6XFFF Beams Research Articles

Planning Considerations for the Primary Lung Tumor Stereotactic Body Radiation Therapy Followed by Concurrent Mediastinal Radiotherapy for Locally-Advanced Non-Small Cell Lung Cancer

Phase III prospective randomized trial of primary lung tumor stereotactic body radiation therapy followed by concurrent mediastinal chemoradiation for locally-advanced non-small cell lung cancer (NRG LU-008), designed to lower the rates of radiation pneumonitis and improve the progression-free survival, is expected to become active nationally in July 2023. Due to the specific nature of the cases selected for this trial, a new approach to treatment planning had to be developed to satisfy the conditions of the trial. Levice Cancer Institute (Atrium Health, North Carolina) ran the initial Phase II trial and investigated several approaches, providing recommendations for the future dosimetry planning approach. A total of 60 patients were selected for the initial trial and treated with a combination of SBRT treatment to the primary tumor (50-54 Gy in 3-5 fractions) and conventional IMRT treatment to 60 Gy to the involved lymph nodes for patients with stage 3 or unresectable stage II NSCLC, combined with chemotherapy. Depending on the location of the primary tumor, all cases could be subdivided into no overlap between the SBRT and IMRT targets, adjacent targets, and overlapping targets. All SBRT plans were done with a 6X-FFF beam, advanced dose calculation algorithm, and 0.1 cm grid size. IMRT targets were primarily treated with VMAT plans, though a minority of cases were planned with the DMLC technique, with a 6X beam, advanced dose calculation algorithm, and 0.25 cm grid size. Various approaches to the planning included target cropping, avoidance via adjusting optimization objectives and utilizing the base dose of the SBRT plan to optimize the dose for the IMRT nodal plan. Various geometries utilized in the plan included a variation in the number of arcs, covered arc angles, and the number of beams. A significant reduction in the side effects was achieved throughout the trial, with only three patients experiencing grade 3 or higher pneumonitis, 3 patients experiencing grade 3 or higher cardiotoxicity, and 1 patient experiencing grade 3 esophagitis. For targets with no significant overlap between the primary tumor and nodal target, standard planning techniques proved to be sufficient. For the overlapping targets, the planning approach of utilizing 2 arcs, >180-degree coverage, for the SBRT plan, and using ¼ of the base SBRT dose to plan the IMRT nodal plan provided the best target coverage while achieving sufficient OAR sparing. Planning the joint SBRT-IMRT plan in the cases of a significant target overlap requires a careful approach, but is feasible with the proposed guideline and should be achievable for any center electing to participate in the NRG LU-008 trial.

Markerless motion tracking with simultaneous MV and kV imaging in spine SBRT treatment—a feasibility study

Objective. Motion tracking with simultaneous MV-kV imaging has distinct advantages over single kV systems. This research is a feasibility study of utilizing this technique for spine stereotactic body radiotherapy (SBRT) through phantom and patient studies. Approach. A clinical spine SBRT plan was developed using 6xFFF beams and nine sliding-window IMRT fields. The plan was delivered to a chest phantom on a linear accelerator. Simultaneous MV-kV image pairs were acquired during beam delivery. KV images were triggered at predefined intervals, and synthetic MV images showing enlarged MLC apertures were created by combining multiple raw MV frames with corrections for scattering and intensity variation. Digitally reconstructed radiograph (DRR) templates were generated using high-resolution CBCT reconstructions (isotropic voxel size (0.243 mm)3) as the reference for 2D–2D matching. 3D shifts were calculated from triangulation of kV-to-DRR and MV-to-DRR registrations. To evaluate tracking accuracy, detected shifts were compared to known phantom shifts as introduced before treatment. The patient study included a T-spine patient and an L-spine patient. Patient datasets were retrospectively analyzed to demonstrate the performance in clinical settings. Main results. The treatment plan was delivered to the phantom in five scenarios: no shift, 2 mm shift in one of the longitudinal, lateral and vertical directions, and 2 mm shift in all the three directions. The calculated 3D shifts agreed well with the actual couch shifts, and overall, the uncertainty of 3D detection is estimated to be 0.3 mm. The patient study revealed that with clinical patient image quality, the calculated 3D motion agreed with the post-treatment cone beam CT. It is feasible to automate both kV-to-DRR and MV-to-DRR registrations using a mutual information-based method, and the difference from manual registration is generally less than 0.3 mm. Significance. The MV-kV imaging-based markerless motion tracking technique was validated through a feasibility study. It is a step forward toward effective motion tracking and accurate delivery for spinal SBRT.

Fetal dose estimation for Virtual Tangential-fields Arc Therapy whole breast irradiation by optically stimulated luminescence dosimeters

Breast cancer is the most frequently diagnosed tumor in pregnant women and radiation therapy should carefully be weighted up because of the dose to the fetus. The aim of this study was to investigate fetal dose in a patient treated with Virtual Tangential-fields Arc Therapy (ViTAT), an innovative technique for whole breast irradiation. Optically stimulated luminescence detectors (OSLDs) were calibrated on a Varian TrueBeam linac, with both a 6X and 6XFFF beam quality, and used for out-of-field measurements. Fetal dose related with ViTAT technique was compared to the standard 3D conformal radiation therapy technique (3DCRT). The fetal dose delivered with a ViTAT technique planned with 6XFFF beam was also investigated. Measurements were taken on a phantom composed of Rando Alderson Phantom slices and solid water slabs. OSLDs were placed in a region identified by the height of the fundus from conception to the twentieth week using a custom made PMMA grid. Due to the higher number of monitor units, the peripheral dose of ViTAT delivered with 6X beams is higher than that of 3DCRT. However, nanoDots measurements prove that ViTAT can be used in place of 3DCRT while maintaining similar fetal dose levels if 6XFFF beams are used.

Assessment of Organ Dose Reduction Using Dynamic Conformal Arc and Static Field with FFF Beams for SBRT in Lung Cancer

Background The aim of this study is to compare the dose delivered to the organs at risk (OAR), using static beams (SF) and a dynamic conformational arc (DCA) with flattening filter free (FFF) beams, for lung stereotactic body radiation therapy (SBRT). Methods 100 patients with lung cancer were treated with SBRT, using FF beams (TrueBeam STx, 6 MV, IQ = 0.67, 600 MU/min), separated into two groups: DCA (50 patients) and SF (50 patients). These patients were retrospectively re-planned using 6XFFF beams, IQ = 0.63, 1400 MU/min. The beam-on time and dosimetric gain on planning target volume (PTV) and OARs (heart, spinal cord, planning risk volume (PRV) of spinal cord, esophagus, lungs and ribs) were analyzed according to tumor location. The comparison of median values was performed using the non-parametric Wilcoxon test (significance level: p < 0.05). Results PTV coverage was 98.90% versus 98.40% (DCA) and 98.8% versus 98.3% (SF) for the FF and FFF beams, respectively. The median dosimetric gain to the heart, spinal cord, PRV spinal cord, esophagus and lungs was 6% (4–11%) in the central region and 8% (2–23%) in the peripheral region, using FFF (p < 0.05). The dose received by the ribs decreases by 5–6 Gy, using FFF beams. The median gain in beam-on time ranged from 31% to 34% for SF and from 44 to 52% for DCA using FFF beams. Conclusions The FFF beams reduce the dose received by all OARs, regardless of the used technique or tumor location, reducing treatment delivery time as well.

Development of a virtual source model for Monte Carlo-based independent dose calculation for varian linac.

Monte Carlo (MC) independent dose calculations are often based on phase‐space files (PSF), as they can accurately represent particle characteristics. PSF generally are large and create a bottleneck in computation time. In addition, the number of independent particles is limited by the PSF, preventing further reduction of statistical uncertainty. The purpose of this study is to develop and validate a virtual source model (VSM) to address these limitations. Particles from existing PSF for the Varian TrueBeam medical linear accelerator 6X, 6XFFF, 10X, and 10XFFF beam configurations were tallied, analyzed, and used to generate a dual‐source photon VSM that includes electron contamination. The particle density distribution, kinetic energy spectrum, particle direction, and the correlations between characteristics were computed. The VSM models for each beam configuration were validated with water phantom measurements as well as clinical test cases against the original PSF. The new VSM requires 67 MB of disk space for each beam configuration, compared to 50 GB for the PSF from which they are based and effectively remove the bottleneck set by the PSF. At 3% MC uncertainty, the VSM approach reduces the calculation time by a factor of 14 on our server. MC doses obtained using the VSM approach were compared against PSF‐generated doses in clinical test cases and measurements in a water phantom using a gamma index analysis. For all tests, the VSMs were in excellent agreement with PSF doses and measurements (>90% passing voxels between doses and measurements). Results of this study indicate the successful derivation and implementation of a VSM model for Varian Linac that significantly saves computation time without sacrificing accuracy for independent dose calculation.

Currently used in clinical practice beam rate changes have no significant effect on the reduction of clonogenic capacity of PNT1A cells in vitro

BackgroundDue to the lack of selectivity of ionizing radiation between normal and cancer cells, it is important to improve the existing radiation patterns. Lowering the risk of cancer recurrence and comfort during treatment are priorities in radiotherapy.Materials and methodsIn the experiment we used dose verification to determine the irradiation time calculated by a treatment planning system for 6XFFF and 10XFFF beams. Cells cultured under standard conditions were irradiated with a dose of 2 Gy at different beam rates 400 MU/min, 600 MU/min, 800 MU/min, 1000 MU/min, 1400 MU/min, 1600 MU/min and 2400 MU/min using 6XFFF, 10XFFF and 6XFF beams.ResultsThe experiment was aimed at comparing the biological response of normal prostate cells after clinically applied radiation patterns. No statistically significant differences in the cellular response were observed. The wide range of beam rates as well as the beam profiles did not significantly affect cell proliferation.ConclusionsHigh beam rates, without significantly affecting the clonogenic capacity of cells, have an impact on the quality of patient’s treatment. With the increasing beam rate the irradiation time is shortened, which has an important impact on patients’ health. This experiment can have a practical significance.

Technical Note: Synthetic treatment beam imaging for motion monitoring during spine SBRT treatments - a phantom study.

One of the biggest challenges in applying megavoltage (MV) treatment beam imaging for monitoring spine motion in stereotactic body radiotherapy (SBRT) is the small beam apertures in the images due to strong beam modulations in IMRT planning. The purpose of this study is to investigate the feasibility of a markerless motion tracking method in spine SBRT delivery using a novel enhanced synthetic treatment beam (ESTB) imaging technique. Three clinical spine SBRT plans using 6XFFF beams and sliding window IMRT technique were transferred to a thorax phantom and delivered by a TrueBeam machine. Before delivery, the phantom was aligned to the plan isocenter using CBCT setup and verified with a second CBCT, and then, 2mm shifts were introduced in both the craniocaudal (CC) and the left-right (LR) directions with the couch. During beam delivery, MV images were continuously taken with an electronic portal imaging device (EPID) and automatically grabbed by Varian iTools Capture software with a frame rate of 11.6Hz. After preprocessing for scatter correction and beam intensity compensation, every 50 frames of MV images were combined to generate a series of ESTB images for each beam. The ESTB images were then registered to the projections of the verification CBCT at the matched beam angles to detect the 2mm shifts. Compared to snapshot MV images, the ESTB images had significantly enlarged fields of view (FOVs) and improved image quality. Based on two-dimensional (2D) rigid registration, the ESTB image to CBCT projection matching showed submillimeter accuracy in detecting motion. Specifically, the root mean square errors in detecting the LR/CC shifts were 0.35/0.28, 0.32/0.35, 0.63/0.44, 0.55/0.51, and 0.69/0.42mm at gantry angles 180, 160, 140, 120, and 100, respectively. Our results in the phantom study suggest that ESTB images from a sliding window IMRT plan can be used to detect spine motion, with submillimeter precision in the 2D plane perpendicular to the beam.

On the use of single‐isocenter VMAT plans for SBRT treatment of synchronous multiple lung lesions: Plan quality, treatment efficiency, and early clinical outcomes

Cone‐beam computed tomography (CT)‐guided volumetric‐modulated arc therapy (VMAT) plans for stereotactic body radiotherapy (SBRT) treatment of synchronous multiple lung lesions with a flattening filter‐free (FFF) beam is a safe and highly effective treatment option for oligometastases lung cancer patients. Fourteen patients with metastatic non–small‐cell lung cancer (NSCLC) lesions (two to five) received a single‐isocenter VMAT SBRT treatment in our clinic. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB‐based dose calculation algorithm using heterogeneity corrections with a single isocenter placed between/among the lesions. Compared to 10X‐FFF and traditional flattened 6X (6X‐FF) beams, 6X‐FFF beam produced highly conformal radiosurgical dose distribution to each target volume, reduced dose to adjacent organs at risk (OAR), and significantly reduced the lung SBRT fraction duration to < 3.5 min/fraction for 54/50 Gy treatments in 3/5 fractions — significantly improving patient convenience and clinic workflow. Early follow‐up CT imaging (mean, 9 months) results show high local control rates (100%) with no acute lung or rib toxicity. Longer clinical follow up in a larger patient cohort is ongoing to further validate the outcomes of this treatment approach.

Effectiveness of robust optimization in volumetric modulated arc therapy using 6 and 10 MV flattening filter-free beam therapy planning for lung stereotactic body radiation therapy with a breath-hold technique.

We investigated the feasibility of a robust optimization with 6 MV X-ray (6X) and 10 MV X-ray (10X) flattening filter-free (FFF) beams in a volumetric modulated arc therapy (VMAT) plan for lung stereotactic body radiation therapy (SBRT) using a breath-holding technique. Ten lung cancer patients were selected. Four VMAT plans were generated for each patient; namely, an optimized plan based on the planning target volume (PTV) margin and a second plan based on a robust optimization of the internal target volume (ITV) with setup uncertainties, each for the 6X- and 10X-FFF beams. Both optimized plans were normalized by the percentage of the prescription dose covering 95% of the target volume (D95%) to the PTV (1050 cGy × 4 fractions). All optimized plans were evaluated using perturbed doses by specifying user-defined shifted values from the isocentre. The average perturbed D99% doses to the ITV, compared to the nominal plan, decreased by 369.1 (6X-FFF) and 301.0 cGy (10X-FFF) for the PTV-based optimized plan, and 346.0 (6X-FFF) and 271.6 cGy (10X-FFF) for the robust optimized plan, respectively. The standard deviation of the D99% dose to the ITV were 163.6 (6X-FFF) and 158.9 cGy (10X-FFF) for the PTV-based plan, and 138.9 (6X-FFF) and 128.5 cGy (10X-FFF) for the robust optimized plan, respectively. Robust optimized plans with 10X-FFF beams is a feasible method to achieve dose certainty for the ITV for lung SBRT using a breath-holding technique.

A simple, yet novel hybrid-dynamic conformal arc therapy planning via flattening filter-free beam for lung stereotactic body radiotherapy.

PurposeDue to multiple beamlets in the delivery of highly modulated volumetric arc therapy (VMAT) plans, dose delivery uncertainties associated with small‐field dosimetry and interplay effects can be concerns in the treatment of mobile lung lesions using a single‐dose of stereotactic body radiotherapy (SBRT). Herein, we describe and compare a simple, yet clinically useful, hybrid 3D‐dynamic conformal arc (h‐DCA) planning technique using flattening filter‐free (FFF) beams to minimize these effects.Materials and MethodsFifteen consecutive solitary early‐stage I‐II non‐small‐cell lung cancer (NSCLC) patients who underwent a single‐dose of 30 Gy using 3–6 non‐coplanar VMAT arcs with 6X‐FFF beams in our clinic. These patients’ plans were re‐planned using a non‐coplanar hybrid technique with 2–3 differentially‐weighted partial dynamic conformal arcs (DCA) plus 4–6 static beams. About 60–70% of the total beam weight was given to the DCA and the rest was distributed among the static beams to maximize the tumor coverage and spare the organs‐at‐risk (OAR). The clinical VMAT and h‐DCA plans were compared via RTOG‐0915 protocol for conformity and dose to OAR. Additionally, delivery efficiency, accuracy, and overall h‐DCA planning time were recorded.ResultsAll plans met RTOG‐0915 requirements. Comparison with clinical VMAT plans h‐DAC gave better target coverage with a higher dose to the tumor and exhibited statistically insignificance differences in gradient index, D2cm, gradient distance and OAR doses with the exception of maximal dose to skin (P = 0.015). For h‐DCA plans, higher values of tumor heterogeneity and tumor maximum, minimum and mean doses were observed and were 10%, 2.8, 1.0, and 2.0 Gy, on average, respectively, compared to the clinical VMAT plans. Average beam on time was reduced by a factor of 1.51. Overall treatment planning time for h‐DCA was about an hour.ConclusionDue to no beam modulation through the target, h‐DCA plans avoid small‐field dosimetry and MLC interplay effects and resulting in enhanced target coverage by improving tumor dose (characteristic of FFF‐beam). The h‐DCA simplifies treatment planning and beam on time significantly compared to clinical VMAT plans. Additionally, h‐DCA allows for the real time target verification and eliminates patient‐specific VMAT quality assurance; potentially offering cost‐effective, same or next day SBRT treatments. Moreover, this technique can be easily adopted to other disease sites and small clinics with less extensive physics or machine support.

Flattening filter free VMAT for a stereotactic, single-dose of 30 Gy to lung lesion in a 15-min treatment slot.

Cone‐beam CT‐guided single dose of lung stereotactic body radiotherapy (SBRT) treatment with a flattening filter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a safe and highly effective treatment modality for selective small lung lesions. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB algorithm for heterogeneity corrections. 6X‐FFF beam produced highly conformal radiosurgical dose distribution to the target and reduced lung SBRT fraction duration to less than 10 min for a single dose of 30 Gy, significantly improving patient comfort and clinic workflow. Early follow‐up CT imaging results (mean, 8 months) show high local control rates (100%) with no acute lung or rib toxicity. Longer clinical follow‐up in a larger patient cohort managed in this fashion is underway to further validate this treatment approach.

Optimized Volumetric Modulated Arc Technique for Noninvasive Cardiac Ablation to Treat Ventricular Tachycardia

Recent publications (Cuculich et al., NEJM-2017, Robinson et al., Circulation-2019) have shown promising outcomes of radioablation using a single-fraction SBRT to treat patients with refractory, life-threatening, VT. This prompted growing interest across the world to use SBRT to treat VT. The primary goal of this study is to explore an alternative VMAT planning technique to achieve greater Non-Target Heart-Tissue (NTHT) sparing while simultaneously allowing for breath hold (BH) treatments. 13 patients were selected who previously received a single fraction dose of 35 Gy to ITV and 25 Gy to PTV utilizing 3 non-coplanar VMAT arcs of 6 MV photon beams using HD-MLC on a radiosurgery-optimized linac. To further improve NTHT sparing we retrospectively re-planned each patient using 10 ipsi-lateral arcs with optimal gantry, couch and collimator angles. Two plans were generated using 6X FFF and 10X FFF beams with dynamic jaw-tracking. Gantry arc lengths were limited to 90° (< 15s/arc) for feasibility of a potential BH treatments. Couch angles were limited to ± 15° to avoid collisions. Plans were optimized to maximize the NTHT sparing while meeting the protocol constraints for the stomach, esophagus and colon (OAR). Plan quality metrics for both clinical and reoptimized plans are summarized in Table 1. Revised plans showed adequate target coverage (V95%Rx=>95%), better dose conformity and gradient, while meeting the dose constraints of the OARs. Excellent NTHT sparing is achieved by reducing the volumes of V20Gy by 43 cc and V5Gy by 250 cc. Average mean dose reduction of 2.5 (range: 0.6 – 6.0) Gy and 2.2 (0.7 – 5.7 Gy) was observed in 6X FFF and 10X FFF plans, respectively. Revised plans showed the possibility of treating each arc in a single BH. The standardized technique used in this study showed improved NTHT sparing and better dose conformity. This result may be important for the group of patients where cardiac toxicity may play a vital role in long-term survival. BH treatments may reduce the required PTV margins resulting in further NTHT sparing.Abstract 3778; Table 1Vol. (cc)Plan TypeTarget CoverageConformityMean D (Gy)%VRx%V95%RxConformity IndexGradient MeasureITV48.1 ± 29Clinical38.4 ± 1.197.6 ± 2.499.7 ± 0.66X FFF36.9 ± 0.494.2 ± 3.199.3 ± 0.810X FFF36.8 ± 0.890.5 ± 11.698.7 ± 2.0PTV138.9 ± 67.8Clinical33.6 ± 1.199.0 ± 0.899.7 ± 0.41.14 ± 0.122.01 ± 0.476X FFF32.3 ± 0.493.8 ± 2.596.8 ± 2.21.02 ± 0.041.76 ± 0.4710X FFF32.3 ± 0.494.2 ± 2.397.1 ± 1.81.03 ± 0.031.55 ± 0.26Non-Target Heart TissueMean D (Gy)%V25%V20%V15%V10%V5%V2Clinical6.2 ± 2.10.9 ± 0.84.9 ± 2.511.2 ± 5.421.4 ± 9.140.5 ± 13.966.7 ± 17.86X FFF3.7 ± 1.10.3 ± 0.31.9 ± 2.74.9 ± 1.610.1 ± 3.622.5 ± 8.945.9 ± 15.7Vol. spared (cc)9 ± 1143.± 3590 ± 76160 ± 113250 ± 146284 ± 20910X FFF4.0 ± 1.10.4 ± 0.32.2 ± 0.75.5 ± 1.711.4 ± 3.825.5 ± 9.649.6 ± 16.5Vol. spared (cc)8 ± 1039 ± 3381 ± 75142 ± 108208 ± 111233 ± 150 Open table in a new tab

Part I: Out-of-field dose mapping for 6X and 6X-flattening-filter-free beams on the TrueBeam for extended distances.

With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out-of-field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening-filter setting. The accurate quantification of out-of-field dose at extended distances could also benefit researchers and detector developers. While data exist for out-of-field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out-of-field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries. Out-of-field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2×2cm2 , 4×4cm2 , 10×10cm2 , and 20×20cm2 ) for a range of distances from the isocenter (0-100cm). The angular dependence of the out-of-field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in-plane direction. All measurements were performed for both 6X and 6X-flattening-filter-free (FFF) beams. The lowest out-of-field doses were observed at 60cm away from isocenter in both in-plane and cross-plane directions for fields smaller than 10×10cm2 . Out-of-field dose decreased with decreasing jaw size (a factor of 4.7 for 6X-FFF and a factor of 3.1 for 6X going from 20×20cm2 to 2×2cm2 at 60cm from isocenter in the in-plane direction). The 6X-FFF beam produced out-of-field doses as low as 64% of the 6X beam. This study presents a comprehensive description of 6X and 6X-FFF out-of-field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter.

Part II: Verification of the TrueBeam head shielding model in Varian VirtuaLinac via out-of-field doses.

A good Monte Carlo model with an accurate head shielding model is important in estimating the long-term risks of unwanted radiation exposure during radiation therapy. The aim of this paper was to validate the Monte Carlo simulation of a TrueBeam linear accelerator (linac) head shielding model. We approach this by evaluating the accuracy of out-of-field dose predictions at extended distances which are comprised of scatter from within the patient and treatment head leakage and thus reflect the accuracy of the head shielding model. We quantify the out-of-field dose of a TrueBeam linac for low-energy photons, 6X and 6X-FFF beams, and compare measurements to Monte Carlo simulations using Varian VirtuaLinac that include a realistic head shielding model, for a variety of jaw sizes and angles up to a distance of 100cm from the isocenter, in both positive and negative directions. Given the high value and utility of the VirtuaLinac model, it is critical that this model is validated thoroughly and the results be available to the medical physics community. Simulations were done using VirtuaLinac, the GEANT4-based Monte Carlo model of the TrueBeam treatment head from Varian Medical Systems, and an in-house GEANT4-based code. VirtuaLinac included a detailed model of the treatment head shielding and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in-house code, which modeled the measurement setup with a solid water buildup, the carbon fiber couch, and the gantry stand. For each jaw size (2×2cm2 , 4×4cm2 , 10×10cm2 , and 20×20cm2 ) and angular setting (0°, 90°, 45°, 135°), the dose was calculated at intervals of 5cm along each measurement direction. For the 10×10cm2 jaw size, both 6X and 6X-FFF showed very good agreement between simulation and measurement in both in-plane directions, with no apparent systematic bias. The percentage deviations for these settings were as follows: (mean, STDEV, maximum) (8.34, 6.44, 24.84) for 6X and (13.21, 8.93, 35.56) for 6X-FFF. For all jaw sizes, simulation agreed well in the in-plane direction going away from the gantry, but, some deviations were observed moving toward the gantry at larger distances. At larger distances, for the jaw sizes smaller than 10×10cm2 , the simulation underestimates the dose compared with measurement, while for jaw sizes larger than 10×10cm2 , it overestimates dose. For all comparisons between ±50cm from isocenter, average absolute agreement between simulation and measurement was better than 28%. We have validated the Varian VirtuaLinac's head shielding model via out-of-field doses and quantified the differences between TrueBeam head shielding model created out-of-field doses and measurements for an extended distance of 100cm.

Investigation the impact of maximum control point on dose calculation in Eclipse treatment planning system for lung SBRT

Purpose: Choosing an appropriate parameter on the computerized treatment planning systems (TPSs) influences on the accuracy of dose calculation. Several dosimetric parameters have been studied to achieve a more accurate dose and qualitative plan. The purpose of this study was to determine the impact of maximum control point on the dose calculation on Eclipse TPSs for lung Stereotactic Body Radiation Therapy (SBRT) considering the plan quality, the computation time and the treatment file size. Methods: Dose distributions for the 8 lung SBRT plans with varying maximum control point of 64, 166, and 320 were calculated by Eclipse TPSs with flattening filter free (FFF) beam. The treatment dose was prescribed at 85% isodose level of 54 Gy to the planning target volume (PTV). The dosimetric impact can be evaluated from target coverage, conformity index (CI), homogeneity index (HI), and organ at risk (OAR) doses, while the computation time and the file storage space were compared with the recommended number of control point. Results: The use of 64 control points per subfields tended to increase the dose at PTV and OARs comparing with the 166 and 320 control point plans, while the HI and CI values were similar. The average increases of OARs doses including the spinal cord, heart, esophagus and total lung depended on the photon beam energy. The higher average control point (AVG) number leaded to increase the computation time and the file size for both 6X-FFF and 10X-FFF photon beams. The correlations between AVG and plan storaage space were observed in the same ratio as the computation time. Conclusion: Using the minimal number of control point, the quantitative analysis in the PTV and OARs showed no clinically significant variation in dose, therefore choosing an optimal number of fixed control points leaded to balance the plan quality, the computation time and the file size.

Treatment of Cervical Cancer With a Prototype Flattening Filter-Free Straight-Through LINAC With Fast Jawless MLC Collimator

To report on the dosimetric and delivery performance of a novel FFF straight-through linac with a jawless MLC system and a high leaf speed capability of 5 cm/sec, intended for faster intensity modulated deliveries (IMRT/VMAT). Comparisons with the Truebeam (TB) Millennium® system were performed. A novel jawless MLC system with 56 leaves of 1 cm width (28x28cm2maximum field size at 100cm), is mounted on a straight-through linac that produces a single 6X-FFF beam. The flexibility of this prototype system is due to the absence of jaws, maximum leaf over-travel with 100% inter-digitization, leaf speed of 5cm/s and a dose rate of 800 MU/min in FFF mode. A group of whole pelvis cases (16 patients) with and without para-aortic volumes were considered for both planning and delivery via IMRT and VMAT and resulting plans and delivery accuracy were compared to a TB linac. Our intention was to evaluate the dosimetric feasibility, the fast delivery capability, workflow, dosimetric differences due to lack of jaws and larger leaf width, and the 100% over-travel and inter-digitization capability compared to TB linac. Dose distributions and DVHs were used for plan evaluation and patient specific QA tests were used to evaluate the delivery accuracy. To study workflow feasibility, large para-aortic cases were planned with two isocenter single and double-optimization techniques, creating a fluence gradient transition region between the upper and lower part of the target volume in the latter case for both IMRT and VMAT modes. Optimization objectives for the prototype system were adjusted in order to obtain at the minimum a plan comparable to the clinical ones from TB system, based on the clinical evaluation by the same clinician. Preliminary results of the dosimetric comparison between the prototype system and the TB system have shown an overall good agreement between the two systems for the cases studied. The prototype system produced higher target doses than the TB system with higher max doses and for some organs-at-risk (OARs) produced lower doses (e.g., kidneys) while higher for others (e.g., rectum). The larger leaf width of the prototype MLC system compared to the TB leaf width, did not result lower plan quality when intensity modulation was involved. OSL measurements were performed on an anthropomorphic phantom to test the accuracy of the delivery. The preliminary results from the pelvis cases study indicated that the prototype linac and MLC system is capable of producing clinically acceptable intensity modulated plans comparable to the TB system plans. The larger leaf width compared to the Millennium® leaf width did not degrade the plan quality perhaps due to the optimization process and to the full inter-digitization flexibility of the new system. The potential for faster delivery by the prototype MLC is under investigation.

Robustness of Extended Field Cervical Target Optimization Techniques to Isocenter Offsets with a Prototype Fast Jawless Mlc System

To study the robustness of treating large cervical target volumes to isocenter offsets with a rapid jawless MLC collimating system, intended for faster intensity modulated deliveries. A jawless MLC system with 56 leaves of 1 cm width (28x28cm2maximum field size at 100cm), is mounted on a straight-through linac that produced a single 6X-FFF beam. The flexibility of this prototype MLC system is due to the absence of jaws, maximum leaf over-travel and 100% inter-digitization and leaf speed of 5cm/s. A group of six patients with cervical CA with para-aortic nodes were planned with intensity modulated deliveries with two isocenters on the craniocaudal direction to cover the extended target volume. A single plan optimization was compared with two sequential optimizations (upper and lower isocenter) to test the plan robustness to isocenter shits of 5mm. For this approach, the PTV was segmented into an upper PTV(U_PTV) and lower PTV(L_PTV) volume with 9 transition volumes (each measured 1.2cm long) between them. The goal of optimization was to achieve a uniform prescription dose to the U_PTV and a smooth dose gradient over the transition volumes. During lower plan optimization, the upper plan was then selected as a base dose plan. The lower plan optimization objectives were then set using the total dose objectives to the full PTV and OARs. A plan sum was created between the resulting lower plan and the upper plan to produce the cumulative dose distribution. Since the single optimization does not allow the control of the dose gradients produced by both fields in the overlapping region, it is possible that the resulting treatment plans will be sensitive to small relative displacements of the isocenters. A potential advantage of the double optimization technique is the enforced smooth gradient produced in each plan. The expectation is that these gradients will reduce the impact of a small relative displacement of the isocenters. For single optimization, the dose maximum increased in the shifted plan sum relative to the un-shifted plan sum. There are bands of increased high dose visible at the edges of the upper and lower fields. For the double optimization, rather than the larger increases in dose at the field edges, moderate dose increases are visible throughout the transition region. The impact was also reduced relative to the single optimization technique. The average dose maximum increase for the single optimization technique was 12.8% while for the double optimization technique is was only 3.6%. The preliminary results on the robustness of extended field cervical target optimization techniques to isocenter offsets with our novel jawless MLC favor the double optimization approach.

EPID-based dosimetry to verify IMRT planar dose distribution for the aS1200 EPID and FFF beams.

We proposed to perform a basic dosimetry commissioning on a new imager system, the Varian aS1200 electronic portal imaging device (EPID) and TrueBeam 2.0 linear accelerator for flattened (FF) and flattening filter‐free (FFF) beams, then to develop an image‐based quality assurance (QA) model for verification of the system delivery accuracy for intensity‐modulated radiation therapy (IMRT) treatments. For dosimetry testing, linearity of dose response with MU, imager lag, and effectiveness of backscatter shielding were investigated. Then, an image‐based model was developed to convert images to planar dose onto a virtual water phantom. The model parameters were identified using energy fluence of the Acuros treatment planning system (TPS) and, reference dose profiles and output factors measured at depths of 5, 10, 15, and 20 cm in water phantom for square fields. To validate the model, its calculated dose was compared to measured dose from MapCHECK 2 diode arrays for 36 IMRT fields at 10 cm depth delivered with 6X, 6XFFF, 10X, and 10XFFF energies. An in‐house gamma function was used to compare planar doses pixel‐by‐pixel. Finally, the method was applied to the same IMRT fields to verify their pretreatment delivery dose compared with Eclipse TPS dose. For the EPID commissioning, dose linearity was within 0.4% above 5 MU and ∼1% above 2 MU, measured lag was smaller than the previous EPIDs, and profile symmetry was improved. The model was validated with mean gamma pass rates (standard deviation) of 99.0% (0.4%), 99.5% (0.6%), 99.3% (0.4%), and 98.0% (0.8%) at 3%/3 mm for respectively 6X, 6XFFF, 10X, and 10XFFF beams. Using the same comparison criteria, the beam deliveries were verified with mean pass rates of 100% (0.0%), 99.6% (0.3%), 99.9% (0.1%), and 98.7% (1.4%). Improvements were observed in dosimetric response of the aS1200 imager compared to previous EPID models, and the model was successfully developed for the new system and delivery energies of 6 and 10 MV, FF, and FFF modes.PACS number(s): 87.53.Oq, 87.53.Xd

SU-G-TeP2-03: Comparison of Standard Dosimetry Protocol in Japan and AAPM TG-51 Addendum in Order to Establish Optimal Dosimetry for FFF Beam

Purpose: Japan Standard Dosimetry of Absorbed dose to water in external beam radiotherapy (JSDP12) is widely used to measure radiation dose in radiotherapy. However, JSDP12 does not take flattening-filter-free (FFF) beam into consideration. In addition, JSDP12 applied TPR20,10 for dose quality index for photon beam. The purpose of this study is to compare JSDP12 with AAPM TG-51 addendum in order to establish optimal dosimetry procedure for FFF beam. Method: We evaluated the ion-recombination factor (ks) and the correction factor of radial beam profile (Prp) in FFF beam dosimetry. The ks was introduced by 2 voltages method and verified by Jaffe's plot. The Prp was given by both film measurement and calculation of treatment planning system, and compared them. Next, we compared the dose quality indexes (kQ) between TPR20,10 method and PDD(10)x method. Finally we considered optimal dosimetry protocol for FFF photon beam using JSDP12 with referring TG-51 addendum protocols. The FFF photon beams of 6 MV (6X-FFF) and 10 MV (10X-FFF) from TrueBeam were investigated in this study. Results: The ks for 6X-FFF and 10X-FFF beams were 1.005 and 1.010, respectively. The Prp of 0.6 cc ionization chamber for 6X-FFF and 10X-FFF beams (Film, TPS) were (1.004, 1.008) and (1.005, 1.008), respectively. The kQ for 6X-FFF and 10X-FFF beams (JSDP12, TG-51 addendum) were (0.9950, 0.9947) and (0.9851, 0.9845), respectively. The most effective factor for uncertainty in FFF photon beam measurement was Prp for JSDP12 formalism. Total dosimetric differences between JSDP12 and TG-51 addendum for 6X-FFF and 10X-FFF were -0.47% and -0.73%, respectively. Conclusion: The total dosimetric difference between JSDP12 and TG-51 addendum was within 1%. The introduction of kQ given by JSDP is feasible for FFF photon beam dosimetry. However, we think Prp should be considered for optimal dosimetry procedure even if JSDP12 is used for FFF photon beam dosimetry.

SU‐F‐T‐502: FFF Beams, Jaw‐Tracking and Treatment Techniques: Out of Field Dose Considerations for Pediatric Radiation Therapy Delivery

Purpose:To compare the extended dose profile delivered by 3DCRT and VMAT techniques for flattened and flattening‐filter‐free(FFF) photon beams (6X, 6XFFF,10XFFF), with and without jaw‐tracking (JT) on Varian TrueBeam linac. The goal is to determine which treatment technique/modality will minimize the peripheral photon dose exposure (and ultimately minimize the risk of second malignant neoplasms (SMN)) in pediatric patients.Methods:3DCRT, VMAT, and jaw tracking VMAT (JTVMAT) plans with 6X, 6XFFF and 10XFFF x‐ray beams were created on a 30×60×22.5cm solid water phantom with a 551 cc PTV. The 3DCRT plans consisted of a 4FLD arrangement. The optimization objectives for the single‐arc VMAT plans was V95%Rx=98% to PTV and minimize dose to a 5cm diameter organ at risk (OAR). The OAR to PTV distance varied from 0–30cm along the long axis at 7.5cm depth. The dose to the center of the OAR was measured using a 0.6cc ion chamber.Results:Relative to the 6X flattened beam, the 10XFFF photon beam had the lowest dose in the penumbra and peripheral region (&gt;15 cm) region by up to 20% and 40%, respectively for all modalities (3DCRT, VMAT, JTVMAT). The 6XFFF beams only showed a dose reduction in the peripheral region (by up to 20%). JT did not significantly affect the peripheral dose for all modalities and energies.Conclusion:Treating pediatric patients with a 10XFFF beam is the most effective way to reduce photon scatter dose in both the penumbra and peripheral regions. However, the neutron dose contribution resulting from the 10MV beam still needs to be considered. For all modalities, 6XFFF was the next effective method to reduce peripheral photon doses. 3DCRT beams had the lowest peripheral doses for all energies compared to VMAT and JTVMAT, however previous publications have shown that this comes at the expense of PTV conformity and OAR sparing.