A Ghebremedhin’s research while affiliated with Loma Linda University and other places

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Publications (40)


SU-F-T-180: Evaluation of a Scintillating Screen Detector for Proton Beam QA and Acceptance Testing
  • Article

June 2016

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30 Reads

A Ghebremedhin

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Purpose:To test the performance of a commercial scintillating screen detector for acceptance testing and Quality Assurance of a proton pencil beam scanning system. Method:The detector (Lexitek DRD 400) has 40cm × 40cm field, uses a thin scintillator imaged onto a 16-bit scientific CCD with ∼0.5mm resolution. A grid target and LED illuminators are provided for spatial calibration and relative gain correction. The detector mounts to the nozzle with micron precision. Tools are provided for image processing and analysis of single or multiple Gaussian spots. Results:The bias and gain of the detector were studied to measure repeatability and accuracy. Gain measurements were taken with the LED illuminators to measure repeatability and variation of the lens-CCD pair as a function with f-stop. Overall system gain was measured with a passive scattering (broad) beam whose shape is calibrated with EDR film placed in front of the scintillator. To create a large uniform field, overlapping small fields were recorded with the detector translated laterally and stitched together to cover the full field. Due to the long exposures required to obtain multiple spills of the synchrotron and very high detector sensitivity, borated polyethylene shielding was added to reduce direct radiation events hitting the CCD. Measurements with a micro ion chamber were compared to the detector’s spot profile. Software was developed to process arrays of Gaussian spots and to correct for radiation events. Conclusion:The detector background has a fixed bias, a small component linear in time, and is easily corrected. The gain correction method was validated with 2% accuracy. The detector spot profile matches the micro ion chamber data over 4 orders of magnitude. The multiple spot analyses can be easily used with plan data for measuring pencil beam uniformity and for regular QA comparison.


SU-F-T-141: Proton Dose Validation in a Phantom Beyond TRUFILL N-BCA Embolization Glue

June 2016

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10 Reads

Purpose:To validate the treatment planning system predicted proton dose beyond a heterogeneity (n-BCA glue) by making a measurement in a custom acrylic phantom. Methods:A custom cubic acrylic phantom was designed for this experiment. A container was designed to fit in the phantom. This container was filled with TRUFILL™ n-Butyl Cyanoacrylate(n-BCA) glue. When the container was placed in the phantom, its center was at a distance of 7.4cm from the entrance. This depth allows us to make measurements around the center of modulation of a 126 MeV proton beam with a 3cm spread-out-Bragg peak. To make measurements at other beam energies, additional acrylic can be added in front of the phantom, to adjust the depth of the heterogeneity. A diamond detector was cross calibrated against a standard cylindrical ion chamber in a 126MeV beam. The diamond detector was then used to make dose measurements beyond the inhomogeneity. The measurement was repeated with the container filled with water. Several measurements were made at each setup, to check reproducibility of measurements. Results:For the same number of Tic3R1 counts, the dose measured with the diamond detector beyond n-BCA glue was 1.053 times the dose measured beyond the water filled container. This result is in agreement with the measured stopping power of glue (1.06). These measurements were in agreement with the dose predicted by the treatment planning system when the electron density of the heterogeneity was replaced with 1.06 before the dose calculation. Conclusion:Our initial measurements validate the dose predicted by our treatment plan in the presence of heterogeneity in a phantom. The material tested (n-BCA glue) is commonly used in the treatment of AVM’s prior to an SRS treatment. An error in dose predicted by the treatment plan in the presence of the glue can be detrimental in a single fraction high dose SRS treatment I received the n-BCA liquid embolic system samples from Codman and Shurtleff, Inc.


Figure 1: Coronal (a) and sagittal (b) images with GTVs expanded with a planning margin. The prostate is in purple; the GTV is in red; the seminal vesicle is in blue
Figure 2: IMRT (a) and proton (b) plans. PGTV(red), PPS(purple), and PSV(blue) are enclosed by various isodose lines
Table 2 Average dosimetric data and plan evaluation parameters for rectum and bladder
Figure 3: IMRT and proton plan comparison in dose volume histogram. a DVH for planning target volumes; b DVH for rectum and bladder; c DVH for femoral heads and penile bulb
Table 3 Average dosimetric data and plan evaluation parameters for other normal tissues

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Passive proton therapy vs. IMRT planning study with focal boost for prostate cancer
  • Article
  • Full-text available

October 2015

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279 Reads

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15 Citations

Radiation Oncology

Exploiting biologic imaging, studies have been performed to boost dose to gross intraprostatic tumor volumes (GTV) while reducing dose elsewhere in the prostate. Interest in proton beams has increased due to superior normal-tissue sparing they afford. Our goal was to dosimetrically compare 3D conformal proton boost plans with intensity-modulated radiation therapy (IMRT) plans with respect to target coverage and avoiding organs at risk. Treatment planning computer tomography scans of ten patients were selected. For each patient, two hypothetical but realistic GTVs each with a fixed volume were contoured in different anatomical locations of the prostate. IMRT and proton beam plans were created with a prescribed dose of 50.4 Gy to the initial planning target volume (PTV) including the PTV of the seminal vesicles (PSV), 70.2 Gy to the PTV of the prostate (PPS), and 90 Gy to the PTV of the gross tumor volumes (PGTVs). For proton plans, uncertainties of range and patient setup were accounted for; apertures were adjusted until the dose-volume coverage of PTVs matched that of the IMRT plan. For both plans, prescribed PTV doses were made identical to allow for comparing normal-tissue doses. Protons delivered more homogeneous but less conformal doses to PGTVs than IMRT did and comparable doses to PSV and PPS. Volumes of bladder and rectum receiving doses higher than 65 Gy were similar for both plans. However, volumes receiving less than 65 Gy were significantly reduced, i.e., protons reduced integral dose by 45.6 % and 26.5 % for rectum and bladder, respectively. This volume-sparing was also seen in femoral heads and penile bulb. Protons delivered comparable doses to targets in dose homogeneity and conformity and spared normal tissues from intermediate-to-low doses better than IMRT did. Further improvement of dose sparing and changes in homogeneity and conformity may be achieved by reducing proton range uncertainties and from implementing intensity modulation.

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Radiographic film dosimetry of proton beams for depth-dose constancy check and beam profile measurement

June 2015

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197 Reads

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7 Citations

Radiographic film dosimetry suffers from its energy dependence in proton dosimetry. This study sought to develop a method of measuring proton beams by the film and to evaluate film response to proton beams for the constancy check of depth dose (DD). It also evaluated the film for profile measurements. To achieve this goal, from DDs measured by film and ion chamber (IC), calibration factors (ratios of dose measured by IC to film responses) as a function of depth in a phantom were obtained. These factors imply variable slopes (with proton energy and depth) of linear characteristic curves that relate film response to dose. We derived a calibration method that enables utilization of the factors for acquisition of dose from film density measured at later dates by adapting to a potentially altered processor condition. To test this model, the characteristic curve was obtained by using EDR2 film and in-phantom film dosimetry in parallel with a 149.65 MeV proton beam, using the method. An additional validation of the model was performed by concurrent film and IC measurement perpendicular to the beam at various depths. Beam profile measurements by the film were also evaluated at the center of beam modulation. In order to interpret and ascertain the film dosimetry, Monte Carlos simulation of the beam was performed, calculating the proton fluence spectrum along depths and off-axis distances. By multiplying respective stopping powers to the spectrum, doses to film and water were calculated. The ratio of film dose to water dose was evaluated. Results are as follows. The characteristic curve proved the assumed linearity. The measured DD approached that of IC, but near the end of the spread-out Bragg peak (SOBP), a spurious peak was observed due to the mismatch of distal edge between the calibration and measurement films. The width of SOBP and the proximal edge were both reproducible within a maximum of 5mm; the distal edge was reproducible within 1 mm. At 5 cm depth, the dose was reproducible within 10%. These large discrepancies were identified to have been contributed by film processor uncertainty across a layer of film and the misalignment of film edge to the frontal phantom surface. The deviations could drop from 5 to 2 mm in SOBP and from 10% to 4.5% at 5 cm depth in a well-controlled processor condition(i.e., warm up). In addition to the validation of the calibration method done by the DD measurements, the concurrent film and IC measurement independently validated the model by showing the constancy of depth-dependent calibration factors. For profile measurement, the film showed good agreement with ion chamber measurement. In agreement with the experimental findings, computationally obtained ratio of film dose to water dose assisted understanding of the trend of the film response by revealing relatively large and small variances of the response for DD and beam profile measurements, respectively. Conclusions are as follows. For proton beams, radiographic film proved to offer accurate beam profile measurements. The adaptive calibration method proposed in this study was validated. Using the method, film dosimetry could offer reasonably accurate DD constancy checks, when provided with a well-controlled processor condition. Although the processor warming up can promote a uniform processing across a single layer of the film, the processing remains as a challenge.


SU-E-T-415: Experimental Determination of the Relative Stopping Power of TRUFILL N-BCA Liquid Embolic System in a Clinical Proton Beam

June 2015

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11 Reads

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1 Citation

Patients who undergo n-BCA glue embolization as part of treatment for AVMs are later referred for proton therapy. Knowing the relative stopping power of the glue accurately allows us to perform accurate dose calculations. In this study we experimentally determine the relative stopping power of an n-BCA mixture in a 126 MeV and 149.6 MeV proton beams. One unit of the TRUFILL™ n-BCA liquid embolic system consists of 1g unit of n-BCA, 1g unit of Tantalum powder and one 10mL vial of Ethiodized oil. The physician mixed 3:1 Ethiodized oil to n-BCA. Five units (20cc) of the n-BCA liquid embolic glue were prepared and placed in a 6cm x 3cm x3cm Lucite container. The container was placed in front of a water tank in the proton beam path. A diamond detector (active volume 0.004mm3) was used to measure distal edge of depth dose of a modulated 126 MeV proton beam collimated using a 3cm brass aperture. The procedure was repeated with a container carrying the same amount of water placed in front of the water tank. The difference in the depth dose measured with glue and with water was used to determine the relative stopping power of the glue. The same determination was done earlier at 149.6 MeV using a different smaller sample (4cc) of n-BCA. The relative stopping power of this particular n-BCA mixture was determined to be 1.06 at both 126 MeV and 149.6 MeV. We are working on obtaining the composition data of the n-BCA glue so we can perform Monte Carlo calculations. Accurate value of the stopping power of the n-BCA glue in the proton beam was determined to be 1.06. It will improve the accuracy of dose calculations in proton radiosurgery procedures on AVM patients with n-BCA embolization.


Commissioning of a proton gantry equipped with dual x-ray imagers and a robotic patient positioner, and evaluation of the accuracy of single-beam image registration for this system

June 2015

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68 Reads

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6 Citations

To check the accuracy of a gantry equipped with dual x-ray imagers and a robotic patient positioner for proton radiotherapy, and to evaluate the accuracy and feasibility of single-beam registration using the robotic positioner. One of the proton treatment rooms at their institution was upgraded to include a robotic patient positioner (couch) with 6 degrees of freedom and dual orthogonal kilovoltage x-ray imaging panels. The wander of the proton beam central axis, the wander of the beamline, and the orthogonal image panel crosswires from the gantry isocenter were measured for different gantry angles. The couch movement accuracy and couch wander from the gantry isocenter were measured for couch loadings of 50-300 lb with couch rotations from 0° to ±90°. The combined accuracy of the gantry, couch, and imagers was checked using a custom-made 30 × 30 × 30 cm(3) Styrofoam phantom with beekleys embedded in it. A treatment in this room can be set up and registered at a setup field location, then moved precisely to any other treatment location without requiring additional image registration. The accuracy of the single-beam registration strategy was checked for treatments containing multiple beams with different combinations of gantry angles, couch yaws, and beam locations. The proton beam central axis wander from the gantry isocenter was within 0.5 mm with gantry rotations in both clockwise (CW) and counterclockwise (CCW) directions. The maximum wander of the beamline and orthogonal imager crosswire centers from the gantry isocenter were within 0.5 and 0.8 mm, respectively, with the gantry rotations in CW and CCW directions. Vertical and horizontal couch wanders from the gantry isocenter were within 0.4 and 1.3 mm, respectively, for couch yaw from 0° to ±90°. For a treatment with multiple beams with different gantry angles, couch yaws, and beam locations, the measured displacements of treatment beam locations from the one based on the initial setup beam registered at the gantry at 0°/180° and couch yaw at 0° were within 1.5 mm in three translations and 0.5° in three rotations for a 200 lb couch loading. Results demonstrate that the gantry equipped with a robotic patient positioner and dual imaging panels satisfies treatment requirements for proton radiotherapy. The combined accuracy of the gantry, couch, and imagers allows a patient to be registered at one setup position and then moved precisely to another treatment position by commanding the robotic patient positioner and delivering treatment without requiring additional image registration.


SU-E-T-251: Developing a Daily Proton Beam Monitoring System

June 2015

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14 Reads

To develop a daily monitoring system for proton beam output check and beam uniformity check. Designed for continuously irradiated photon and electron beams with a field size of 20 cm x 20 cm, the daily output checker (Sun Nuclear, Inc.) is not suitable for monitoring proton beams with inter-pulse beam-off and a field size smaller than 14-16 cm in diameter. To allow such proton beam monitoring, the following tests were performed. 1. Absolute dose and array calibrations which accept continuous irradiation only, were performed using photon beams. 2. Five ion chambers within the central area of 8 cm x 8 cm were utilized to check constancy of output at the center of beam modulation and at distal edge and to check beam symmetry and flatness. 3. To simplify our evaluation, the array calibration was manually modified, such that all five chambers report equal values in spite of their differences in build-up thicknesses. 4. The chamber at the lower-right corner is placed under a buildup thickness that can offer dose measurement at the distal edge. This buildup thickness was determined by proton beam range measurements, which established buildup thickness for beam output measurement at the central chamber and range measurement at the corner chamber. 5. The beam-off delay which allows receipt of pulsed irradiation was activated and optimal delay times were determined for each proton beam at 149.6, 185.6, and 249.5 MeV. The above system was tested by miss-steering proton beams and altering phantom thickness by 1 mm at a time. The system reliably monitored the beam with: 3% tolerance for beam flatness, symmetry and output. The range difference of 0.5 mm could be detected at all energies by setting a tolerance of 20%. A quick daily proton beam monitoring system was feasible.


SU-E-T-134: Assessing the Capabilities of An MU Model for Fields as Small as 2cm in a Passively Scattered Proton Beam

June 2015

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8 Reads

To assess and expand the capabilities of the current MU model for a passively scattered proton beam. The expanded MU model can potentially be used to predict the dose/MU for fields smaller than 2cm in diameter and reduce time needed for physical calibrations. The current MU model accurately predicted the dose/MU for more than 800 fields when compared to physical patient calibrations. Three different ion chambers were used in a Plastic Water phantom for physical measurements: T1, PIN, and A-16. The original MU model predicted output for fields that were affected by the bolus gap factor (BGF) and nozzle extension factor (NEF). As the system was tested for smaller treatment fields, the mod wheel dependent field size factor (MWDFSF) had to be included to describe the changes observed in treatment fields smaller than 3cm. The expanded model used Clarkson integration to determine the appropriate value for each factor (field size factor (FSF), BGF, NEF, and MWDFSF), to accurately predict the dose/MU for fields smaller than 2.5cm in effective diameter. The expanded MU model demonstrated agreement better than 2% for more than 800 physical calibrations that were tested. The minimum tested fields were 1.7cm effective diameter for 149MeV and 2.4cm effective diameter for 186MeV. The inclusion of Clarkson integration into the MU model enabled accurate prediction of the dose/MU for very small and irregularly shaped treatment fields. The MU model accurately predicted the dose/MU for a wide range of treatment fields used in the clinic. The original MU model has been refined using factors that were problematic to accurately predict the dose/MU: the BGF, NEF, and MWDFSF. The MU model has minimized the time for determining dose/MU and reduced the time needed for physical calibrations, improving the efficiency of the patient treatment process.




Citations (18)


... Results reporting good performance and reliability of this type of dosimeter in multiple studies have increased the interest in this technology. 40,[61][62][63][64] Diamonds have a high radiation sensitivity, and hence very small diamonds, of the order of a few cubic millimeters, can be manufactured. They can be considered water equivalent as the water to carbon mass energy absorption coefficient and mass collision stopping power ratios are constant, minimizing the need for considering corrections to energy fluence perturbations. ...

Reference:

A review of diamond dosimeters in advanced radiotherapy techniques
SU-E-T-104: Evaluation of the Dosimetric Properties of a Synthetic Single Crystal Diamond Detector in Clinical Proton Beams
  • Citing Article
  • June 2013

... Proton therapy planning was investigated to deliver not only a uniform dose to prostate but also an escalated dose to dominant intraprostatic lesions (DILs) in prostate with conventional fractionation. 8,9 Proton therapy planning with passive scattering has been investigated with an escalated dose to the DILs in prostate in its comparison with intensitymodulated radiation therapy (IMRT) planning. 8 Proton therapy is provided to targets with more homogeneous but less conformal doses and to normal tissues with better intermediate-to-low doses than IMRT. ...

Passive proton therapy vs. IMRT planning study with focal boost for prostate cancer

Radiation Oncology

... The beam energy was carefully selected by examining existing APBI proton plans to represent an energy that could be greatly affected by the BZs. It also simplified an issue of quenching (underresponse of detector at the Bragg peak due to high linear energy transfer) for the film measurement by avoiding superimposing multiple Bragg peaks at the measured depths [15]. A range shifter for shallow targets generally makes larger spots and may contain more low-energy scattered protons. ...

Radiographic film dosimetry of proton beams for depth-dose constancy check and beam profile measurement

... 5-6 mm) visibly perturbed the SOBP, while for the BaSO 4 -marker the size was restricted to a cylinder shape of 1 Â 3 mm. A major difference of this work compared to other work on markers in proton therapy is that we perform actual physical dose measurements in a clinical proton beam and include real patient data, while previous works have been performed by Monte Carlo simulation [10,29,32]. ...

SU-E-T-415: Experimental Determination of the Relative Stopping Power of TRUFILL N-BCA Liquid Embolic System in a Clinical Proton Beam
  • Citing Article
  • June 2015

... These can be moved into and out of the beam path prior to treatment. Daily images can be taken during setup compared to the digitally reconstructed radiographs (DRRs) from the time of the simulation, while the corrective moves can be performed based on the bony anatomy [17], often on a robotic 6-degree-of-freedom couch [18][19][20]. One caveat is that these can lengthen the path and increase the spot size for pencil beam scanning (PBS) gantries if downstream of the beamline vacuum window. ...

Commissioning of a proton gantry equipped with dual x-ray imagers and a robotic patient positioner, and evaluation of the accuracy of single-beam image registration for this system
  • Citing Article
  • June 2015

... The Elekta's RTsafe, [103] CIRS's STEEV, [104] Standard Imaging's Lucy 3D QA, [105] PTW's Ruby [106] cranial phantoms, as well as high-resolution array detectors such as PTW's OCTAVIUS Detector 1600 SRS, [107] Sun Nuclear's SRS MapCHECK, [108] and IBA Dosimetry's myQA SRS [109] are among the advanced equipment in this field. Some of these detector arrays also have an evaluation gamma index resolution of up to 2%/2 mm. ...

SU-E-T-268: Proton Radiosurgery End-To-End Testing Using Lucy 3D QA Phantom
  • Citing Article
  • June 2014

... 14 Diamond's low dielectric constant and high bandgap result in minimal noise and high signal integrity, crucial for precise dosimetric readings. While diamond dosimeters, and in particular the microDiamond detector (model 60 019, PTW Freiburg), developed by the University of Rome Tor Vergata in collaboration with PTW-Freiburg, is widely diffused and well-established in clinical practice worldwide, 15,16 diamond microdosimeters have been recently proposed for particle therapy applications in various configurations. [17][18][19] The microdosimetric properties of diamond, along with its excellent dosimetric performance, enhance the potential for diamond detectors that can perform both dosimetric and microdosimetric characterizations of radiation beams as recently reported in the literature. ...

SU‐E‐T‐104: Evaluation of the Dosimetric Properties of a Synthetic Single Crystal Diamond Detector in Clinical Proton Beams
  • Citing Article
  • December 2013

... Wroe et al. investigated treatment couches as well as a bite block insert, compared measured to TPS-modeled WET values, and reported a maximum discrepancy <1 mm. 10 Fellin and coworkers expanded the list of devices by including new treatment couches, thermoplastic masks, and headrests. 11 They found a good agreement between measured and TPS-predicted WET for most devices. ...

Water Equivalent Thickness Analysis of Immobilization Devices for Clinical Implementation in Proton Therapy

... First, the intensity throughout the spill has known variations. These are characterized by ripple of known magnitude (Coutrakon et al 1996) at 60, 120, 180, 360 and 720 Hz. The beam intensity has been modeled with a Gaussian envelope and ripple from 60 Hz harmonics based on earlier measurements (Coutrakon et al 1996).Figure 3(a) ...

Spill uniformity measurements for a raster scanned proton beam
  • Citing Article
  • January 2000

... It often tends to overestimate the maximum dose to OARs, complicating subsequent plan design and optimization. Therefore, a 4D reconstructed dose approach is necessary to evaluate dose uncertainties due to cardiorespiratory motion (24,48). ...

Evaluation and comparison of New 4DCT based strategies for proton treatment planning for lung tumors

Radiation Oncology