272Chinese Journal of Cancer2009; Vol. 28 Issue 3
[Chinese Journal of Cancer 28:3, 272-276; March 2009]; ©2009 Landes Bioscience
0.01 cc ion chamber for the 1 cm × 1 cm beam. For the 1 cm ×
1 cm beam, the TMR of the depth deeper than 15 cm measured
with the 0.01 cc ion chamber was about 4% different compared
with that measured with the 0.13 cc ion chamber; for radiation
fields of ≥ 2 cm × 2 cm, the differences of TMR between the
0.01 cc and 0.13 cc chambers were within 1%. For the radia-
tion fields of ≥ 3 cm × 3 cm, the measured TMR values had a
good consistency with the calculated values obtained from the
percentage depth doses (PDDs) at the depth of 0 to 15 cm; but
the two values were obviously different at the depths of deeper
than 15 cm ( > 2%). Conclusions: For the measurement of small
fields, the choice of a suitable detector is important due to the
lack of lateral electron equilibrium. Misuse of the detector may
affect the accuracy of the measurements for small radiation fields.
When the lateral electron equilibrium is not established, the size
of the detector used to measure the absorbed dose on the central
axis should be considerably smaller than the field size.
With the development of the radiotherapy apparatuses and the
computer techniques, such as the application of the stereotactic
radiotherapy (SRT) and intensity modulated radiation therapy
(IMRT),1,2 the measurement and calculation of beam data for
small radiation fields become important. In the use of some treat-
ment planning systems (TPSs) and physical validation software,
the beam data for small fields or even the zero-point radiation
field are required to construct the dose calculation model. These
beam data include the percentage depth doses (PDDs) (or tissue-
maximum ratio (TMR), total scatter factor (Scp) and collimator
scatter factor (Sc). For the step-and-shoot IMRT using multileaf
collimator (MLC), the minimum radiation field is 1 cm x 1 cm or
even smaller. For the circular radiation field of SRT, the minimum
diameter for the radiation field of the collimator of the Elekta
-knife (Elekta, Sweden) at the focal plane is 4mm. When the lateral
electron equilibrium cannot be established, the size of the detector
used to measure the absorbed dose on the central axis should be
considerably smaller than the radiation field for the measurement
of beam data for small radiation fields ( ≤ 3 cm × 3 cm).3,4 Using
the 6 MV x-rays, the results from the Monte Carlo simulation
method suggest that the minimum radiation radius of the beam to
establish the lateral electron equilibrium is about 1.0 cm.5 When
different detectors are used, such as the ionization chamber or the
Background and Objective: Accurate data acquisition is
very important to establish a reliable dose calculation model
of the treatment planning system for small radiation fields in
intensity modulated radiation therapy (IMRT) and stereotactic
radiotherapy (SRT). This study was to analyze and compare
small-field measurements using different methods and ionization
chambers. Methods: Three types of farmer chambers were used,
with active volumes of 0.65 cc, 0.13 cc and 0.01cc respectively.
The beam data, including the total scatter factor (Scp), collimator
scatter factor (Sc), tissue-maximum ratio (TMR), were acquired
in a 30 × 30 × 30 cm3 water phantom under two linear accelera-
tors. Measurements were performed at accelerating potentials of
4, 6, and 8 MV with the beam size ranging from 1 cm × 1cm to
10 cm × 10 cm. The measurements were analyzed and compared.
Results: For the beam size of ≥ 3 cm × 3 cm, the differences in Scp
and Sc measurements of the 0.65 cc, 0.13 cc and 0.01 cc ion cham-
bers were within 0.8%, while the differences were much greater
for the beam size of less than 3 cm × 3 cm (the maximum differ-
ence reached 64%). Using 4, 6 and 8 MV x-rays, Sc measured by
the 0.13cc chamber with an elongated source-to-surface distance
(SSD) ( > 150cm) were 25.4%, 6.9%, 24.6%, and 1.4%, 1.4%,
2.2% greater than those measured by a standard SSD (100 cm)
for 1 cm × 1 cm and 2 cm × 2 cm beams respectively; although
there was no significant difference in Sc measurements for the
beams of ≥ 2 cm × 2 cm using the elongated SSD of the 0.13 cc
and the 0.01 cc ion chambers, Sc measured by the 0.13 cc ion
chamber were 0.2%, 8.5%, 3.4% less than those measured by the
*Correspondence to: Li-Xin Chen; State Key Laboratory of Oncology in South
China; Guangzhou, Guangdong 510060 P.R. China; and Department of Radiation
Oncology; Cancer Center; Sun Yat-sen University; Guangzhou, Guangdong
510060 P.R. China; Tel.:86.20.87343089; Fax: 86.20.87343394; Email:
Submitted: 05/26/08; Revised: 10/08/08; Accepted: 12/25/08
This paper was translated into English from its original publication in Chinese.
Translated by: Beijing Xinglin Meditrans Center (http://www.58medtrans.com)
and Hua He on 02/20/09.
The original Chinese version of this paper is published in: Ai Zheng (Chinese
Journal of Cancer), 28(3); http://www.cjcsysu.cn/cn/article.asp?id=14984
Previously published online as a Chinese Journal of Cancer E-publication:
Methods and Technology
Measurements and comparisons for data of small beams
of linear accelerators
Li Chen,1 Li-Xin Chen,1,* Hong-Qiang Sun,1 Shao-Min Huang,1 Wen-Zhao Sun1, Xing-Wang Gao2 and Xiao-Wu Deng1
1State Key Laboratory of Oncology in South China; Guangzhou, Guangdong P.R. China; and Department of Radiation Oncology; Cancer Center; Sun Yat-sen University;
Guangzhou, Guangdong P.R. China; 2Department of Radiation Oncology; No. 458 Hospital of People’s Liberation Army; Guangzhou, Guangdong P.R. China
Key words: ionization chamber, total scatter factor, collimator scatter factor, tissue-maximum ratio, small field
www.landesbioscience.comChinese Journal of Cancer 273
Measurements and comparisons for data of small beams of linear accelerators
semiconductor detector, to measure the data of small beams, the
size and the form of the detectors should suitable for the small radi-
ation fields.6-10 Characteristics such as the directional dependence,
stability and energy response should be taken into account as well.
This study analyzed and compared small-field measurements using
different methods and ionization chambers.
Materials and Methods
The ionization chambers with an active volume of 0.6 cc
(FC65, IBA company), 0.13cc (CC13, IBA company) and 0.01
cc (CC01, IBA company) and the dosemeter (DOSE1, IBA
company) were applied to measure Scp, Sc and TMR of radiation
fields ranging from 1 cm × 1 cm–10 cm × 10 cm. All the beam
data were acquired using the ELEKTA linear accelerator (ELEKTA
Precise Desktop) with accelerating potentials of 4 MV and 8 MV,
and the Varian linear accelerator (Varian 600 C) with an acceler-
ating potential of 6 MV. The water-equivalent phantom (RW3
with a density of 1.05 g·cm-3) was used. The parameters of the
ionization chambers are listed in Table 1.
Measurements of Scp. Scp is defined as the ratio of the dose, at
the maximum dose depth (dmax) on the central axis for an interest
field size, to the dose measured at the same point in a phantom
for a reference of 10 cm × 10 cm field, when both are measured
with the same SAD setup using the same number of monitor units
(MU). The measurement depth in the water-equivalent phantom
was adjusted before the measurement. The measurement depth in
the water-equivalent phantom (dm) is related to the measurement
depth in the water [dw=dm × pm × (Z/A)m/(Z/A)W] is the physical
density, and Z and A are the atomic number and atomic mass
Measurement of Sc. Generally, when the ionization chamber
with the build-up cap is placed at the mechanical isocenter (SSD
= SAD = 100 cm), Sc is calculated using the output dose rate in
air for different radiation fields and normalized to the value for
the reference radiation field (10 cm × 10 cm). Given the outer
radius of the build-up cap (made of copper) is 18 mm using 8 MV
rays (16 mm for 6 MV), the small radiation field of 1 cm × 1 cm
could not totally cover the build-up cap of the ionization chamber.
Therefore, the SSD were elongated to allow the radiation field to
cover the ionization chamber and the build-up cap.12,14 Sc values
for normal conditions (SSD = 100 cm) and for elongated SSD
(SSD ≥ 150 cm) were measured.
Measurements of TMR. Two methods were used to measure
TMR, the SAD method and PDD method. Using the SAD method,
the TMR was measured in RW3 water-equivalent phantom and
the depths were transformed according to Formula (1). The PDDs
were measured in the 3-D water phantom (RFA300, Scanditronix
Medical AB Company) for radiation fields ranging from 2 × 2 cm2
to 10 × 10 cm2. The PDDs were transformed to TMRs using the
OminiPro Accept software (version 6.1, Scanditronix Medical AB
company). TMRs were calculated by the OminiPro Accept using
PDDs and the peak scatter factors (PSFs) from British Journal of
Radiology (BJR).15 However, TMRs for the minimum beam of
the PDDs could not be calculated using PDDs. TMRs of smaller
beams could only be directly measured or extrapolated from the
measurement data of larger beams.
Measurement results of Scp. The measured data of Scp using
the three chambers with accelerating potentials of 4, 6, 8 MV are
shown in Figure 1.
In reference to the data measured by the CC01 ionization
chamber, the relative errors between the other two ionization
chambers and the CC01 chamber are listed in Table 2. The relative
error (%) = (measurement data by the CC13 or FC65 chamber –
measurement date by the CC01 chamber)/(measurements by the
CC01 chamber) × 100. The results measured by the three ioniza-
tion chambers were similar for the radiation fields of ≥ 4 cm × 4
Figure 1. The total scatter factors measured by the three chambers under
4, 6, 8 MV x-rays.
Geometrical parameters of the ionization
chambers used in dose measurements
Measurements and comparisons for data of small beams of linear accelerators
274Chinese Journal of Cancer 2009; Vol. 28 Issue 3
cm. The relative errors using 4, 6, 8 MV rays were within 0.4%,
0.6% and 0.7%, respectively. Greater relative errors were obtained
from the three ionization chambers for the radiation fields of ≤ 3
cm × 3 cm, because the lateral electron equilibrium could not be
established. The relative error was negatively correlated to the size
of the radiation field. The measurement data increased with the
decrease of the active volume of the ionization chamber.
Measurement results of Sc. Because the volume of the FC65
ionization chamber and its build-up cap were large, Sc was
only measured using the CC01 and CC13 chambers with their
build-up caps. Using 4, 6 and 8MV rays, Sc measured by the CC13
and CC01 chamber with an elongated source-to-surface distance
(SSD) are shown in Figure 2. Using 4, 6 and 8 MV rays, the Sc
measured by the CC13 chamber were 0.2%, 8.5% and 3.4%
smaller than those measured by the CC01 chamber respectively,
for the radiation field of 1 cm × 1 cm. The relative errors of the
radiation field of 2 cm × 2 cm or even bigger ones were within
0.8%. When the SSD was elongated, the radiation field of 2 cm ×
2 cm could cover the ionization chamber and the build-up cap.
The ratios of Sc for the small radiation fields measured with the
elongated SSD to that measured with the standard SSD (100 cm)
are listed in Table 3. Significant differences in the relative errors
were observed between the two methods for the measurement of
radiation field of 1 cm × 1 cm or 2 cm × 2 cm, with the maximal
relative error of 25%. In contrast, the relative errors for the radia-
tion field of ≥ 3 cm × 3 cm were within 1%.
Results of TMR. TMRs for radiation fields of ≥ 3 cm × 3 cm
measured by the CC13 and CC01 ionization chambers had no
significant difference ( < 1%). TMRs for the radiation fields of
1 cm × 1 cm and 2 cm × 2 cm using 4, 6 and 8 MV rays by the
CC01 chamber is listed in Table 4. TMRs were normalized to the
value at dmax, the dmax for 4, 6 and 8MV rays was 1.3 cm, 1.5 cm
and 2.0 cm respectively. Relative ratios of TMR measured by the
two ionization chambers at different depths are shown in Table 5.
The measurement data for the radiation field of 2 cm × 2 cm were
consistent using the two ionization chambers (with the relative
error below 0.8%). However, TMR showed great relative errors for
the radiation field of 1 cm × 1 cm at a depth of more than 15 cm
(with the maximum value of 4%). PDDs measured in the radia-
tion fields ranging from 2 cm × 2 cm to 10 cm × 10 cm in the
RFA300 water phantom were used to calculate the values of TMR
The ratios of collimator scatter factors
measured with an elongated source-to-surface
distance (SSD) to those measured with the
standard SSD by the CC13 or CC01 chamber
The elongated source-chamber distances equal to 150 cm, 176 cm, 181 cm at 4, 6, 8 MV potentials,
The tissue maximum ratios measured
by the CC01 ion chamber
The relative errors of the total scatter factors
measured by the CC13 or FC65 chamber to
those measured by the CC01 chamber under
4, 6, 8 MV x-rays
Figure 2. The diagrams of collimator scatter factors measured by the
CC13 and CC01 ionization chambers.
www.landesbioscience.com Chinese Journal of Cancer275
Measurements and comparisons for data of small beams of linear accelerators
for the radiation fields ranging from 3 cm × 3 cm to 10 cm × 10 cm
by the OminiPro Software. The ratios of TMR measured by the
CC13 ionization chamber using 8MV rays to those calculated with
PDD curves and BSF values are listed in Table 6. TMRs directly
measured by the ionization chamber for the radiation fields ranging
from 3 cm × 3 cm to 10 cm × 10 cm were consistent with those
calculated with PDDs at the depth of ≤ 15 cm. However, with an
increase in the depth, the measured TMR were 2% to 5% higher
than those calculated values, which may be due to the different
detectors used for measuring PDD and TMR.
For the radiation fields of ≥ 4 cm × 4 cm, no differences existed
in the data measured by the three ionization chambers. However,
for small radiation fields, especially for those as small as 1 cm ×
1 cm, great differences were observed in Scp and Sc, even though
the CC13 and CC01 chambers with small active volumes were
used. The standard SSD (SSD = 100 cm) setup is usually used to
measure Sc for normal radiation fields, but elongated SSD should
be used for small radiation fields to guarantee that the radiation
fields can cover the build-up cap, otherwise, great errors would
occur. The difference in TMR was relatively small measured by the
CC01 and CC13 chambers at a depth of less than 15 cm. However,
with the increase of the depth, the difference in measured values by
the two chambers was increased. If TMRs were normalized to the
values at the maximum depth, the relative errors were within 1%,
which might be acceptable in clinical practice. As the measured
data for TMR were relative ratios, the relative deviations could
be eliminated to some extents, though the lateral electron equi-
librium could not be established at some depths. As a result, the
relative errors of TMR were much smaller than those of Scp values
measured by the two ionization chambers.
Positioning is important for the measurement of small radia-
tion fields. Using the ELEKTA 8MV rays, the measurement data
of Scp by the CC01 chamber with the SAD method revealed that,
a deviation of the measurement setup from the central axis of the
radiation fields by 1mm would result in 13.3% errors for the radia-
tion field of 1 cm × 1 cm. For radiation fields of ≥ 2 cm × 2 cm,
such a deviation was within 1%, implying that accurate positioning
is needed for the accurate measurement of small radiation fields.
Pilot studies are needed for more accurate measurement data.
The tissue maximum ratios measured by the
CC13 chamber to those measured by the CC01
The tissue maximum ratios measured by the
CC13 chamber to those calculated by OminiPro
software under a 8MV x-ray
Monte Carlo study claimed that the minimum radiation radius
for 6 MV rays was from 1.0–1.3 cm when the lateral electron
equilibrium could be established. The minimum radiation radius
for 10 MV rays was about 1.7 cm.4,5 The radiation fields for the
lateral electron equilibrium is increased with an increase in energy.
Therefore, radiation to small fields should be carefully planned,
with appropriate positioning and detectors.3,9,16-18 Regarding to
the choice of detectors, besides directional dependence, stability
and energy response, the form and volume of the detector should
also be seriously considered. The measurement of small radia-
tion fields needs a lot of work, including the measurement of the
off-axis ratio (OAR) and so on. Based on the current study, the
measurement and calculation of data for small radiation fields
should be further investigated using other detectors and Monte
Grant: Medical Sci-Tech Research Foundation from Health
Bureau of Guangdong Province (No. A2007215)
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