Output Factors For Field Sizes Research Articles

A Monte Carlo model of an agility head for a 6 MV Elekta photon beam

IntroductionWith the advance in radiotherapy delivery, the need for a method allowing for a precise dose calculation increased. Monte Carlo modelling of radiotherapy dosages has become an important methodology for the evaluation of dose and beam performance, especially given recent advances in radiotherapy. In this study, an Elekta equipped with an Agility multileaf collimator will be modelled and validated in order to be used in further studies. MethodologyA 6 MV photon beam for the Elekta Synergy linear accelerator with an Agility head was simulated, adjusted, and confirmed using the BEAMnrc/EGSnrc algorithm. Tuning involved comparing percentage depth dose with measured data using different incident electron energies, as analyzed using the gamma index. The best energy so calculated was then used with different full half width maximums (FWHMs) and different angular divergences in order to tune the lateral profiles. Output factors for different field sizes were validated with measurements. Then, the validated model was checked for different depths. ResultsFor PDD comparison, the best electron energy was found to be 6.6 MeV, where the gamma analysis is 100%. A 0.15 cm circle FWHM was found to have the best FWHM. with an angular diverge of 0.03°. The output factors match measured values, proving that the model can achieve a suitably high accuracy. ConclusionThe validated model can be used in advanced radiotherapy dose calculation such as in VMAT, IMRT, and Stereotactic radiotherapy. The model needs to be validated in future studies for smaller field sizes, surface dose, and out-of-field dosimetry.

Validation of a 10 MV photon beam Elekta Synergy linear accelerator using the BEAMnrc MC code

Objective: The Monte Carlo (MC) technique can accurately model any linear accelerator where accurate details about the treatment head and the incident electron beam have been provided. However, manufacturers generally do not provide such details as the energy, radial intensity (spot size), or the angular spread of the incident electron beam. The aim of this study is to predict these details and validate a MC Linac model with the measurements.Methodology: A 10 MV photon beam from an Elekta Synergy Linac was modelled using the BEAMnrc code. The percentage depth dose of the MC model was generated using different electron energies and compared with the measurements using a Gamma index (γ) with two different criteria sets. The dose profiles of the fine-tuned electron energy were generated using different spot sizes and compared with appropriate measurements. In addition, the fine-tuned electron energy and spot size dose profiles were generated using different angular spreads to minimise any differences. Finally, the output factors for different field sizes, with and without a wedge, and also the quality index, were compared.Results: The fine-tuned electron energy of the MC model was found to be 9.8 MeV, where 94.12% of the calculation points pass the γ test. For the spot size, a circular radial intensity of 0.35 cm best described the 10 MV photon beam. Furthermore, a mean angular spread of 0.05° minimised the cross-field profile differences between calculation and measurement. The largest differences between the output factors from the MC model and measurements were -0.8% and 4.7% for an open and wedged field, respectively. Ultimately, a difference of 0.82% in the quality index was achieved.Conclusion: A reliable MC model of a 10 MV photon beam Elekta Synergy Linac can be achieved as presented in this study using the BEAMnrc code. This model can be reliably used to calculate dose distributions.

Characterization of ferrous-agarose-xylenol gel dosimeter at 60Co γ-rays beam therapy unit.

Fricke gel dosimetry is a valuable technique used for recording 3D dose distribution in radiotherapy. Herein, we present the dosimetric characteristics of a synthesized ferrous-agarose-xylenol orange gel dosimeter in a clinical 60Co beam. Experimental data were obtained using a secondary standards dosimetry laboratory 60Co therapy unit. The dosimeter was calibrated using the ionization chamber as a reference, and its total mass attenuation coefficient, absorption spectrum, optical density-dose relationship, sensitivity, and dose rate dependency were evaluated. The potentiality of the ferrous-agarose-xylenol gel dosimeter was investigated to measure output factors for different field sizes. The gel dosimeter readings were measured using a spectrophotometer. The ferrous-agarose-xylenol gel dosimeter exhibited a linearity in the range of 3-15Gy, indicating that the dosimeter is tissue-equivalent and dose rate-independent and yield reproducible results. The measured output factors and those published in the literature showed good agreement.

Investigation of the Dependency of the Output Factor on the Field Size in Passive Scattering Proton Therapy at the National Cancer Center in Korea

The dependency of the output factor on the field size in the passive scattering proton therapy at the National Cancer Center (NCC) in Korea was investigated through a comparison of the measured output factors. The output factor of the fields of the representative beam options was measured using a Pinpoint chamber with standard circular blocks of less than 5 cm in diameter. The measured output factors for different field sizes show that the dependency of the output factor on the field size is not negligible even in the field for the 5 cm in diameter for long-range proton beams. In addition, the conflicting dependency in shallow fields indicates that the potential contribution of scattered protons should be considered in predicting and modelling of the output factors in small fields.

Technical Note: Dosimetric characterization of the dynamic beam flattening MLC sequence on a ring shaped, Jawless Linear Accelerator with double stacked MLC.

To characterize the dosimetric features and limitations of the dynamic beam flattening (DBF) on the Halcyon 2.0 linear accelerator (Varian Medical Systems). A predefined multi-leaf collimator (MLC) sequence was introduced and used to flatten the 6MV flattening filter free (FFF) beam on the Halcyon 2.0. Dosimetric characterizations of the flattened beams, including beam flatness, symmetry, percent depth dose (PDD), output factor and MU linearity, were investigated. Flatness and symmetry were obtained from profile measurements with both radiographic films (EDR2) and a two dimensional ion-chamber array (IC Profiler, Sun Nuclear Corporation). MU linearity, output factors, and PDDs were measured in a water tank with a CC13 ion chamber (Scanditronix Wellhöfer, Nuremburg, Germany). In addition, the effect of the DBF sequence on 3D plan quality was evaluated by creating DBF plans for a 4-field box rectum and an AP/PA spine plan. Patient specific QA was performed on these plans. At 100cm SSD and 10cm depth, a flatness of <3% was observed on both transversal and radial profiles for all square field sizes ≥10cm with DBF. For both larger and smaller field sizes the flatness showed a tendency to increase as the fields got bigger or smaller, respectively. Similar trends in flatness were observed at all depths measured. All measured output factors for square field sizes ≥5cm were within 1% of the TPS prediction. Linearity was ≤2.02% for all measurements. For both treatment sites, the MD judged the plans created for the Halcyon without the use of DBF not to be clinically acceptable, however considered both the TrueBeam plan and the Halcyon plan with the DBF sequence to be clinically acceptable. The DBF sequence on the Halcyon and its characteristics were investigated. The analysis indicates that the DBF sequence can be used on the Halcyon to generate clinically acceptable 3D treatment plans.

Characterization of Fricke-gelatin dosimeters for intraoperative Radiation Therapy dosimetry

In this work, the usability of Fricke gels in Intraoperative Radiation Therapy (IORT) dosimetry was investigated. Irradiated gel systems are characterized by means of UV/Vis spectrophotometric analysis in order to define the main dosimetric indexes of interest and to evaluate their post-irradiation stability. A measurement of Output Factors for different field sizes is also attempted to study the application of this system for routine quality control. Ad-hoc manufactured thin-layers of gel are employed to perform acquisitions of planar transverse dose profiles and to quantify the effect of Fe(III) diffusion. The Fricke gel system presents an adequate dosimetric performance, with excellent response linearity up to 30 Gy. Comparison of dosimetric response over a wide range of irradiation dose rates allows to conclude that the gel response is not influenced significantly by this parameter. Transverse dose profiles acquired with thin-layers present an agreement within 3% with reference values, even in very steep dose gradient regions. Finally, computed values for the diffusion coefficient of ferric ions are in good agreement with those reported in literature for dosimetric gels of similar composition. Overall performance of the gel system appears promising for further applications in IORT dosimetry.

Application of the TRS 483 code of practice for reference and relative dosimetry in tomotherapy.

To apply the recent code of practice from the IAEA/AAPM, TRS 483, to helical tomotherapy (HT) for reference and relative dosimetry obtaining correction factors for the Exradin A1SL ionization chamber. The beam quality correction factor for the A1SL chamber was obtained through three different approaches following TRS 483 concepts and compared with published values. The determination of the reference absolute dose for the machine-specific reference (msr) field was complemented with relative dosimetry through the determination of output factors of small fields using different detectors. The response of A1SL was compared with correction-free film results and corrected output factors of other detectors. A weighted mean beam quality correction factor of 0.9945±0.0073 was obtained for the A1SL chamber which is in agreement with values reported in the literature. Output factors obtained with different detectors were in agreement, given the uncertainty level. Considering the film output factors as free of corrections, the average value for A1SL output factors corrections was 1.000±0.007. The beam quality correction factors for the A1SL chamber obtained through the three different pathways recommended by TRS 483 agreed with each other and also with published values. The measurements from the A1SL chamber normalized to the msr field in HT can be taken as output factors for small clinical field sizes without further corrections.

Technical Note: Experimental characterization of the dose deposition in parallel MRI-linacs at various magnetic field strengths.

Dose deposition measurements for parallel MRI-linacs have previously only shown comparisons between 0T and a single available magnetic field. The Australian MRI-Linac consists of a magnet coupled with a dual energy linear accelerator and a 120 leaf Multi-Leaf Collimator with the radiation beam parallel to the magnetic field. Two different magnets, with field strengths of 1 and 1.5T, were used during prototyping. This work aims to characterize the impact of the magnetic field at 1 and 1.5T on dose deposition, possible by comparing dosimetry measured at both magnetic field strengths to measurements without the magnetic field. Dose deposition measurements focused on a comparison of beam quality (TPR20/10 ), PDD, profiles at various depths, surface doses, and field size output factors. Measurements were acquired at 0, 1, and 1.5T. Beam quality was measured using an ion chamber in solid water at isocenter with appropriate TPR20/10 buildup. PDDs and profiles were acquired via EBT3 film placed in solid water either parallel or perpendicular to the radiation beam. Films at surface were used to determine surface dose. Output factors were measured in solid water using an ion chamber at isocenter with 10cm solid water buildup. Beam quality was within ±0.5% of the 0T value for the 1 and 1.5T magnetic field strengths. PDDs and profiles showed agreement for the three magnetic field strengths at depths beyond 20mm. Deposited dose increased at shallower depths due to electron focusing. Output factors showed agreement within 1%. Dose deposition at depth for a parallel MRI-linac was not significantly impacted by either a 1 or 1.5T magnetic field. PDDs and profiles at shallow depths and surface dose measurements showed significant differences between 0, 1, and 1.5T due to electron focusing.

Development and Validation of End-to-End Test for Stereotactic Radiosurgery

The purpose of this study is to evaluate End to End test and dosimetric test of small field SRS MapCHECK dosimeter. The Fundamental properties of the SRS MapCHECK Diode Detector such as reproducibility, linearity, Energy dependence, Dose rate dependency and output factor are studied for 6MV FFF energy photon beams. All dosimetric test carried out on a medical accelerator consisting High definition 120 multileaf collimator (HD120 MLC), provide stereotactic treatments. End to End test of SRSMapCHECK carried out with StereoPHAN Cylindrical Phantom with CT and MRI inserts. The E2E test includes MRI scan, CT scan, Target contouring on CT/MRI scan, MRI/CT image fusion, Treatment planning and Dose delivery. SRS MapCHECK comprise of 1013 solid state diode detector, are equally spaced at 2.47mm. The active detector area is 0.48mm x 0.48mm and resolution is 0.007mm3. The maximum field size is 7 X 7 cm2. Before to start dosimetric measurement the calibration of SRS MapCHECK and StereoPHAN is necessary. The calibration process includes background measurement, array calibration and StereoPHAN factor calculation. The reproducibility of SRS MapCHECK checked by measuring output for 5 consecutive days for 6MV FFF beam with field size 5x5cm2. The Linearity of dose measurement also evaluated. The response of SRS MapCHECK to different energy (6, 10 & 15MV) also studied. Dose rate dependency also studied by varying dose rate from 400 to 1400 MU/min. Output Factor for different field sizes ranging from 1 x 1 cm2 to 7 x 7 cm2 was measured. Geometric accuracy of inserts volumes are find with CT and MRI fusion. Actual Insert volumes of Central Sphere, Right Sphere, Left Sphere are 4.17cc and 0.52 cc, 0.52cc respectively. The TPS measured volume on CT scan is 4.09cc, 0.52cc and 0.53 cc respectively similarly on MRI TPS measured volume is 4.22cc, 0.50cc and 0.52cc. The % variation on CT volume is less than 2%. For MRI the % difference for right sphere is -4.0%, central and left side are less than 2% The reproducibility of SRS MapCHECK output is well within less than 2% of the first day measurement. The SRS MapCHECK showed linear dose response for energy 6 MV FFF. Energy Dependency checked for 6MV, 10MV and 15MV with fixed dose rate 600MU/min it showed linear response to energy. We found SRS MapCHECK are independent on dose rate. Output factor was measured and compared with pinpoint chamber 0.015cc. SRS MapCHECK showed under response compared to pinpoint chamber for field size 1 x 1 cm2 to 4 x 4 cm2 and showed over response from the 5 x 5 cm2 to 7 x 7 cm2 The gamma analysis done with Dose difference 3% and Dose to agreement 3mm, we also checked with DAT 2mm, 1mm with threshold 10%. The result of gamma passing is 97.6%, 96.8% and 95.8 % respectively. All dosimetric test performed on SRS MapCHECK are comparable with ionization chamber measurement and clinically acceptable. It is an effective tool for E 2 E test and patient’s specific QA for SRS Patients.

First measurements with a plastic scintillation dosimeter at the Australian MRI-LINAC

MRI-LINACs combine MRI and LINAC technologies with the potential for image guided radiation therapy with optimal soft-tissue contrast. In this work, we present the advantages and limitations of plastic scintillation dosimeters (PSDs) for relative dosimetry with MRI-LINACs. PSDs possess many desirable qualities, including magnetic field insensitivity and irradiation angle independence, which are expected to make them suitable for dosimetry with MRI-LINACs. An in-house PSD was used to measure field size output factors as well as a percent depth dose distribution and the beam quality index TPR20/10 at a cm2 field size. Measurements were repeated with a Scanditronix/Wellhofer FC65-G ionisation chamber and PTW 60019 microDiamond detector for comparison. Relative differences were calculated between the three detectors, where the mean difference in dose was 1.2% between the PSD and ionisation chamber, 1.9% between the PSD and microDiamond detector and 1.3% between the microDiamond detector and the ionisation chamber. The closeness between the three mean differences in doses suggests that PSDs are feasible for relative dosimetry with MRI-LINACs.

Radiation Therapy Deficiencies Identified During On-Site Dosimetry Visits by the Imaging and Radiation Oncology Core Houston Quality Assurance Center

To review the dosimetric, mechanical, and programmatic deficiencies most frequently observed during on-site visits of radiation therapy facilities by the Imaging and Radiation Oncology Core Quality Assurance Center in Houston (IROC Houston). The findings of IROC Houston between 2000 and 2014, including 409 institutions and 1020 linear accelerators (linacs), were compiled. On-site evaluations by IROC Houston include verification of absolute calibration (tolerance of ±3%), relative dosimetric review (tolerances of ±2% between treatment planning system [TPS] calculation and measurement), mechanical evaluation (including multileaf collimator and kilovoltage-megavoltage isocenter evaluation against Task Group [TG]-142 tolerances), and general programmatic review (including institutional quality assurance program vs TG-40 and TG-142). An average of 3.1 deficiencies was identified at each institution visited, a number that has decreased slightly with time. The most common errors are tabulated and include TG-40/TG-142 compliance (82% of institutions were deficient), small field size output factors (59% of institutions had errors ≥3%), and wedge factors (33% of institutions had errors ≥3%). Dosimetric errors of ≥10%, including in beam calibration, were seen at many institutions. There is substantial room for improvement of both dosimetric and programmatic issues in radiation therapy, which should be a high priority for the medical physics community. Particularly relevant was suboptimal beam modeling in the TPS and a corresponding failure to detect these errors by not including TPS data in the linac quality assurance process.

Characterisation of small photon field outputs in a heterogeneous medium using X-ray voxel Monte Carlo dose calculation algorithm

AbstractAimTo characterise small photon beams using the Monte Carlo dose calculation algorithm for small field ranges in a heterogeneous medium.Materials and methodAn in-house phantom constructed with three different mediums, foam, polymethyl methacrylate and delrin resembling the densities of lung, soft tissue and bone respectively, was used in this study. Photon beam energies of 6 and 15 MV and field sizes of 8×8, 16×16, 24×24, 32×32 and 40×40 mm using X-ray voxel Monte Carlo (XVMC) algorithm using different detectors were validated. The relative output factor was measured in three different mediums having six different tissue interfaces; at the depth of 0, 1, 2 and 3 cm. The planar dose verification was undertaken using gafchromic films and considered dose at the lung and bone medium interfaces. For all the measurements, 104×104 mm was taken as the reference field size. The relative output factor for all other field sizes was taken and compared with planning system calculated values.ResultsFrom field size 16×16 mm and above, the relative output factors were analysed in bone and soft tissue medium having lung as first medium. The maximum deviations were observed as 1·8 and 1·3% for 6 MV and 2·5 and 1·1% for 15 MV photon beams for bone and soft tissue, respectively. For lung as measurement medium, the maximum deviation of 14·8 and 19·2% were observed and having bone as first medium with 8×8 mm for 6 and 15 MV photon beams, respectively. The fluence verification of dose spectrum for the lung–bone interface scenarios with smaller field sizes were found within 2% of deviation with treatment planning system (TPS).ConclusionThe accuracy of dose calculations for small field sizes in XVMC-based treatment planning algorithm was studied in different inhomogeneous mediums. It was found that the results correlated with measurement data for field size 16×16 mm and above. Noticeable deviation was observed for the smallest field size of 8×8 mm with interfaces of significant change in density. The observed results demands further analysis of work with smaller field sizes.

A 2D silicon detector array for quality assurance in small field dosimetry: DUO.

Nowadays, there are many different applications that use small fields in radiotherapy treatments. The dosimetry of small radiation fields is not trivial due to the problems associated with lateral disequilibrium and source occlusion and requires reliable quality assurance (QA). Ideally such a QA tool should provide high spatial resolution, minimal beam perturbation and real time fast measurements. Many different types of silicon diode arrays are used for QA in radiotherapy; however, their application in small filed dosimetry is limited, in part, due to a lack of spatial resolution. The Center of Medical Radiation Physics (CMRP) has developed a new generation of a monolithic silicon diode array detector that will be useful for small field dosimetry in SRS/SRT. The objective of this study is to characterize a monolithic silicon diode array designed for dosimetry QA in SRS/SRT named DUO that is arranged as two orthogonal 1D arrays with 0.2 mm pitch. DUO is two orthogonal 1D silicon detector arrays in a monolithic crystal. Each orthogonal array contains 253 small pixels with size 0.04 × 0.8 mm2 and three central pixels are with a size of 0.18 × 0.18 mm2 each. The detector pitch is 0.2 mm and total active area is 52 × 52 mm2 . The response of the DUO silicon detector was characterized in terms of dose per pulse, percentage depth dose, and spatial resolution in a radiation field incorporating high gradients. Beam profile of small fields and output factors measured on a Varian 2100EX LINAC in a 6 MV radiation fields of square dimensions and sized from 0.5 × 0.5 cm2 to 5 × 5 cm2 . The DUO response was compared under the same conditions with EBT3 films and an ionization chamber. The DUO detector shows a dose per pulse dependence of 5% for a range of dose rates from 2.7 × 10-4 to 1.2 × 10-4 Gy/pulse and 23% when the rate is further reduced to 2.8 × 10-5 Gy/pulse. The percentage depth dose measured to 25 cm depth in solid water phantom beyond the surface and for a field size of 10 × 10 cm2 agrees with that measured using a Markus IC within 1.5%. The beam profiles in both X and Y orthogonal directions showed a good match with EBT3 film, where the FWHM agreed within 1% and penumbra widths within 0.5 mm. The effect of an air gap above the DUO detector has also been studied. The output factor for field sizes ranging from 0.5 × 0.5 cm2 to 5 × 5 cm2 measured by the DUO detector with a 0.5 mm air gap above silicon surface agrees with EBT3 film and MOSkin detectors within 1.8%. The CMRP's monolithic silicon detector array, DUO, is suitable for SRS/SRT dosimetry and QA because of its very high spatial resolution (0.2 mm) and real time operation.

Measurement and Analysis of PDDs Profile and Output Factors for Small Field Sizes by cc13 and Micro-Chamber cc01

Small radiation fields are abundantly used in modern radiotherapy techniques like in IMRT and SRS. In order to commission these techniques, dosimetric data for small fields is required. The purpose of this study is to compare dosimetric measurements with two different ion chambers cc13, and cc01 for smaller fields. Dosimetric measurements are beam profile, output factor, pdds, and collimator factor. Dosimetric data is acquired in water phantom for two different photon beam energies 6 MV and 15 MV with zero gantry angle. In beam profiles cc13 chamber, measure wider penumbra as compare to cc01. And this wider measurement of penumbra occurs for smaller as well as for larger field sizes. Accumulated relative error in the measurement of penumbra for number of field sizes and 6 MV at dmax, and at 10 cm depth are 34.32% and 27.72% respectively. Accumulated relative error in the measurement of penumbra for number of field sizes and 15 MV at dmax, and at 10 cm depth are 28.49% and 23.92%. In case of output factor for smaller fields cc13 underestimates the output factor relative to cc01, with non-linear increase for smaller fields. But for larger fields, this increase in output factor is almost linear difference of two chambers is decreased. For very smaller fields × 2 cm, relative error in output factor of cc13 and cc01 is greater than 5% and rapidly increases with decreasing field size. But for lager fields, this relative error is negligible. In measurement of pdds after the buildup region difference occurs in the response of two chambers cc13 and cc01 for smaller fields. For field sizes ≤2 cm × 2 cm average cc13-cc01 at various depths 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, and 80 cm is almost greater than 0.5 cm. And similarly as output factor, this difference (cc13-cc01) increases with field size decreasing.

CyberKnife beam output factor measurements: A multi-site and multi-detector study

PurposeNew promising detectors are available for measuring small field size output factors (OFs). This study focused on a multicenter evaluation of two new generation detectors for OF measurements on CyberKnife systems. MethodsPTW-60019 microDiamond and W1 plastic scintillation detector (PSD) were used to measure OFs on eight CyberKnife units of various generations for 5–60mm fixed cones. MicroDiamond and PSD OF were compared to routinely used silicon diodes data corrected applying published Monte Carlo (MC) factors. PSD data were corrected for Čerenkov Light Ratio (CLR). The uncertainties related to CLR determination were estimated. ResultsConsidering OF values averaged over all centers, the differences between MC corrected diode and the other two detectors were within 1.5%. MicroDiamond exhibited an over-response of 1.3% at 7.5mm and a trend inversion at 5mm with a difference of 0.2%. This behavior was consistent among the different units. OFs measured by PSD slightly under-responded compared to MC corrected diode for the smaller cones and the differences were within 1%. The observed CLR variability was 2.5% and the related variation in OF values was 1.9%. ConclusionThis study indicates that CyberKnife microDiamond OF require corrections below 2%. The results are enhanced by the consistency observed among different units. Scintillator shows a good agreement to MC corrected diode but CLR determination remains critical requiring further investigations. The results emphasized the value of a multi-center validation over a single center approach.

SU-F-T-610: Comparison of Output Factors for Small Radiation Fields Used in SBRT Treatment

Purpose: In order to fundamentally understand our previous dose verification results between measurements and calculations from treatment planning system (TPS) for SBRT plans for different sized targets, the goal of the present work was to compare output factors for small fields measured using EDR2 films with TPS and Monet Carlo (MC) simulations. Methods: 6MV beam was delivered to EDR2 films for each of the following field sizes; 1×1 cm2, 1.5×1.5 cm2, 2×2 cm2, 3×3 cm2, 4×4 cm2, 5×5 cm2 and 10×10 cm2. The films were developed in a film processer, then scanned with a Vidar VXR-16 scanner and analyzed using RIT113 version 6.1. A standard calibration curve was obtained with the 6MV beam and was used to get absolute dose for measured field sizes. Similar plans for all fields sizes mentioned above were generated using Eclipse with the Analytical Anisotropic Algorithm. Similarly, MC simulations were carried out using the MCSIM, an in-house MC code for different field sizes. Output factors normalized to 10×10 cm2 reference field were calculated for different field sizes in all the three cases and compared. Results: For field sizes ranging from 1×1 cm2 to 2×2 cm2, the differences in output factors between measurements (films), TPS and MC simulations were within 0.22%. For field sizes ranging from 3×3cm2 to 5×5cm2, differences in output factors were within 0.10%. Conclusion: No clinically significant difference was obtained in output factors for different field sizes acquired from films, TPS and MC simulations. Our results showed that the output factors are predicted accurately from TPS when compared to the actual measurements and superior dose calculation Monte Carlo method. This study would help us in understanding our previously obtained dose verification results for small fields used in the SBRT treatment.

SU‐F‐T‐260: Using Portal Image Device for Pre‐Treatment QA in Volumetric Modulated Arc Plans with Flattening Filter Free (FFF) Beams

Purpose:To implement and validate a method of using electronic portal image device (EPID) for pre‐treatment quality assurance (QA) of volumetric modulated arc therapy (VMAT) plans using flattering filter free (FFF) beams for stereotactic body radiotherapy (SBRT).Methods:On Varian Edge with 6MV FFF beam, open field (from 2×2 cm to 20×20 cm) EPID images were acquired with 200 monitor unit (MU) at the image device to radiation source distance of 150cm. With 10×10 open field and calibration unit (CU) provided by vendor to EPID image pixel, a dose conversion factor was determined by dividing the center dose calculated from the treatment planning system (TPS) to the corresponding CU readout on the image. Water phantom measured beam profile and the output factors for various field sizes were further correlated to those of EPID images. The dose conversion factor and correction factors were then used for converting the portal images to the planner dose distributions of clinical fields. A total of 28 VMAT fields of 14 SBRT plans (8 lung, 2 prostate, 2 liver and 2 spine) were measured. With 10% low threshold cutoff, the delivered dose distributions were compared to the reference doses calculated in water phantom from the TPS. A gamma index analysis was performed for the comparison in percentage dose difference/distance‐to‐agreement specifications.Results:The EPID device has a linear response to the open fields with increasing MU. For the clinical fields, the gamma indices between the converted EPID dose distributions and the TPS calculated 2D dose distributions were 98.7%±1.1%, 94.0%±3.4% and 70.3%±7.7% for the criteria of 3%/3mm, 2%/2mm and 1%/1mm, respectively.Conclusion:Using a portal image device, a high resolution and high accuracy portal dosimerty was achieved for pre‐treatment QA verification for SBRT VMAT plans with FFF beams.

Multicenter evaluation of a synthetic single-crystal diamond detector for CyberKnife small field size output factors

PurposeThe aim of the present work was to evaluate small field size output factors (OFs) using the latest diamond detector commercially available, PTW-60019 microDiamond, over different CyberKnife systems. OFs were measured also by silicon detectors routinely used by each center, considered as reference. MethodsFive Italian CyberKnife centers performed OFs measurements for field sizes ranging from 5 to 60mm, defined by fixed circular collimators (5 centers) and by Iris™ variable aperture collimator (4 centers). Setup conditions were: 80cm source to detector distance, and 1.5cm depth in water. To speed up measurements two diamond detectors were used and their equivalence was evaluated. MonteCarlo (MC) correction factors for silicon detectors were used for comparing the OF measurements. ResultsConsidering OFs values averaged over all centers, diamond data resulted lower than uncorrected silicon diode ones. The agreement between diamond and MC corrected silicon values was within 0.6% for all fixed circular collimators. Relative differences between microDiamond and MC corrected silicon diodes data for Iris™ collimator were lower than 1.0% for all apertures in the totality of centers. The two microDiamond detectors showed similar characteristics, in agreement with the technical specifications. ConclusionsExcellent agreement between microDiamond and MC corrected silicon diode detectors OFs was obtained for both collimation systems fixed cones and Iris™, demonstrating the microDiamond could be a suitable detector for CyberKnife commissioning and routine checks. These results obtained in five centers suggest that for CyberKnife systems microDiamond can be used without corrections even at the smallest field size.

SU‐E‐T‐246: MU Model Implementation for a Passively Scattered Proton Beam: Inclusion of the Bolus Gap and Nozzle Extension Factors

Purpose:To develop and implement an MU model for a passively scattered proton beam, and eliminate the need for patient specific calibrations for field sizes from 3cm to 15cm. This would enable consistent and timely calibrations for a wide variety of patient portals, streamlining the treatment process.Methods:Measurements were initially made using a Standard Imaging A‐16 ion chamber and a modified water tank to determine the bolus gap factors (BGF) for multiple combinations of aperture size, bolus thickness, and air gap. The BGF was then separated into two component factors: the bolus thickness factor (BTF) and the nozzle extension factor (NEF). Polynomial curves were generated using the measured data to produce BTF tables for air gaps from 0cm to 30cm and for bolus thicknesses from 0cm to 10cm, and NEF tables for the full range of clinically used nozzle extensions. Additionally, data tables were created for every factor that affects beam output in the MU model. The MUs were then modeled for 487 patient portals and retrospectively compared to the MUs generated from the physical calibrations previously performed.Results:Of the 487 patient portals tested, 100% of the portals used for the comparison were within 2.5% from the MUs generated using a physical calibration, and 95.9% of the MU model portals tested were within 2%. The patient portals tested had field sizes ranging from 2.1cm to 10.1cm, with air gaps from 2cm to 25cm. Output factors for field sizes below 3cm with irregularly shaped fields demonstrated inconsistent results and will be further studied.Conclusion:The most problematic output factor, the BGF, was modeled accurately and consistently using the lowest order polynomial curve fits and interpolation between measured data. The study results demonstrate the robustness of the MU model and the potential for saving valuable personnel and beam time.

Monte Carlo simulation of the transit dosimetric response of an a-Si electronic portal imaging device

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) are x-ray detectors frequently used in radiotherapy imaging and dosimetry applications. EPIDs employ a copper plate and gadolinium oxysulfide phosphor screen with an array of a-Si photodiodes to indirectly detect incident radiation. In this study, a previously developed Monte Carlo (MC) model of an a-Si EPID has been extended for transit dosimetry. The GEANT4 MC toolkit was used to integrate an a-Si EPID model with two phantoms and a 6 MV x-ray source. A solid water phantom was used to simulate EPID transmission factors, field size output factors and relative dose profiles and results were compared to experimental measurements. An anthropomorphic head phantom was used to qualitatively compare simulated and measured portal images of humanoid anatomy. Calculated transmission factors and field size output factors agreed to within 2.0% and 1.9% of experimental measurements, respectively. A comparison of calculated and measured relative dose profiles yielded >98% of points passing a gamma analysis with 3%/3 mm criterion for all field sizes. The simulated anthropomorphic head phantom image shows macroscopic anatomical features and qualitatively agrees with the measured image. Results validate the suitability of the MC model for predicting EPID response in transit dosimetry.