TrueBeam Linear Accelerator Research Articles

SU‐E‐T‐406: Evaluation of the Stealth Chamber by IBA Under Large Field Conditions

Purpose:To evaluate the performance of the stealth chamber by IBA under large field conditions.Methods:Measurement of PDDs and profiles for fields of 10×10, 15×15, 20×20 and 25×25 cm2 were acquired in a blue phantom2 scanning system for 6 and 15 MV beams in a Varian TrueBeam linear accelerator. The stealth chamber has a field size limit of 25×25 cm2. Measurements with two different detector set were performed. First a cc13 chamber as field and reference chamber was acquired. And with a cc13 chamber as field detector and the stealth chamber as the reference chamber. PDDs for the fields mentioned above were conducted and profiles with the same fields at depth of dmax, 5, 10 and 20 cm were acquired for comparison.Results:Examination of the PDDs and profile comparison with a criterion of 1% and 1 mm agreement was performed. Very good agreement between the cc13 and stealth chamber set was observed within the chosen criterion.Conclusion:This work indicates the comparable results obtained with the use of the stealth chamber and cc13 as the reference chamber in relative dosimetry measurements.

SU‐E‐T‐796: Variation of Surface Photon Energy Spectra On Bone Heterogeneity and Beam Obliquity Between Flattened and Unflattened Beam

Purpose:This study investigates the spectra of surface photon energy and energy fluence in the bone heterogeneity and beam obliquity using flattened and unflattened photon beams. The spectra were calculated in a bone and water phantom using Monte Carlo simulation (the EGSnrc code).Methods:Spectra of energy, energy fluence and mean energy of the 6 MV flattened and unflattened photon beams (field size = 10 × 10 cm2) produced by a Varian TrueBEAM linear accelerator were calculated at the surfaces of a bone and water phantom using Monte Carlo simulations. The spectral calculations were repeated with the beam angles turned from 0° to 15°, 30° and 45° in the phantoms.Results:It is found that the unflattened photon beams contained more photons in the low‐energy range of 0 – 2 MeV than the flattened beams with a flattening filter. Compared to the water phantom, both the flattened and unflattened beams had slightly less photons in the energy range < 0.4 MeV when a bone layer of 1 cm is present under the phantom surface. This shows that the presence of the bone decreased the low‐energy photons backscattered to the phantom surface. When the photon beams were rotated from 0° to 45°, the number of photon and mean photon energy increased with the beam angle. This is because both the flattened and unflattened beams became more hardened when the beam angle increased. With the bone heterogeneity, the mean energies of both photon beams increased correspondingly. This is due to the absorption of low‐energy photons by the bone, resulting in more significant beam hardening.Conclusion:The photon spectral information is important in studies on the patient's surface dose enhancement when using unflattened photon beams in radiotherapy.

The evaluation of popliteal fossa fixed method to reduce setup errors for postoperative cervical cancer via CBCT

Objective To evaluate the effect of popliteal fossa fixed method to reduce the setup errors in patients with postoperative cervical carcinoma by CBCT of TrueBeam Linear accelerator. Methods 30 cases of postoperative cervical cancer patients were randomly divided into two groups, group A with popliteal fossa fixed method by trapezoidal fixation, group B with traditional vacuum pad fixation. CBCT was used to record both setup errors and rotational errors, Stroom extension formula was used to calculate the PTV expansion value coming from the two different fixation methods. Results There was significant difference in setup errors between group A and group B. The setup errors in the left-right direction (X), cranial-caudal direction (Y) and anterior-posterior direction (Z) were (0.19±0.14) cm, (0.17±0.12) cm and (0.13±0.11) cm in group A, respectively. On the contrary, the setup errors in X, Y and Z were (0.24±0.19) cm, (0.25±0.21) cm and (0.22±0.18) cm in group B, respectively. The rotational errors were 0.05°±0.02° in group A, comparing with 0.5°±0.21° in group B (P=0.00). The PTV expanded margin in group A was 0.56 cm in X direction, 0.51 cm in Y direction, 0.40 cm in Z direction, in comparing with 0.73 cm, 0.78 cm and 0.67 cm in group B, respectively. Group A remarkably reduced the PTV, pelvis, small intestine, bladder and rectum irradiated volumes [(1 167±271) mm3 vs (1 379±297) mm3, (84±12) mm3 vs (130±17) mm3, (81±51) mm3 vs (117±64) mm3, (62±40) mm3 vs (75±47) mm3, (21±16) mm3 vs (31±21) mm3]. Conclusion Popliteal fossa fixed method can reduce setup errors and improve the stability of positioning, more suitable in precise radiotherapy for postoperative cervical cancer patients, which has the value of further validation. Key words: Image-guided radiotherapy; Cervical cancer; Fixed position; Setup errors

A Monte Carlo investigation of cumulative dose measurements for cone beam computed tomography (CBCT) dosimetry

Many studies have shown that the computed tomography dose index (CTDI100) which is considered as a main dose descriptor for CT dosimetry fails to provide a realistic reflection of the dose involved in cone beam computed tomography (CBCT) scans. Several practical approaches have been proposed to overcome drawbacks of the CTDI100. One of these is the cumulative dose concept. The purpose of this study was to investigate four different approaches based on the cumulative dose concept: the cumulative dose (1) f(0,150) and (2) f(0,∞) with a small ionization chamber 20 mm long, and the cumulative dose (3) f100(150) and (4) f100(∞) with a standard 100 mm pencil ionization chamber. The study also aimed to investigate the influence of using the 20 and 100 mm chambers and the standard and the infinitely long phantoms on cumulative dose measurements. Monte Carlo EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc codes were used to simulate a kV imaging system integrated with a TrueBeam linear accelerator and to calculate doses within cylindrical head and body PMMA phantoms with diameters of 16 cm and 32 cm, respectively, and lengths of 150, 600, 900 mm. f(0,150) and f100(150) approaches were studied within the standard PMMA phantoms (150 mm), while the other approaches f(0,∞) and f100(∞) were within infinitely long head (600 mm) and body (900 mm) phantoms. CTDI∞ values were used as a standard to compare the dose values for the approaches studied at the centre and periphery of the phantoms and for the weighted values. Four scanning protocols and beams of width 20–300 mm were used. It has been shown that the f(0,∞) approach gave the highest dose values which were comparable to CTDI∞ values for wide beams. The differences between the weighted dose values obtained with the 20 and 100 mm chambers were significant for the beam widths <120 mm, but these differences declined with increasing beam widths to be within 4%. The weighted dose values calculated within the infinitely long phantoms with both the chambers for the beam widths ≤140 were within 3% of those within the standard phantoms, but the differences rose to be within 15% at wider beams. By comparing the approaches studied in this investigation with other methodologies taking into account the efficiency of the approach as a dose descriptor and the simplicity of the implementation in the clinical environment, the f(0,150) method may be the best for CBCT dosimetry combined with the use of correction factors.

Feasibility of a unified approach to intensity-modulated radiation therapy and volume-modulated arc therapy optimization and delivery.

To study the feasibility of unified intensity-modulated arc therapy (UIMAT) which combines intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) optimization and delivery to produce superior radiation treatment plans, both in terms of dose distribution and efficiency of beam delivery when compared with either VMAT or IMRT alone. An inverse planning algorithm for UIMAT was prototyped within the pinnacle treatment planning system (Philips Healthcare). The IMRT and VMAT deliveries are unified within the same arc, with IMRT being delivered at specific gantry angles within the arc. Optimized gantry angles for the IMRT and VMAT phases are assigned automatically by the inverse optimization algorithm. Optimization of the IMRT and VMAT phases is done simultaneously using a direct aperture optimization algorithm. Five treatment plans each for prostate, head and neck, and lung were generated using a unified optimization technique and compared with clinical IMRT or VMAT plans. Delivery verification was performed with an ArcCheck phantom (Sun Nuclear) on a Varian TrueBeam linear accelerator (Varian Medical Systems). In this prototype implementation, the UIMAT plans offered the same target dose coverage while reducing mean doses to organs at risk by 8.4% for head-and-neck cases, 5.7% for lung cases, and 3.5% for prostate cases, compared with the VMAT or IMRT plans. In addition, UIMAT can be delivered with similar efficiency as VMAT. In this proof-of-concept work, a novel radiation therapy optimization and delivery technique that interlaces VMAT or IMRT delivery within the same arc has been demonstrated. Initial results show that unified VMAT/IMRT has the potential to be superior to either standard IMRT or VMAT.

Planning study of flattening filter free beams for volumetric modulated arc therapy in squamous cell carcinoma of the scalp.

PurposeFlattening filter free (FFF) beams show the potential for a higher dose rate and lower peripheral dose. We investigated the planning study of FFF beams with their role for volumetric modulated arc therapy (VMAT) in squamous cell carcinoma of the scalp.Methods and MaterialsOne patient with squamous cell carcinoma which had involvement of entire scalp was subjected to VMAT using TrueBeam linear accelerator. As it was a rare skin malignancy, CT data of 7 patients with brain tumors were also included in this study, and their entire scalps were outlined as target volumes. Three VMAT plans were employed with RapidArc form: two half-field full-arcs VMAT using 6 MV standard beams (HFF-VMAT-FF), eight half-field quarter-arcs VMAT using 6 MV standard beams (HFQ-VMAT-FF), and HFQ-VMAT using FFF beams (HFQ-VMAT-FFF). Prescribed dose was 25×2 Gy (50 Gy). Plan quality and efficiency were assessed for all plans.ResultsThere were no statistically significant differences among the three VMAT plans in target volume coverage, conformity, and homogeneity. For HFQ-VMAT-FF plans, there was a significant decrease by 12.6% in the mean dose to the brain compared with HFF-VMAT-FF. By the use of FFF beams, the mean dose to brain in HFQ-VMAT-FFF plans was further decreased by 7.4% compared with HFQ-VMAT-FF. Beam delivery times were similar for each technique.ConclusionsThe HFQ-VMAT-FF plans showed the superiority in dose distributions compared with HFF-VMAT-FF. HFQ-VMAT-FFF plans might provide further normal tissue sparing, particularly in the brain, showing their potential for radiation therapy in squamous cell carcinoma of the scalp.

Evaluation of the 4D octavius with associated 1000 SRS for SBRT plans delivered by flattened and flattening filter free beams

Introduction: The aim of this work was to study the characteristics of the ionization chamber array 1000 SRS (PTW) for flattening filter (FF) and flattening free filter (FFF) beams delivered by the TrueBeam linear accelerator (Varian). The use of the 1000 SRS with the 4D octavius was assessed for verification of dynamic arctherapy stereotactic treatment plans.

An assessment of the efficiency of methods for measurement of the computed tomography dose index (CTDI) for cone beam (CBCT) dosimetry by Monte Carlo simulation

The IEC has introduced a practical approach to overcome shortcomings of the CTDI100 for measurements on wide beams employed for cone beam (CBCT) scans. This study evaluated the efficiency of this approach (CTDIIEC) for different arrangements using Monte Carlo simulation techniques, and compared CTDIIEC to the efficiency of CTDI100 for CBCT. Monte Carlo EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc codes were used to simulate the kV imaging system mounted on a Varian TrueBeam linear accelerator. The Monte Carlo model was benchmarked against experimental measurements and good agreement shown. Standard PMMA head and body phantoms with lengths 150, 600, and 900 mm were simulated. Beam widths studied ranged from 20–300 mm, and four scanning protocols using two acquisition modes were utilized. The efficiency values were calculated at the centre (εc) and periphery (εp) of the phantoms and for the weighted CTDI (εw). The efficiency values for CTDI100 were approximately constant for beam widths 20–40 mm, where εc(CTDI100), εp(CTDI100), and εw(CTDI100) were 74.7 ± 0.6%, 84.6 ± 0.3%, and 80.9 ± 0.4%, for the head phantom and 59.7 ± 0.3%, 82.1 ± 0.3%, and 74.9 ± 0.3%, for the body phantom, respectively. When beam width increased beyond 40 mm, ε(CTDI100) values fell steadily reaching ~30% at a beam width of 300 mm. In contrast, the efficiency of the CTDIIEC was approximately constant over all beam widths, demonstrating its suitability for assessment of CBCT. εc(CTDIIEC), εp(CTDIIEC), and εw(CTDIIEC) were 76.1 ± 0.9%, 85.9 ± 1.0%, and 82.2 ± 0.9% for the head phantom and 60.6 ± 0.7%, 82.8 ± 0.8%, and 75.8 ± 0.7%, for the body phantom, respectively, within 2% of ε(CTDI100) values for narrower beam widths. CTDI100,w and CTDIIEC,w underestimate CTDI∞,w by ~55% and ~18% for the head phantom and by ~56% and ~24% for the body phantom, respectively, using a clinical beam width 198 mm. The CTDIIEC approach addresses the dependency of efficiency on beam width successfully and correction factors have been derived to allow calculation of CTDI∞.

Effect of varying dose-per-pulse and average dose rate in X-ray beam irradiation on cultured cell survival.

Characterizing the biological effects of flattening filter-free (FFF) X-ray beams from linear accelerators is of importance, due to their increasing clinical availability. The purpose of this work is to determine whether in vitro cell survival is affected by the higher dose-per-pulse present in FFF beams in comparison with flattened X-ray beams. A Varian TrueBeam(®) linear accelerator was used to irradiate the T98G, V79-4 and U87-MG cell lines with a single fraction of 5 Gy or 10 Gy doses of X-rays. Beams with energies of 6 MegaVolt (MV), 6 MV FFF and 10 MV FFF were used, with doses-per-pulse as measured at the monitor chamber of 0.28, 0.78 and 1.31 mGy/pulse for 6 MV, 6 MV FFF and 10 MV FFF, respectively. The dose delivered to each Petri dish was verified by means of ionization chamber measurements. No statistically significant effects on survival fraction were observed for any of the cell lines considered, either as a function of dose-per-pulse, average dose rate or total dose delivered. Biological effects of higher instantaneous rates should not be excluded on the basis of in vitro experimental results such as the ones presented in this work. The next step toward an assessment of the biological impact of FFF beams will require in vivo studies.

Poster — Thur Eve — 31: Dosimetric Effect of Respiratory Motion on RapidArc Lung SBRT Treatment Delivered by TrueBeam Linear Accelerator

Volumetric modulated arc therapy (VMAT) allows fast delivery of stereotactic radiotherapy. However, the discrepancies between the calculated and delivered dose distributions due to respiratory motion and dynamic multileaf collimators (MLCs) interplay are not avoidable. The purpose of this study is to investigate RapidArc lung SBRT treatment delivered by the flattening filter‐free (FFF) beam and flattened beam with Varian TrueBeam machine. CIRS Dynamic Thorax Phantom with in‐house made lung tumor insertion was CT scanned both in free breathing and 4DCT. 4DCT was used to determine the internal target volume. The free breathing CT scan was used for treatment planning. A 5 mm margin was given to ITV to generate a planning target volume. Varian Eclipse treatment planning was used to generate RapidArc plans based on the 6 MV flattened beam and 6MV FFF beam. The prescription dose was 48 Gy in 4 fractions. At least 95% of PTV was covered by the prescribed dose. The RapidArc plans with 6 MV flattened beam and 6MV FFF beam were delivered with Varian TrueBeam machine. The dosimetric measurements were performed with Gafchromic XR‐RV3 film, which was placed in the lung tumor insertion. The interplay between the dynamic MLC‐based delivery of VMAT and the respiratory motion of the tumor degraded target coverage and created undesired hot or cold dose spots inside the lung tumor. Lung SBRT RapidArc treatments delivered by the FFF beam of TrueBeam linear accelerator is superior to the flattened beam. Further investigation will be performed by Monte Carlo simulation.

Sci-Fri AM: Mountain - 05: Unified Optimization and Delivery of Intensity-modulated Radiation Therapy and Volume-modulated Arc Therapy

Purpose: To study the feasibility of a unified intensity-modulated arc therapy (UIMAT) that combines IMRT and VMAT optimization and delivery in order to produce efficient and superior radiation treatment plans. Methods: Inverse planning for UIMAT was prototyped on the Pinnacle treatment planning system (Philips Medical Systems). UIMAT integrates IMRT and VMAT delivery in the same arc where IMRT was delivered with gantry speed close to zero. Optimal gantry angles for the IMRT phases were selected automatically by the inverse optimization algorithm. Optimization of the VMAT phases and IMRT phases were done simultaneously using Pinnacle's direct machine parameter optimization algorithm. Five treatment plans each for prostate, head and neck, and lung were generated using our unified technique and compared with clinical VMAT or IMRT plans. Delivery verification was performed on an ArcCheck phantom (Sun Nuclear) and delivered in clinical mode on a Varian TrueBeam linear accelerator. Results: In this prototype implementation, compared to the VMAT or IMRT plans, with the plans normalized to the same dose coverage to the planning target volumes, the UIMAT plans produced improved OAR sparing for head and neck cases, while for lung and prostate cases, the dosimetric improvements for OARs were not as significant. In this proof-of-concept work, we demonstrated that a novel radiation therapy delivery technique combining VMAT and IMRT delivery in the same arc is feasible. Initial results showed UIMAT has the potential to be superior to either standard IMRT or VMAT.

Poster — Thur Eve — 28: Enabling trajectory‐based radiotherapy on a TrueBeam accelerator with the Eclipse treatment planning system

The TrueBeam linear accelerator platform has a developer's mode which permits the user dynamic control over many of the machine's mechanical and radiation systems. Using this research tool, synchronous couch and gantry motion can be programmed to simulate isocentric treatment with a shortened SAD, with benefits such as smaller projected MLC leaf widths and an increased dose rate. In this work, water tank measurements were used to commission a virtual linear accelerator with an 85 cm SAD in Eclipse, from which several arc‐based radiotherapy treatments were generated, including an inverse optimized VMAT delivery. For each plan, the pertinent treatment delivery information was extracted from control points specified in the Eclipse‐exported DICOM files using the pydicom package in Python, allowing construction of an XML control file. The dimensions of the jaws and MLC positions, defined for an 85 cm SAD in Eclipse, were scaled for delivery on a conventional SAD linear accelerator, and translational couch motion was added as a function of gantry angle to simulate delivery at 85 cm SAD. Ionization chamber and Gafchromic film measurements were used to compare the radiation delivery to dose calculations in Eclipse. With the exception of the VMAT delivery, ionization chamber measurements agreed within 3.3% of the Eclipse calculations. For the VMAT delivery, the ionization chamber was located in an inhomogeneous region, but gamma evaluation of the Gafchromic film plane resulted in a 94.5% passing rate using criteria of 3 mm/3%. The results indicate that Eclipse calculation infrastructure can be used.

SU‐E‐T‐438: Commissioning of An In‐Vivo Quality Assurance Method Using the Electronic Portal Imaging Device

Purpose:Patient specific pre‐treatment quality assurance (QA) using arrays of detectors or film have been the standard approach to assure the correct treatment is delivered to the patient. This QA approach is expensive, labor intensive and does not guarantee or document that all remaining fractions were treated properly. The purpose of this abstract is to commission and evaluate the performance of a commercially available in‐vivo QA software using the electronic portal imaging device (EPID) to record the daily treatments.Methods:The platform EPIgray V2.0.2 (Dosisoft), which machine model compares ratios of TMR with EPID signal to predict dose was commissioned for an Artiste (Siemens Oncology Care Systems) and a Truebeam (Varian medical systems) linear accelerator following the given instructions. The systems were then tested on three different phantoms (homogeneous stack of solid water, anthropomorphic head and pelvis) and on a library of patient cases. Simple and complex fields were delivered at different exposures and for different gantry angles. The effects of the table attenuation and the EPID sagging were evaluated. Gamma analysis of the measured dose was compared to the predicted dose for complex clinical IMRT cases.Results:Commissioning of the EPIgray system for two photon energies took 8 hours. The difference between the dose planned and the dose measured with EPIgray was better than 3% for all phantom scenarios tested. Preliminary results on patients demonstrate an accuracy of 5% is achievable in high dose regions for both 3DCRT and IMRT. Large discrepancies (&gt;5%) were observed due to metallic structures or air cavities and in low dose areas. Flat panel sagging was visible and accounted for in the EPIgray model.Conclusion:The accuracy achieved by EPIgray is sufficient to document the safe delivery of complex IMRT treatments. Future work will evaluate EPIgray for VMAT and high dose rate deliveries.This work is supported by Dosisoft, Cachan, France.

SU-E-T-109: Development of An End-To-End Test for the Varian TrueBeamtm with a Novel Multiple-Dosimetric Modality H&amp;N Phantom

Purpose: To develop a comprehensive end-to-end test for Varian's TrueBeam linear accelerator for head and neck IMRT using a custom phantom designed to utilize multiple dosimetry devices. Methods: The initial end-to-end test and custom H&N phantom were designed to yield maximum information in anatomical regions significant to H&N plans with respect to: i) geometric accuracy, ii) dosimetric accuracy, and iii) treatment reproducibility. The phantom was designed in collaboration with Integrated Medical Technologies. A CT image was taken with a 1mm slice thickness. The CT was imported into Varian's Eclipse treatment planning system, where OARs and the PTV were contoured. A clinical template was used to create an eight field static gantry angle IMRT plan. After optimization, dose was calculated using the Analytic Anisotropic Algorithm with inhomogeneity correction. Plans were delivered with a TrueBeam equipped with a high definition MLC. Preliminary end-to-end results were measured using film and ion chambers. Ion chamber dose measurements were compared to the TPS. Films were analyzed with FilmQAPro using composite gamma index. Results: Film analysis for the initial end-to-end plan with a geometrically simple PTV showed average gamma pass rates >99% with a passing criterion of 3% / 3mm. Film analysis of a plan with a more realistic, ie. complex, PTV yielded pass rates >99% in clinically important regions containing the PTV, spinal cord and parotid glands. Ion chamber measurements were on average within 1.21% of calculated dose for both plans. Conclusion: trials have demonstrated that our end-to-end testing methods provide baseline values for the dosimetric and geometric accuracy of Varian's TrueBeam system.

TH-C-17A-03: Dynamic Visualization and Dosimetry of IMRT and VMAT Treatment Plans by Video-Rate Imaging of Cherenkov Radiation in Pure Water

Purpose: A novel optical dosimetry technique for the QA and verification of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) radiotherapy plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: An intensified CCD camera (ICCD) was used to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank. The ICCD acquisition was gated to the Linac, operated for single pulse imaging, and binned to a resolution of 512×512 pixels. The resulting videos were analyzed temporally for regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR) and summed to obtain an overall light distribution, which was compared to the expected dose distribution from the TPS using a gammaindex analysis. Results: The chosen camera settings resulted in data at 23.5 frames per second. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.2% and 95.6% agreement between the light distribution and expected TPS dose distribution based upon a 3% / 3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans respectively. Conclusion: The results from this initial study demonstrate the first documented use of Cherenkov radiation for optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications. NIH R01CA109558 and R21EB017559.

SU‐E‐T‐305: Study of the Eclipse Electron Monte Carlo Algorithm for Patient Specific MU Calculations

Purpose:To evaluate the Eclipse electron Monte Carlo (eMC) algorithm based on patient specific monitor unit (MU) calculations, and to propose a new factor which quantitatively predicts the discrepancy of MUs between the eMC algorithm and hand calculations.Methods:Electron treatments were planned for 61 patients on Eclipse (Version 10.0) using the eMC algorithm for Varian TrueBeam linear accelerators. For each patient, the same treatment beam angle was kept for a point dose calculation at dmax performed with the reference condition, which used an open beam with a 15×15 cm2 size cone and 100 SSD. A patient specific correction factor (PCF) was obtained by getting the ratio between this point dose and the calibration dose, which is 1 cGy per MU delivered at dmax. The hand calculation results were corrected by the PCFs and compared with MUs from the treatment plans.Results:The MU from the treatment plans were in average (7.1±6.1)% higher than the hand calculations. The average MU difference between the corrected hand calculations and the eMC treatment plans was (0.07±3.48)%. A correlation coefficient of 0.8 was found between (1‐PCF) and the percentage difference between the treatment plan and hand calculations. Most outliers were treatment plans with small beam opening (&lt; 4 cm) and low energy beams (6 and 9 MeV).Conclusion:For CT‐based patient treatment plans, the eMC algorithm tends to generate a larger MU than hand calculations. Caution should be taken for eMC patient plans with small field sizes and low energy beams. We hypothesize that the PCF ratio reflects the influence of patient surface curvature and tissue inhomogeneity to patient specific percent depth dose (PDD) curve and MU calculations in eMC algorithm.

SU-E-T-110: Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials

Purpose: To develop a comprehensive end-to-end test for Varian's TrueBeam linear accelerator for head and neck IMRT using a custom phantom designed to utilize multiple dosimetry devices. Purpose: To commission a multiple-source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV. Methods: A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x-ray beams was developed. Energy spectra of two photon sources corresponding to primary photons created in the target and scattered photons originating in the linear accelerator head were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue-Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in a water tank for a 10×10cm2 field. Off-axis effects were modeled by a 3rd degree polynomial used to describe the off-axis half-value layer as a function of off-axis angle and fitting the off-axis fluence to a piecewise linear function to match calculated dose profiles with measured dose profiles for a 40×40cm2 field. The model was validated by comparing calculated PDDs and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 to those obtained from measurements. A benchmarking study compared calculated data to measurements for IMRT plans delivered to anthropomorphic phantoms. Results: Along the central axis of the beam 99.6% and 99.7% of all data passed the 2%/2mm gamma criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, through 25cm agreed with measured data for 99.4% and 99.6% of data tested for 6 and 10MV models, respectively. A comparison of calculated dose to film measurement in a head and neck phantom showed an average of 85.3% and 90.5% of pixels passing a 3%/2mm gamma criterion for 6 and 10MV models respectively. Conclusion: A Monte Carlo multiple-source model for Elekta 6 and 10MV therapeutic x-ray beams has been developed as a quality assurance tool for clinical trials.

SU‐C‐BRE‐06: Radiobiological Advantage of Very Rapid Irradiation

Purpose:To characterize the effect of very rapid dose delivery as compared to conventional therapeutic irradiation times on clonogenic cell survival.Methods:We used a Varian Trilogy linear accelerator to deliver doses up to 10 Gy using a 6 MV SRS photon beam. We irradiated four cancer cell lines in times ranging from 30 sec to 30 min. We also used a Varian TrueBeam linear accelerator to deliver 9 MeV electrons at 10 Gy in 10 s to 30 min to determine the effect of irradiation time on cell survival. We then evaluated the effect of using 60 and 120 MeV electrons on cell survival using the Next Linear Collider Test Accelerator (NLCTA) beam line at the SLAC National Accelerator Laboratory. During irradiation, adherent cells were maintained at 37oC with 20%O2/5%CO2. Clonogenic assays were completed following irradiation to determine changes in cell survival due to dose delivery time and beam quality, and the survival data were fitted with the linear‐quadratic model.Results:Cell lines varied in radiosensitivity, ranging from two to four logs of cell kill at 10 Gy for both conventional and very rapid irradiation. Delivering radiation in shorter times decreased survival in all cell lines. Log differences in cell kill ranged from 0.2 to 0.7 at 10 Gy for the short compared to the long irradiation time. Cell kill differences between short and long irradiations were more pronounced as doses increased for all cell lines.Conclusion:Our findings suggest that shortening delivery of therapeutic radiation doses to less than 1 minute may improve tumor cell kill. This study demonstrates the potential advantage of technologies under development to deliver stereotactic ablative radiation doses very rapidly.Bill Loo and Peter Maxim have received Honoraria from Varian and Research Support from Varian and RaySearch.

SU-E-T-382: Influence of Compton Currents On Profile Measurements in Small- Volume Ion Chambers

Purpose: Ionization chambers in electron radiation fields are known to exhibit polarity effects due to Compton currents. Previously we have presented a unique manifestation of this effect observed with a microionization chamber. We have expanded that investigation to include three micro-ionization chambers commonly used in radiation therapy. The purpose of this project is to determine what factors influence this polarity effect for micro-chambers and how it might be mitigated. Methods: Three chambers were utilized: a PTW 31016, an Exradin A-16, and an Exradin A- 26. Beam profile scans were obtained on a Varian TrueBeam linear accelerator in combination with a Wellhofer water phantom for 6, 9, and 12 MeV electrons. Profiles were obtained parallel and perpendicular to the chamber's long axis, with both positive and negative collecting bias. Profiles were obtained with various chamber components shielded by 5 mm of Pb at 6 MeV to determine their relative contributions to this polarity effect. Results: The polarity effect was observed for all three chambers, and the ratio of the polarity effect for the Exradin chambers is proportional to the ratio of chamber volumes. Shielding the stem of both Exradin chambers diminished, but did not remove the polarity effect. However, they demonstrated no out-of-field effect when the cable was shielded with Pb. The PTW chamber demonstrated a significantly reduced polarity effect without any shielding despite its comparable volume with the A-26. Conclusions: The sensitive volume of these micro-chambers is relatively insensitive to collecting polarity. However, charge deposition within the cable can dramatically alter measured ionization profiles. This is demonstrated by the removal of the out-of-field ionization when the cable is shielded for the Exradin chambers. We strongly recommend analyzing any polarity dependence for small-volume chambers used in characterization of electron fields.

Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation.

A novel technique for optical dosimetry of dynamic intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 (TG-119) C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank doped with the fluorophore quinine sulfate. The ICCD acquisition was gated to the Linac target trigger pulse to reduce background light artifacts, read out for a single radiation pulse, and binned to a resolution of 512 × 512 pixels. The resulting videos were analyzed temporally for various regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR), and summed to obtain an overall light intensity distribution, which was compared to the expected dose distribution from the TPS using a gamma-index analysis. The chosen camera settings resulted in 23.5 frames per second dosimetry videos. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.9% and 96.2% agreement between the experimentally captured Cherenkov light distribution and expected TPS dose distribution based upon a 3%/3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans, respectively. The results from this initial study demonstrate the first documented use of Cherenkov radiation for video-rate optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications.