects.
Int J Radiat Oncol Biol Phys. 2010;76(3):656–65.
7. Soltys SG, Kalani MY, Cheshier SH, Szabo KA, Lo A, Chang SD. Stereotactic radiosurgery for a cardiac sarcoma:
a case report. Technol Cancer Res Treat. 2008;7(5):363–68.
8. Schweikard A, Shiomi H, Adler J. Respiration tracking in radiosurgery. Med Phys. 2004;31(10):2738–41.
9. Dieterich S, Cleary K, D’Souza W, Murphy M, Wong KH, Keall P. Locating and targeting moving tumors with
radiation beams. Med Phys. 2008;35(12):5684–94.
10. Sawant A, Venkat R, Srivastava V, et al. Management of three-dimensional intrafraction motion through real-time
DMLC tracking. Med Phys. 2008;35(5):2050–61.
11. Lu XQ, Shanmugham LN, Mahadevan A, et al. Organ deformation and dose coverage in robotic respiratory-
tracking radiotherapy. Int J Radiat Oncol Biol Phys. 2008;71(1):281–89.
12. Scarantino CW and Beyer GP. The Dose Verication System (DVS) for cancer patients receiving radiation therapy.
Expert Rev Med Devices. 2008;5(6):679–85.
13. Scarantino CW, Prestidge BR, Anscher MS, et al. The observed variance between predicted and measured
radiation dose in breast and prostate patients utilizing an in vivo dosimeter. Int J Radiat Oncol Biol Phys.
2008;72(2):597–604.
14. Scalchi P, Righetto R, Cavedon C, Francescon P, Colombo F. Direct tumor in vivo dosimetry in highly-conformal
radiotherapy: a feasibility study of implantable MOSFETs for hypofractionated extracranial treatments using the
Cyberknife system. Med Phys. 2010;37(4):1413–23.
15. Burgkhardt B, Herrera R, Piesch E. Fading characteristics of different thermoluminescent dosimeters. Nuclear
Instruments and Methods. 1976;137(1):41–47.
16. Moscovitch M and Horowitz YS. Thermoluminescent materials for medical applications: LiF:Mg, Ti and
LiF:Mg,Cu,P. Radiation Measurements. 2006;41:S71–S77.
17. Soubra M, Cygler J, Mackay G. Evaluation of a dual bias dual metal oxide-silicon semiconductor eld effect
transistor detector as radiation dosimeter. Med Phys. 1994;21(4):567–72.
18. Antolak JA, Cundiff JH, Ha CS. Utilization of thermoluminescent dosimetry in total skin electron beam radio-
therapy of mycosis fungoides. Int J Radiat Oncol Biol Phys. 1998;40(1):101–08.
19. Consorti R, Petrucci A, Fortunato F, et al. In vivo dosimetry with MOSFETs: dosimetric characterization and
rst clinical results in intraoperative radiotherapy. Int J Radiat Oncol Biol Phys. 2005;63(3):952–60.
- CitationsCitations9
- ReferencesReferences21
- A study on mini swine using the Cyberheart system (Portola Valley, CA), where fiducials were implanted next to the target volume, demonstrated the feasibility of using stereotactic robotic radiosurgery to create cardiac fibrotic lesions as well as to induce a significant decrease in voltage at the pulmonary vein–left atrial junction at a dose of 25 Gy [7]. In another animal study targeting the right pulmonary vein ostia, the use of in-vivo thermoluminescent dosimeter near the right pulmonary vein showed that the accuracy of the CyberKnife radiosurgery system within 5% of the predicted dose [9]. Using an internal target volume method accounting for both respiratory and cardiac motion, Bode et al., [11] investigated the feasibility of lesion formation in a porcine model.
[Show abstract] [Hide abstract] ABSTRACT: Purpose: The purpose of the study was to determine the extent of displacement of the pulmonary vein antrums resulting from the intrinsic motion of the heart using 4D cardiac dual-source computed tomography (DSCT). Methods: Ten consecutive female patients were enrolled in this prospective planning study. In breath-hold, a contrast-injected cardiac 4-dimensional (4D) computed tomography (CT) synchronized to the electrocardiogram was obtained using a prospective sequential acquisition method including the extreme phases of systole and diastole. Right and left atrial fibrillation target volumes (CTVR and CTVL) were defined, with each target volume containing the antral regions of the superior and inferior pulmonary veins. Four points of interest were used as surrogates for the right superior and inferior pulmonary vein antrum (RSPVA and RIPVA) and the left superior and inferior pulmonary vein antrum (LSPVA and LIPVA). On our 4D post-processing workstation (MIM Maestro™, MIM Software Inc.), maximum displacement of each point of interest from diastole to systole was measured in the mediolateral (ML), anteroposterior (AP), and superoinferior (SI) directions. Results: Median age of the enrolled patients was 60 years (range, 56-71 years). Within the CTVR, the mean displacements of the superior and inferior surrogates were 3 mm vs. 1 mm (p=0.002), 2 mm vs. 0 mm (p= 0.001), and 3 mm vs. 0 mm (p=0.00001), in the ML, AP, and SI directions, respectively. On the left, mean absolute displacements of the LSPVA vs. LIPVA were similar at 4 mm vs. 1 mm (p=0.0008), 2 mm vs. 0 mm (p= 0.001), and 3 mm vs. 1 mm (p=0.00001) in the ML, AP, and SI directions. Conclusion: When isolated from breathing, cardiac contraction is associated with minimal inferior pulmonary veins motion and modest (1-6 mm) motion of the superior veins. Target deformation was thus of a magnitude similar or greater than target motion, limiting the potential gains of cardiac tracking. Optimal strategies for cardiac radiosurgery should thus either incorporate the generation of an internal target or cardiac gating. In either case, cardiac 4D DSCT would allow for personalized margin definition.- Initial lesion contouring was performed with the CardioPlan® Software (CyberHeart Inc, Sunnyvale, USA) and an isotropic margin of 3 mm was added to generate the Right and Left Planning Target Volumes (RPTV / LPTV) based on the accuracy of the CyberKnife [14], and previous studies [1-3, 5, 6, 9-11, 15]. An extra margin for the cardiac motion was not generated by the cardiac motion at the PV antrum is small [5-7, 18, 21, 23] and has a likely neglectable impact on the dosimetry [15, 36]. OAR contouring, as well as treatment planning, was performed with the MultiPlan® Software (Version 4.x, Accuray) with final Monte Carlo dose calculation [37] and density overwrites of the contrast enhancement in the heart on the planning computed tomography (CT).
[Show abstract] [Hide abstract] ABSTRACT: Purpose Robotic guided stereotactic radiosurgery has recently been investigated for the treatment of atrial fibrillation (AF). Before moving into human treatments, multiple implications for treatment planning given a potential target tracking approach have to be considered. Materials & Methods Theoretical AF radiosurgery treatment plans for twenty-four patients were generated for baseline comparison. Eighteen patients were investigated under ideal tracking conditions, twelve patients under regional dose rate (RDR = applied dose over a certain time window) optimized conditions (beam delivery sequence sorting according to regional beam targeting), four patients under ultrasound tracking conditions (beam block of the ultrasound probe) and four patients with temporary single fiducial tracking conditions (differential surrogate-to-target respiratory and cardiac motion). Results With currently known guidelines on dose limitations of critical structures, treatment planning for AF radiosurgery with 25 Gy under ideal tracking conditions with a 3 mm safety margin may only be feasible in less than 40% of the patients due to the unfavorable esophagus and bronchial tree location relative to the left atrial antrum (target area). Beam delivery sequence sorting showed a large increase in RDR coverage (% of voxels having a larger dose rate for a given time window) of 10.8-92.4% (median, 38.0%) for a 40-50 min time window, which may be significant for non-malignant targets. For ultrasound tracking, blocking beams through the ultrasound probe was found to have no visible impact on plan quality given previous optimal ultrasound window estimation for the planning CT. For fiducial tracking in the right atrial septum, the differential motion may reduce target coverage by up to -24.9% which could be reduced to a median of -0.8% (maximum, -12.0%) by using 4D dose optimization. The cardiac motion was also found to have an impact on the dose distribution, at the anterior left atrial wall; however, the results need to be verified. Conclusion Robotic AF radiosurgery with 25 Gy may be feasible in a subgroup of patients under ideal tracking conditions. Ultrasound tracking was found to have the lowest impact on treatment planning and given its real-time imaging capability should be considered for AF robotic radiosurgery. Nevertheless, advanced treatment planning using RDR or 4D respiratory and cardiac dose optimization may be still advised despite using ideal tracking methods.- The scar tissue in this lesion blocks the transmission of electrical impulses in the heart in the same way that scar produced by Radio Frequency (RF) ablation isolates aberrant signals. Tests of this technique have included phantom studies and animal studies [1][2][3]. This method has been used in humans for treatment of both ventricular tachycardia [4] and atrial fibrillation [5].
[Show abstract] [Hide abstract] ABSTRACT: In a treatment planning study, radiosurgical treatment plans designed to produce lesions on the left atrium were created using two different methodologies. In one, structures in the heart (mitral valve and coronary arteries) were designated as critical structures while this was not done in the second plan. The treatment plans that were created were compared with standards for heart dose used when treating spine tumors. Although the dosage for the whole heart greatly exceeded the dose standards, when only the dose to the ventricles was considered, the plan where the mitral valve was spared was very close to the dose standards. The ventricles received a substantially higher dose in the plan where the mitral valve was not a critical structure. Although neither treatment plan was delivered, this study demonstrated the feasibility of treating the heart while minimizing dose to the ventricles.- Concerning cardiac motion and radiation dose delivery precision, Gardner, et al. implanted thermoluminescent dosimeter (TLD) crystals onto the surface of canine hearts and transferred metal-oxide-semiconductor field-effect transistor (MOSFET) sensors via a catheter in canine and porcine models close to an ablation target volume to measure the difference between the delivered and planned radiation doses [23]. Planned with an internal left atrium target volume derived from cardiac-gated CT scans, they found the measured doses were within 10% of the planned doses using either of the dose-monitoring techniques.
[Show abstract] [Hide abstract] ABSTRACT: Purpose: To explore the feasibility of using stereotactic body radiotherapy (SBRT) to irradiate the antra of the four pulmonary veins while protecting nearby critical organs, such as the esophagus. Materials and Methods: Twenty patients who underwent radiofrequency catheter ablation for atrial fibrillation were selected. For each patient, the antra of the four pulmonary veins were identified as the target volumes on a pre-catheterization contrast or non-contrast CT scan. On each CT scan, the esophagus, trachea, heart, and total lung were delineated and the esophagus was identified as the critical organ. For each patient, three treatment plans were designed with 0, 2, and 5 mm planning margins around the targets while avoiding overlap with a planning organ at risk volume (PRV) generated by a 2 mm expansion of the esophagus. Using three non-coplanar volumetric modulated arcs (VMAT), 60 plans were created to deliver a prescription dose of 50 Gy in five fractions, following the SBRT dose regimen for central lung tumors. With greater than 97% of the planning target volumes (PTV) receiving the prescription doses, we examined dosimetry to 0.03 cc and 5 cc of the esophagus PRV volume as well as other contoured structures. Results: The average PTV-0 mm, PTV-2 mm, and PTV-5 mm volumes were 3.05 ± 1.90 cc, 14.70 ± 5.00 cc, and 40.85 ± 10.20 cc, respectively. With three non-coplanar VMAT arcs, the average conformality indices (ratio of prescription isodose volume to the PTV volume) for the PTV-0 mm, PTV-2 mm and PTV-5 mm were 4.81 ± 2.0, 1.71 ± 0.19, and 1.23 ± 0.08, respectively. Assuming patients were treated under breath-hold with 2 mm planning margins to account for cardiac motion, all plans met esophageal PRV maximum dose limits < 50 Gy to 0.03 cc and 16 plans (80%) met < 27.5 Gy to 5 cc of the esophageal PRVs. For PTV-5 mm plans, 18 plans met the maximum dose limit < 50 Gy to 0.03 cc and only two plans met the maximum dose limit < 27.5 Gy to 5 cc of the esophageal PRV. Conclusions: The anatomical relationship between the antra of the four pulmonary veins and the esophagus varies from patient to patient. Adding 2 mm planning margins and a 2 mm PRV to the esophagus can meet the dose constraints developed for SBRT central lung tumors. Future studies are needed to validate the safety and efficacy of the planning dose, tolerance dose to normal cardiac tissue, and adequate planning margins.- Additional clinical trials that aim to provide further evidence of the efficacy of CyberHeart Systemare ongoing. [59] (Table 1).
- The detectors were calibrated [25] at the surface of phantom XWU-IMRT (ρ = 1.18 g.cm -3 ; 20×20×20 cm 3 ) with the dome oriented towards the radiation source using the accelerator output at zmax.The sensitivity coefficients SCzeq,A×A in mV.cGy -1 were obtained at the zeq equivalent depth and with a given field size A×A. As the MOSFET detectors are of the dual type, temperature dependence is mini- mal [26,27]. The maximal discrepancy in sensitivity measured was 1.8%.
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.
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