Flattening Filter Research Articles

Safety of high-dose-rate stereotactic body radiotherapy.

Background and purposeFlattening filter free (FFF) beams with high dose rate are increasingly used for stereotactic body radiotherapy (SBRT), because they substantially shorten beam-on time. The physical properties of these beams together with potentially unknown radiobiological effects might affect patient safety. Therefore here we analyzed the clinical outcome of our patients.Material and methodsBetween 3/2010 and 2/2014 84 patients with 100 lesions (lung 75, liver 10, adrenal 6, lymph nodes 5, others 4) were treated with SBRT using 6 MV FFF or 10 MV FFF beams at our institution. Clinical efficacy endpoints and toxicity were assessed by Kaplan-Meier analysis and CTCAE criteria version 4.0.ResultsMedian follow-up was 11 months (range: 3–41). No severe acute toxicity was observed. There has been one case of severe late toxicity (1%), a grade 3 bile duct stricture that was possibly related to SBRT. For all patients, the 1-year local control rate, progression free survival and overall survival were 94%, 38% and 80% respectively, and for patients with lung lesions 94%, 48% and 83%, respectively.ConclusionsNo unexpected toxicity occurred. Toxicity and treatment efficacy are perfectly in range with studies investigating SBRT with flattened beams. The use of FFF beams at maximum dose rate for SBRT is time efficient and appears to be safe.

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.

A comparison of phantom scatter from flattened and flattening filter free high-energy photon beams

Flattening filter free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this work was to characterize the collimator scatter (Sc) and total scatter (Scp) from 3 FFF beams of differing quality indices and use the resulting mathematical fits to generate phantom scatter (Sp) data. The similarities and differences between Sp of flattened and FFF beams are described. Sc and Scp data were measured for 3 flattened and 3 FFF high-energy photon beams (Varian 6 and 10MV and Elekta 6MV). These data were fitted to logarithmic power law functions with 4 numerical coefficients. The agreement between our experimentally determined flattened beam Sp and published data was within ± 1.2% for all 3 beams investigated and all field sizes from 4 × 4 to 40 × 40cm2. For the FFF beams, Sp was only within 1% of the same flattened beam published data for field sizes between 6 × 6 and 14 × 14cm2. Outside this range, the differences were much greater, reaching − 3.2%, − 4.5%, and − 4.3% for the fields of 40 × 40cm2 for the Varian 6-MV, Varian 10-MV, and Elekta 6-MV FFF beams, respectively. The FFF beam Sp increased more slowly with increasing field size than that of the published and measured flattened beam of a similar reference field size quality index, i.e., there is less Phantom Scatter than that found with flattened beams for a given field size. This difference can be explained when the fluence profiles of the flattened and FFF beams are considered. The FFF beam has greatly reduced fluence off axis, especially as field size increases, compared with the flattened beam profile; hence, less scatter is generated in the phantom reaching the central axis.

IMRT and RapidArc commissioning of a TrueBeam linear accelerator using TG-119 protocol cases.

The purpose of this study is to evaluate the overall accuracy of intensity‐modulated radiation therapy (IMRT) and RapidArc delivery using both flattening filter (FF) and flattening filter‐free (FFF) modalities based on test cases developed by AAPM Task Group 119. Institutional confidence limits (CLs) were established as the baseline for patient specific treatment plan quality assurance (QA). The effects of gantry range, gantry speed, leaf speed, dose rate, as well as the capability to capture intentional errors, were evaluated by measuring a series of Picket Fence (PF) tests using the electronic portal imaging device (EPID) and EBT3 films. Both IMRT and RapidArc plans were created in a Solid Water phantom (30 × 30 × 15 cm3) for the TG‐119 test cases representative of normal clinical treatment sites for all five photon energies (6X, 10X, 15X, 6X‐FFF, 10X‐FFF) and the Exact IGRT couch was included in the dose calculation. One high‐dose point in the PTV and one low‐dose point in the avoidance structure were measured with an ion chamber in each case for each energy. Similarly, two GAFCHROMIC EBT3 films were placed in the coronal planes to measure planar dose distributions in both high‐ and low‐dose regions. The confidence limit was set to have 95% of the measured data fall within the tolerance. The mean of the absolute dose deviation for variable dose rate and gantry speed during RapidArc delivery was within 0.5% for all energies. The corresponding results for leaf speed tests were all within 0.4%. The combinations of dynamic leaf gap (DLG) and MLC transmission factor were optimized based on the ion chamber measurement results of RapidArc delivery for each energy. The average 95% CLs for the high‐dose point in the PTV were 0.030 ± 0.007 (range, 0.022–0.038) for the IMRT plans and 0.029 ± 0.011 (range, 0.016–0.043) for the RapidArc plans. For low‐point dose in the avoidance structures, the CLs were 0.029 ± 0.006 (range, 0.024–0.039) for the IMRT plans and 0.027 ± 0.013 (range, 0.017–0.047) for the RapidArc plans. The average 95% CLs using 3%/3 mm gamma criteria in the high‐dose region were 5.9 ± 2.7 (range, 1.4–8.6) and 3.9 ± 2.9 (range, 1.5–8.8) for IMRT and RapidArc plans, respectively. The average 95% CLs in the low‐dose region were 5.3 ± 2.6 (range, 1.2–7.4) and 3.7 ± 2.8 (range, 1.8–8.3) for IMRT and RapidArc plans, respectively. Based on ion chamber, as well as film measurements, we have established CLs values to ensure the high precision of IMRT and RapidArc delivery for both FF and FFF modalities.PACS number: 87

Poster — Thur Eve — 43: Monte Carlo Modeling of Flattening Filter Free Beams and Studies of Relative Output Factors

Flattening filter free (FFF) beams have been adopted by many clinics and used for patient treatment. However, compared to the traditional flattened beams, we have limited knowledge of FFF beams. In this study, we successfully modeled the 6 MV FFF beam for Varian TrueBeam accelerator with the Monte Carlo (MC) method. Both the percentage depth dose and profiles match well to the Golden Beam Data (GBD) from Varian. MC simulations were then performed to predict the relative output factors. The in‐water output ratio, Scp, was simulated in water phantom and data obtained agrees well with GBD. The in‐air output ratio, Sc, was obtained by analyzing the phase space placed at isocenter, in air, and computing the ratio of water Kerma rates for different field sizes. The phantom scattering factor, Sp, can then be obtained from the traditional way of taking the ratio of Scp and Sc. We also simulated Sp using a recently proposed method based on only the primary beam dose delivery in water phantom. Because there is no concern of lateral electronic disequilibrium, this method is more suitable for small fields. The results from both methods agree well with each other. The flattened 6 MV beam was simulated and compared to 6 MV FFF. The comparison confirms that 6 MV FFF has less scattering from the Linac head and less phantom scattering contribution to the central axis dose, which will be helpful for improving accuracy in beam modeling and dose calculation in treatment planning systems.

TH‐E‐BRE‐04: An Online Replanning Algorithm for VMAT

Purpose:To develop a fast replanning algorithm based on segment aperture morphing (SAM) for online replanning of volumetric modulated arc therapy (VMAT) with flattening filtered (FF) and flattening filter free (FFF) beams.Methods:A software tool was developed to interface with a VMAT planning system ((Monaco, Elekta), enabling the output of detailed beam/machine parameters of original VMAT plans generated based on planning CTs for FF or FFF beams. A SAM algorithm, previously developed for fixed‐beam IMRT, was modified to allow the algorithm to correct for interfractional variations (e.g., setup error, organ motion and deformation) by morphing apertures based on the geometric relationship between the beam's eye view of the anatomy from the planning CT and that from the daily CT for each control point. The algorithm was tested using daily CTs acquired using an in‐room CT during daily IGRT for representative prostate cancer cases along with their planning CTs. The algorithm allows for restricted MLC leaf travel distance between control points of the VMAT delivery to prevent SAM from increasing leaf travel, and therefore treatment delivery time.Results:The VMAT plans adapted to the daily CT by SAM were found to improve the dosimetry relative to the IGRT repositioning plans for both FF and FFF beams. For the adaptive plans, the changes in leaf travel distance between control points were < 1cm for 80% of the control points with no restriction. When restricted to the original plans' maximum travel distance, the dosimetric effect was minimal. The adaptive plans were delivered successfully with similar delivery times as the original plans. The execution of the SAM algorithm was < 10 seconds.Conclusion:The SAM algorithm can quickly generate deliverable online‐adaptive VMAT plans based on the anatomy of the day for both FF and FFF beams.

SU-E-T-377: Inaccurate Positioning Might Introduce Significant MapCheck Calibration Error in Flatten Filter Free Beams

Purpose: This study investigates the calibration error of detector sensitivity for MapCheck due to inaccurate positioning of the device, which is not taken into account by the current commercial iterative calibration algorithm. We hypothesize the calibration is more vulnerable to the positioning error for the flatten filter free (FFF) beams than the conventional flatten filter flattened beams. Methods: MapCheck2 was calibrated with 10MV conventional and FFF beams, with careful alignment and with 1cm positioning error during calibration, respectively. Open fields of 37cmx37cm were delivered to gauge the impact of resultant calibration errors. The local calibration error was modeled as a detector independent multiplication factor, with which propagation error was estimated with positioning error from 1mm to 1cm. The calibrated sensitivities, without positioning error, were compared between the conventional and FFF beams to evaluate the dependence on the beam type. Results: The 1cm positioning error leads to 0.39% and 5.24% local calibration error in the conventional and FFF beams respectively. After propagating to the edges of MapCheck, the calibration errors become 6.5% and 57.7%, respectively. The propagation error increases almost linearly with respect to the positioning error. The difference of sensitivities between the conventional and FFF beams was small (0.11 ±more » 0.49%). Conclusion: The results demonstrate that the positioning error is not handled by the current commercial calibration algorithm of MapCheck. Particularly, the calibration errors for the FFF beams are ~9 times greater than those for the conventional beams with identical positioning error, and a small 1mm positioning error might lead to up to 8% calibration error. Since the sensitivities are only slightly dependent of the beam type and the conventional beam is less affected by the positioning error, it is advisable to cross-check the sensitivities between the conventional and FFF beams to detect potential calibration errors due to inaccurate positioning. This work was partially supported by a DOD Grant No.; DOD W81XWH1010862.« less

SU-E-T-218: Investigating the Differences in Out-Of-Field Dose Between An Inherently Uniform Beam and a Flattening Filter Free Beam Made Uniform Through Inverse Modulation

Purpose: To characterize with physical measurements the out-of-field dose for flattening filter free (FFF) and IMRT flattened FFF photon beams less than 10 MV from a Siemens Artiste LINAC, field size dependence, thereof, and comparison with the Pinnacle planning system calculation. Methods: Single field plans were developed for 5×5cm^2 and 20×20cm^2 field sizes for 6MV and 7MV-FFF. An additional 7MV-FFF plan was developed for each field size with the purpose of delivering a uniform dose (7MV-FFFUniform), similar to 6MV, utilizing inverse modulation. The calculated and measured dose was normalized to achieve equivalence at 10cm depth in water. Out-of-field dose measurements were acquired with a farmer-type ionization chamber in solid water at incremental distances up to 90cm from the field edge at physical depths of 1.5cm and 5.0cm. The dose contribution from head leakage was determined by closing the jaws symmetrically and the MLC off-axis Results: Data collected show that the out-of-field dose for 6MV is generally greater than 7MV-FFF and 7MV-FFF-Uniform. The 7MV-FFF-Uniform dose is less than 7MV-FFF for the small field size, but similar for the larger field and may be attributed to the small IMRT segments of the 5x5 field developing narrow beam geometry with reduced scatter. The out-of-field dose from 7MV-FFF-Uniform for the 20×20 field was greater than 6MV and 7MV-FFF beyond 50cm from the field edge. Head leakage from the 7MV-FFF-Uniform segmental MU modulation was greater than the un-modulated beams. The calculated out-of-field dose was consistently −200% different than measured for a subset of points considered. Conclusion: The modulation required to deliver a uniform large field dose with 7MV-FFF will increase the peripheral dose to the patient in comparison to already physically flattened fields; however, the dose near the field edge will be reduced. If reliable out-of-field doses are needed physical measurements should be used.

Use of Flattening Filter-Free Photon Beams in Treating Medulloblastoma: A Dosimetric Evaluation

Aim. To evaluate the dosimetric benefits of flattening filter-free (FFF) photon beams in intensity modulated radiation therapy (IMRT) and Rapid Arc (RA) over conventional CSI methods. Methods and Materials. Five patients treated with IMRT using static multileaf collimators (MLC) were randomly selected for this retrospective study. Dynamic MLC IMRT, RA, and conformal therapy (3DCRT) were iterated with the same CT data sets with and without flattening filter photons. Total dose prescribed was 28.80 Gy in 16 fractions. Dosimetric parameters such as D max⁡, D min⁡, D mean, V 95%, V 107%, DHI, and CI for PTV and D max⁡, D mean, V 80%, V 50%, V 30%, and V 10% for OARs were extracted from DVHs. Beam on time (BOT) for various plans was also compared. Results. FFF RA therapy (6F_RA) resulted in highly homogeneous and conformal doses throughout the craniospinal axis. 3DCRT resulted in the highest V 107% (SD) 46.97 ± 28.6, whereas flattening filter (FF) and FFF dynamic IMRT had a minimum V 107%. 6F_RA and 6F_DMLC resulted in lesser doses to thyroid, eyes, esophagus, liver, lungs, and kidneys. Conclusion. FFF IMRT and FFF RA for CSI have definite dosimetric advantages over 3DCRT technique in terms of target coverage and OAR sparing. Use of FFF in IMRT resulted in 50% reduction in BOT, thereby increasing the treatment efficiency.

Comparison of the Structure and Composition of the Branchial Filters in Suspension Feeding Elasmobranchs

The four, evolutionarily independent, lineages of suspension feeding elasmobranchs have two types of branchial filters. The first is a robust, flattened filter pad akin to a colander (e.g., whale sharks, mantas and devil rays) while the second more closely resembles the comb-like gill raker structure found in bony fishes (e.g., basking and megamouth sharks). The structure and the presence of mucus on the filter elements will determine the mechanical function of the filter and subsequent particle transport. Using histology and scanning electron microscopy, we investigated the anatomy of the branchial filters in 12 of the 14 species of Chondrichthyian filter-feeding fishes. We hypothesized that mucus producing cells would be abundant along the filter epithelium and perform as a sticky mechanism to retain and transport particles; however, we found that only three species had mucus producing goblet cells. Two of these (Mobula kuhlii and Mobula tarapacana) also had branchial cilia, indicating sticky retention and transport. The remaining filter-feeding elasmobranchs did not have a sticky surface along the filter for particles to collect and instead must employ alternative mechanisms of filtration (e.g., direct sieving, inertial impaction or cross-flow). With the exception of basking sharks, the branchial filter is composed of a hyaline cartilage skeleton surrounded by a layer of highly organized connective tissue that may function as a support. Megamouth sharks and most of the mobulid rays have denticles along the surface of the filter, presumably to protect against damage from large particle impactions. Basking sharks have branchial filters that lack a cartilaginous core; instead they are composed entirely of smooth keratin.

Practical approach for pretreatment verification of IMRT with flattening filter free(FFF) beams using Varian Portal Dosimetry.

Patient‐specific pretreatment verification of intensity‐modulated radiation therapy (IMRT) or volumetric‐modulated arc therapy (VMAT) is strongly recommended for all patients in order to detect any potential errors in treatment planning process and machine deliverability, and is thus performed routinely in many clinics. Portal dosimetry is an effective method for this purpose because of its prompt setup, easy data acquisition, and high spatial resolution. However, portal dosimetry cannot be applied to IMRT or VMAT with flattening filter‐free (FFF) beams because of the high dose‐rate saturation effect of the electronic portal imaging device (EPID). In our current report, we suggest a practical QA method of expanding the conventional portal dosimetry to FFF beams with a QA plan generated by the following three steps: 1) replace the FFF beams with flattening filtered (FF) beams of the same nominal energy; 2) reduce the dose rate to avoid the saturation effect of the EPID detector; and 3) adjust the total MU to match the gantry and MLC leaf motions. Two RapidArc plans with 6 and 10 MV FFF beams were selected, and QA plans were created by the aforementioned steps and delivered. The trajectory log files of TrueBeam obtained during the treatment and during the delivery of QA plan were analyzed and compared. The maximum discrepancies in the expected trajectories between the treatment and QA plans were within 0.002 MU for the MU, 0.06° for the motion of gantry rotation, and 0.006 mm for the positions of the MLC leaves, indicating much higher levels of accuracy compared to the mechanical specifications of the machine. For further validation of the method, direct comparisons of the delivered QA FF beam to the treatment FFF beam were performed using film dosimetry and show that gamma passing rates under 2%/2 mm criteria are 99.0%–100% for the all four arc beams. This method can be used on RapidArc plans with FFF beams without any additional procedure or modifications on the conventional portal dosimetry of IMRT and is, therefore, a practical option for routine clinical use.PACS numbers: 87.53.Kn, 87.55.T‐, 87.56.bd, 87.59.‐e

STUDY ON MONITORING UNIT EFFICIENCY OF FLATTENING-FILTER FREE PHOTON BEAM IN ASSOCIATION WITH TUMOR SIZE AND LOCATION

TM (ver. A10, Varian Medical Systems, Palo Alto, USA) treatment planning system. The length of major and minor axis in axial view was 50 cm and 30 cm, respectively. Cylindrical-shaped targets were generated inside that patient at the center (symmetric target) and in the periphery (asymmetric target, 7.5 cm away from the center of the patient to the right direction) of the virtual patient. The longitudinal length was 10 cm and the diameters were 2, 5, 10 and 15 cm. Total 8 targets were generated. RapidArc TM plans using TrueBeam STx TM were generated for each target. Two full arcs were used and the axis of rotation of the gantry was set to be at the center of the virtual patient. Total MU, homogeneity index (HI), target mean dose, the value of gradient measure and body mean dose were calculated. In the case of symmetric targets, averaged total MU of FFF plan was 23% and 19% higher than that of flattening filter (FF) plan when using 6 MV and 10 MV photons, respectively. The difference of HI, target mean dose, gradient measure and body mean dose between FF and FFF was less than 0.04, 2.6%, 0.1 cm and 2.2%, respectively. For the asymmetric targets, total MU of FFF plan was 21% and 32% was higher than that of FF when using 6 MV and 10 MV photons, respectively. The homogeneity of the target was always worse when using FFF than using FF. The maximum difference of HI was 0.22. The target mean dose of FFF was 3.2% and 4.1% higher than that of FF for the 6 MV and 10 MV, respectively. The difference of gradient measure was less than 0.1 cm. The body mean dose was higher when using FFF than FF about 4.2% and 2.8% for the 6 MV and 10 MV, respectively. No significant differences between VMAT plans of FFF beam and FF beam were observed in terms of quality of treatment plan. The HI was higher when using FFF 10 MV photons for the asymmetric targets. The MU was increased noticeably when using FFF photon beams.

Evaluation of treatment plan quality of IMRT and VMAT with and without flattening filter using Pareto optimal fronts

PurposeTo investigate the differences in treatment plan quality of IMRT and VMAT with and without flattening filter using Pareto optimal fronts, for two treatment sites of different anatomic complexity. Materials and MethodsPareto optimal fronts (POFs) were generated for six prostate and head-and-neck cancer patients by stepwise reduction of the constraint (during the optimization process) of the primary organ-at-risk (OAR). 9-static field IMRT and 360°-single-arc VMAT plans with flattening filter (FF) and without flattening filter (FFF) were compared. The volume receiving 5Gy or more (V5Gy) was used to estimate the low dose exposure. Furthermore, the number of monitor units (MUs) and measurements of the delivery time (T) were used to assess the efficiency of the treatment plans. ResultsA significant increase in MUs was found when using FFF-beams while the treatment plan quality was at least equivalent to the FF-beams. T was decreased by 18% for prostate for IMRT with FFF-beams and by 4% for head-and-neck cases, but increased by 22% and 16% for VMAT. A reduction of up to 5% of V5Gy was found for IMRT prostate cases with FFF-beams. ConclusionsThe evaluation of the POFs showed an at least comparable treatment plan quality of FFF-beams compared to FF-beams for both treatment sites and modalities. For smaller targets the advantageous characteristics of FFF-beams could be better exploited.

Monte Carlo investigation into feasibility and dosimetry of flat flattening filter free beams

Flattening filter free (FFF) beams due to their non-uniformity, are sub-optimal for larger field sizes. The purpose of this study was to investigate the incident electron beam distributions that would produce flat FFF (F4) beams without the use of a flattening filter (FF). Monte Carlo (MC) simulations with BEAMnrc and DOSXYZnrc codes have been performed to evaluate feasibility of this approach. The dose distributions in water for open 6 MV beams were simulated using the Varian 21EX linac head model, which will be called the FF model. The FF was then removed from the FF model, and MC simulations were performed using (1) 6 MeV electrons incident on the target and (2) a 6 MeV electron beam with electron angular distributions optimized to provide as flat dose profiles as possible. Configuration (1) represents FFF beam while configuration (2) allowed producing a F4 beam. Optimizations have also been performed to produce flattest profiles for a set of dose rates (DRs) in the range from 1.25 to 2.4 of the DR of FF beam. Profiles and percentage depth doses (PDDs) from 6 MV F4 beams have been calculated and compared to those from the FF beam. Calculated profiles demonstrated improved flatness of the FFF beams. In fact, up to field sizes within the circle of 35 cm diameter the flatness of F4 beam at dmax was better or comparable to that of FF beam. At 20 cm off-axis the dose increased from 52% for FFF to 92% for F4 beam. Also, profiles of F4 beams did not change considerably with depth. PDDs from F4 beams were similar to those of the FFF beam. The DR for the largest modeled (44 cm diameter) F4 beam was higher than the DR from FF beam by a factor of 1.25. It was shown that the DR can be increased while maintaining beam flatness, but at the cost of reduced field size.

Analysis and applications of phase shifts for multichannel fiber Bragg grating

A novel phase compensation method to improve comb filters performance by increasing DC refractive index of linearly chirped fiber Bragg grating (LCFBG) is proposed. Nonlinear chirp introduced by linearly chirped mask is discussed, and then DC phase shift compensate method is proposed. Two kinds of DC phase shift LCFBGs are analyzed, which shows a good agreement with traditional discrete phase shift method. A DC phase shift comb filter is realized with the help of a micrometer-precision translation stage, which shows the feasibility to introduce DC phase shift in LCFBG. A high flattened comb filter with precise 0.8 nm channel spacing is realized with the help of a micrometer precision translation stage.

Comparison of Monitoring Unit Efficiency Between With and Without Flattening Filter Using Volumetric Modulated Arc Therapy

To investigate the monitoring unit (MU) efficiency and plan quality of volumetric modulated radiation therapy (VMAT) using flattening filter free (FFF) beam compared with flattening filter (FF) beam by the target size and location.

Linac based SBRT for prostate cancer in 5 fractions with VMAT and flattening filter free beams: preliminary report of a phase II study

BackgroundTo evaluate the feasibility and early side effects of a short course hypo-fractionated SBRT programme with Volumetric Modulated Arc Therapy (VMAT) and Flattening Filter Free (FFF) beams.MethodsA prospective phase I-II study, started on February 2012. Inclusion criteria were: age ≤ 80 years, WHO-PS ≤ 2, PSA ≤ 20 ng/ml, histologically proven prostate adenocarcinoma, T1-T2 stage, no distant metastases, no previous surgery other than TURP, no malignant tumours in the previous 5 years, IPSS 0–7. The schedule was 35 Gy in 5 alternative days. SBRT was delivered with RapidArc VMAT, with 10MV FFF photons. Toxicity assessment was performed according to CTCAE v4.0 scale. EPIC questionnaires assessed Quality-of-Life. Neo-adjuvant/concomitant hormonal-therapy was prescribed according to risk classification. SpaceOAR™ gel was optionally implanted to increase the separation space between the prostate and the rectal wall.ResultsMedian follow-up was 11 months (range: 5–16); 40 patients were recruited in the protocol and treated. According to NCCN criteria, 26/40 patients were low-risk and 14/40 were intermediate risk. Median age was 70 years (56–80), median initial PSA was 6.25 ng/ml (0.50-13.43 ng/ml). Median Gleason score was 6 (6–7). All patients completed the treatment as programmed (median 11.8 days (9–22). Acute Toxicities were as follow: Rectum G0: 30/40 cases (75%); G1: 6/40 (15%); G2: 4/40 (10%). Genito-urinary: G0: 16/40 (40%); G1: 8/40 (20%); G2: 16/34 (40%). In two G2 urinary retention cases, intermittent catheter was needed. No acute G3 or greater toxicity was found. Median treatment time was 126 sec (120–136). SpaceOAR™ was implanted in 8 patients. PSA reduction from the pre-treatment value of the marker was documented in all patients.ConclusionsEarly findings suggest that SBRT with RapidArc and FFF beams for prostate cancer in 5 fractions is feasible and tolerated in acute setting. Longer follow-up is needed for assessment of late toxicity and outcome.

SU‐E‐T‐33: A Comparative Study of Commissioning Data of Two TrueBeam LINACs with TrueBeam Representative Beam Data

Purpose: To compare the commissioning water tank data of the two TrueBeam type linacs with TrueBeam Representative Beam Data. Methods: Three flattening filtered (FF), 6MV, 10MV, and 15MV, and two flattening filtered free (FFF), 6FFF and 10FFF, photon beams of one TrueBeam and one TrueBeam STX Linacs (Varian Medical Systems) are compared to the TrueBeam Representative Beam Data (TRBD) as provided by Varian. All measurements are scanned in Blue Phantom (IBA) with cc04 ion chamber at SSD 100cm at the highest dose rate available for each beam. The TRBD set is the average values of three TrueBeams at a single institution acquired by Iba cc13 ion chamber. To compare the beam quality, the PDD at d10 and d20 and the corresponding PDD 10/20 ratio at field sizes of 3×3, 10×10 and 30×30 cm2 are compared. The flatness of 10×10 and 30×30 cm2 open field profiles at depth of dmax, 10cm and 20cm are also compared. The output factors for square fields are also compared. Results: The PDD difference at both d10 and d20 between all FF and FFF beams and TRBD are within 0.6% and 0.9% respectively. The average PDD ratio difference between all beams and TRBD is within 0.5%. The flatness of all the FF and FFF beams are within 0.4% and 1.0% of TRBD respectively. The output factor differences between the two TrueBeams and RTBD for all beams and field sizes are within 0.8%. Conclusion: All the beam parameters of both the FF and FFF beams of the two Truebeams are within 1% of TRBD. Research grant from Varian Medical Systems

SU-E-T-145: Scanned Percent Depth Dose Curve Discrepancy for Varian TrueBeam Flattening-Filter-Free Photon Beams Using Very Small Sensitive Volume Ion Chamber

Purpose: To investigate the discrepancy of scanned percent depth dose (PDD) for Varian TrueBeam flattening‐filter‐free (FFF) photon beams when using ion chambers with different sensitive volumes.Methods/Materials: PDD curves were scanned for both Flattening Filtered (FF) and FFF photon beams (6MV and 10MV) by using two common scanning chambers with different sensitive volumes — Exradin A1 (0.053cm3) and PTW23323 (0.1cm3). Both Pion and Ppol of two chambers for FFF beams were measured based on TG‐51 protocol at depth of dmax, 10cm and 20cm. The PDD curves from the two chambers were compared, as along with the Pion and Ppol at different depths. Results: Scanned PDD curves of the same energy FF beams were almost identical between two chambers, but the FFF beams showed non‐trivial differences. The differences increased with depth, and were more pronounced for 10MV beam. PDD of FFF beams scanned with different dose rates showed very little difference.For FF beams, all PDDs agreed well with each other at 10cm depth. For FFF beams, PDD of PTW23323 agreed with Varian specifications, but PDD of Exradin A1 showed significant differences at 10cm depth. The PDDs at 10 cm depth of the PTW23323 agreed with Varian specifications, golden beam data provided by RPC and the published TrueBeam commissioning data.Both chambers can be seen to yield Pion and Ppol that agree within 0.6%. Conclusion: We observed a significant difference in scanned PDD curves for FFF beams obtained when using different sensitive volume ion chambers. The PDD curves scanned with the very small sensitive volume ion chamber showed significant difference from both larger chamber results and golden beam data, beyond 10cm depth. If calibration of the FFF beam output was performed based on PDD scanned with a small sensitive volume ion chamber, the output calibration difference could reach 3%.

SU‐E‐T‐265: Measurement of Secondary Neutrons Produced in the (gamma,n) Reaction From the TrueBeam Linac Head

Purpose: The purpose of this study was to measure secondary neutrons produced via the (γ,n) reaction in the linac head of the Varian TrueBeam system for 10 MV and 15 MV x‐ray beams, respectively. Of note is that there is currently no measured data available in the literature on secondary neutron production from the TrueBeam treatment head. Methods: The linac was operated in flattening filter‐free (FFF) mode for the 10 MV beam and flattening filter (FF) mode for the 15 MV beam with the jaws and multileaf collimators in the fully closed position. A nominal setting of monitor units per minute (MU/min) were delivered for each beam energy and the secondary neutron dose rate was measured in triplicate using a LudLum neutron survey meter (model 12‐4) positioned at multiple locations including isocenter, longitudinal (gun‐target) and lateral to the isocenter. Results: For the 15 MV x‐ray beam (FF), 600, 300 and 100 MU/min were delivered. The secondary neutron dose rate peaked at a distance of 20 cm longitudinal to the isocenter at 600 MU/min and decreased with distance from the isocenter. Fluctuations in the longitudinal neutron dose rate were negligible; however, the lateral neutron dose rate varied up to nearly a factor of 2 away from the isocenter with decreasing MU/min. For the 10 MV x‐ray beam (FFF), 2400, 1200 and 400 MU/min were delivered and the secondary neutron dose rate was again observed to peak around 20 cm longitudinal to the isocenter with fluctuations varying by about 50% with decreasing MU/min in positions lateral to the isocenter. Conclusion: Our preliminary results suggest a secondary neutron dose rate on the order of a 45 mSv/hr or less for all measurements considered in this study indicating a relatively low contribution of produced secondary neutrons to the primary therapeutic beam.