L Rosen

SU-E-T-188: Commission of World 1st Commercial Compact PBS Proton System

Purpose: ProteusONE is the 1st commercial compact PBS proton system with an upstream scanning gantry and C230 cyclotron. We commissioned XiO and Raystation TPS simultaneously. This is a summary of beam data collection, modeling, and verification and comparison without range shiter for this unique system with both TPS. Methods: Both Raystation and XiO requires the same measurements data: (i) integral depth dose(IDDs) of single central spot measured in water tank; (ii) absolute dose calibration measured at 2cm depth of water with mono-energetic 10×10 cm2 field with spot spacing 4mm, 1MU per spot; and (iii) beam spot characteristics in air at 0cm and ± 20cm away from ISO. To verify the beam model for both TPS, same 15 cube plans were created to simulate different treatment sites, target volumes and positions. PDDs of each plan were measured using a Multi-layer Ionization Chamber(MLIC), absolute point dose verification were measured using PPC05 in water tank and patient-specific QA were measured using MatriXX PT, a 2D ion chamber array. Results: All the point dose measurements at midSOBP were within 2% for both XiO and Raystation. However, up to 5% deviations were observed in XiOs plans at shallow depth while within 2% in Raystation plans. 100% of the ranges measured were within 1 mm with maximum deviation of 0.5 mm. 20 patient specific plan were generated and measured in 3 planes (distal, proximal and midSOBP) in Raystation. The average of gamma index is 98.7%±3% with minimum 94% Conclusions: Both TPS were successfully commissioned and can be safely deployed for clinical use for ProteusONE. Based on our clinical experience in PBS planning, user interface, function and workflow, we preferably use Raystation as our main clinical TPS. Gamma Index >95% at 3%/3 mm criteria is our institution action level for patient specific plan QAs.

SU‐E‐T‐136: Comparison of TomoScanner™ 2D Water Phantom versus IBA Helix for Tomotherapy Profile Measurements

PURPOSE To compare the differences in measured data using Tomotherapys TomoScanner™ 2D Water Phantom versus IBAs Blue Phantom Helix taken during commissioning of the Tomotherapy HD unit. METHODS IBA Helix was used with CC04 ion chamber to measure Inline (jaw-size width), Crossline (40 cm width) and Percent Depth Dose (PDD). Data was analyzed using IBA Omni-Pro 7.3 software. Measurements were performed at 85 SSD for field sizes 5×40 cm2 , 2.5×40 cm2 , and 1.0×40 cm2 . All field sizes were measured at 1.5 cm (nominal dmax), 5 cm, 10 cm and 15 cm depths. Scans were performed at a continuous speed setting of 0.5 cm/sec. Setup and data measurements were performed twice on separate occasions for consistency and repeatability. Data measurements were normalized to 10 cm depth and compared to commissioning data taken from TomoScanner™ and Tomotherapys Twinning Data. RESULTS For all jaws, the profiles measured at 5 cm, 10 cm, and 15 cm depth using IBA Helix matched the profiles of the TomoScanner™ data within 1%. Profile variance at 1.5 cm depth showed a deviation of up to 3%. For all jaws, the PDD comparisons displayed 3-5% deviation from the surface to 2 cm depth and within 1% deviation at 2 cm to 15 cm depth. Measurements performed using CC04 ion chamber versus A1SL ion chamber (used by Standard Imaging) showed data differences of less than 0.5%. CONCLUSIONS Discrepancies in Tomotherapy beam profiles were observed between different water phantoms. Further investigation is required to determine the cause of variances between IBA and Tomotherapy data sets such as investigating geometrical differences between the water tanks and software dissimilarities in collecting and correcting raw data. It is recommended that independent commissioning data be taken when TomoScanner™ is not the clinical sites standard water phantom.

SU-E-T-780: Use Robustness Optimization (RO) Method to Improve the Planning Efficiency for Pencil Beam Scanning Cranial Spinal Irradiation

Purpose: We evaluate the feasibility of using robustness optimization (RO) function to improve the planning efficiency of pencil beam scanning (PBS) craniospinal irradiation (CSI) with gradient matching technique. Methods: A CSI patient was planned with 2 lateral brain fields and 4 posterior fields to cover the entire spine to maximal field of 24 cm × 20 cm on a compact PBS gantry, ProteusONE. CSI plans were generated using traditional volumetric gradient dose optimization (VGDO) and robustness optimization (RO) method respectively. In traditional VGDO, besides the sectioned spine target volumes, gradient volume (GV) were generated as 4 equally spaced structures e.g. 80%, 60%, 40%, and 20% of prescription dose. In RO method, only sectioned spine target volumes with an overlap of 4cm were created. In the robustness optimization settings, 5mm uncertainty in superior and inferior direction was defined for auto gradient optimization. Dosimetric metrics of conformity number (CN), homogeneity index (HI), and maximal cord doses were compared in Raystation version 4.6.100.6. Results: In VGDO method, total 16 GV structures and five 100% dose level target structures were contoured compared to total 5 target structures in RO method which saves 30 min in contour. With the same PTV coverage (95% volume receive 30.6Gy prescription dose), maximum cord dose is 32.64Gy in VGDO and 31.94Gy in RO. HI is 1.03 and 1.04 for VGDO and RO respectively. CN is 0.93 and 0.94 for VGDO and RO respectively. Conclusions: The dosimetric comparison demonstrated both methods are equivalent in terms of plan quality. With robust optimization for CSI gradient matching, it efficiently reduces the amount of planning target contour structure by factor of 4 and thus improves the planning efficiency especially for 4 or more gradient junction area.

SU-E-T-189: Commission Range Shifter On a Spot Scanning Proton System Using Raystation Treatment Planning System

Purpose: To treat superficial target e.g. chest wall, head&neck or cranial cases, we commissioned two range shifter(RS) in Raystation4.0 with 7.37cm(RS1) and 4.1cm(RS2) Water Equivalent Thickness(WET) respectively. However, current beam model has limitations due to the secondary scattered proton. This study provides a detailed and critical commission data and provides suggestions for using RS in clinic. Methods: RS’ WET was verified by Multi-Layer Ionization Chamber from 120MeV to 226.7MeV before TPS modeling. Spot characteristics were measured using 2D scintillate detector at ISO with different air gap. A 8×8×10cm3 cube is created in 8cm depth of water to verify the absolute dose accuracy. Plans were created with different air gap using both RS. Absolute dose verification was measured along the central axis from distal end to surface using PPC05. 10 clinical RS2 plans were measured using MatriXXPT in 3 planes (proximal, distal and midSOBP). Results: RS material’s proton stopping power is energy dependent(from 70MeV to 226.7MeV) ranging from 7.42 to 7.31cm and from 4.10 to 4.03cm respectively. We chose 7.37cm (RS1) and 4.10cm (RS2) to favor the low and median proton energy. With different air gap(3cm to 32cm), spot size expands from 3.2mm to 5.5mm(RS1) and from 3.1mm to 4.1mm(RS2) respectively(226.7MeV in air, 1-sigma). For the absolute dose verification, the larger air gap and shallower depth causes larger discrepancy between TPS and measurements. All 10 clinical plans with 5–10cm air gap passed gamma index 95% with 3%/3mm criteria and outputs differences were within 3%. Conclusion: We strongly recommend each institution to verify the WET independently and choose the value to fit the clinical needs. To minimize the output difference in Raystation4.0 while avoid potential collision to the patient, we recommend to use 5–10cm air gap to minimize the output difference within 2% and preferably use RS with smaller WET if possible.

SU‐E‐T‐73: A Robust Proton Beam Therapy Technique for High‐Risk Prostate Cancer Whole Pelvis Irradiation: Bilateral Opposed Single Field Uniform Dose (SFUD) Plan with Lateral Penumbra Gradient Matching

Purpose: To develop a clinical feasible and robust proton therapy technique to spare bowel, bladder and rectum for high-risk prostate cancer patients Methods: The study includes 3 high-risk prostate cancer cases treated with bilateral opposed SFUD with lateral penumbra gradient matching technique prescribed to 5400cGyE in 30 fx in our institution. To treat whole pelvic lymph node chain, the complicated ‘H’ shape, using SFUD technique, we divided the target into two sub-targets (LLAT beam treating ‘90 degree T-shape’ and RLAT beam treating ‘: shape’) in Plan A and use lateral penumbra gradient matching at patient’s left side. Vice verse in Plan B. Each plan deliver half of the prescription dose. Beam-specific PTVs were created to take range uncertainty and setup error into account. For daily treatment, patient received four fields from both plan A and B per day. Robustness evaluation were performed in the worst case scenario with 3.5% range uncertainty and 1, 2, 3mm overlap or gap between LLAT and RLAT field matching in Raystation 4.0. All of cases also have a Tomotherapy backup plan approved by physician as a dosimetric comparison. Results: The total treatment time take 15–20mins including IGRT and four fields delivery on ProteusONE, a compact size PBS proton system, compared to 25–30min in traditional Tomotherapy. Robustness analysis shows that this plan technique is insensitive to the range uncertainties. With the lateral gradient matching, 1, 2, 3mm overlap renders only 2.5%, 5.5% and 8% hot or cool spot in the junction areas. Dosimetric comparisons with Tomotherapy show a significant dose reduction in bladder D50%(14.7±9.3Gy), D35%(7.3±5.8Gy); small bowel and rectum average dose(19.6±7.5Gy and 14.5±6.3Gy respectively). Conclusion: The bilateral opposed(SFUD) plan with lateral penumbra gradient matching has been approved to be a safe, robust and efficient treatment option for whole pelvis high-risk prostate cancer patient which significantly spares the OARs.

SU-E-T-143: Automatic PBS Treatment Planning for Prostate Cancer

Purpose: This study presents a novel idea of using automated planning technique for parallel-opposed prostate plans with pencil beam scanning proton therapy. Methods: A random 5 clinical prostate patients treated with parallel-opposed modulated-scanning proton beam in our proton center were selected in this study. All of the 5 cases were re-planned with in-house developed python scripts integrated into RayStation 4.0 clinical version. This automatic tool was designed to perform patient DICOM data import, external structure contours, CT density assignment, prescription to PTV assignment, beam placement, optimization, and dose calculation. Results: The mean time to generate a complete treatment plan was 1 minute per plan. For the automatically generated plans, 5 out of 5 plans (100%) were deemed clinically acceptable by one experienced radiation oncologist. In addition, the automated plans were overall dosimetrically equivalent to the clinical plans when scored for target coverage, rectum and bladder dose. Mann-Whitney U tests did not show significant dosimetric differences for normal tissue structures between the clinical and automated plans. Conclusion: We have developed a robust and automated method for fully inversed planned paralleled-opposed prostate proton treatment planning. This efficient method can be readily integrated into clinical practice. The tool produces clinically acceptable plans using only manually contoured anatomic structures and simulation CT data set. We anticipate that this tool will not only improve patient access to high-quality proton treatment by simplifying the planning process and will also reduce the effort and cost of incorporating more advanced planning into clinical practice in the near future.

SU‐E‐T‐280: Dose Evaluation in Using CT Density Versus Relative Stopping Power for Pencil Beam Planning and Treating IROC Proton Phantom

Purpose: The purpose of this study is to evaluate any effects of converted CT density variation in treatment planning system (TPS) of spot scanning proton therapy with an IROC proton prostate phantom at our new ProteusOne Proton Therapy Center. Methods: A proton prostate phantom was requested from the Imaging and Radiation Oncology Core Houston (IROC), The University of Texas MD Anderson Cancer Center, Houston, TX, where GAF Chromic films and couples of thermo luminescent dosemeter (TLD) capsules in target and adjacent structures were embedded for imaging and dose monitoring. Various material such as PVC, PBT HI polystyrene as dosimetry inserts and acrylic were within phantom. Relative stopping power (SP) were provided. However our treatment planning system (TPS) doesn’t require SP instead relative density was converted relative to water in TPS. Phantom was irradiated and the results were compared with IROC measurements. The range of relative density was converted from SP into relative density of water as a new assigned material and tested. Results: The summary of TLD measurements of the prostate and femoral heads were well within 2% of the TPS and met the criteria established by IROC. The film at coronal plane was found to be shift in superior-inferior direction due to locking position of cylinder insert was off and was corrected. The converted CT density worked precisely to correlated relative stopping power. Conclusion: The proton prostate phantom provided by IROC is a useful methodology to evaluate our new commissioned proton pencil beam and TPS within certain confidence in proton therapy. The relative stopping power was converted into relative physical density relatively to water and the results were satisfied.

SU-E-T-10: A Clinical Implementation and the Dosimetric Evidence in High Dose Rate Vaginal Multichannel Applicator Brachytherapy

Purpose: The multichannel cylindrical applicator has a distinctive modification of the traditional single channel cylindrical applicator. The novel multichannel applicator has additional peripheral channels that provide more flexibility both in treatment planning process and outcomes. To protect by reducing doses to adjacent organ at risk (OAR) while maintaining target coverage with inverse plan optimization are the goals for such novel Brachytherapy device. Through a series of comparison and analysis of reults in more than forty patients who received HDR Brachytherapy using multichannel vaginal applicator, this procedure has been implemented in our institution. Methods: Multichannel planning was CT image based. The CTV of 5mm vaginal cuff rind with prescribed length was well reconstructed as well as bladder and rectum. At least D95 of CTV coverage is 95% of prescribed dose. Multichannel inverse plan optimization algorithm not only shapes target dose cloud but set dose avoids to OAR’s exclusively. The doses of D2cc, D5cc and D5; volume of V2Gy in OAR’s were selected to compare with single channel results when sole central channel is only possibility. Results: Study demonstrates plan superiorly in OAR’s doe reduction in multi-channel plan. The D2cc of the rectum and bladder were showing a little lower for multichannel vs. single channel. The V2Gy of the rectum was 93.72% vs. 83.79% (p=0.007) for single channel vs. multichannel respectively. Absolute reduced mean dose of D5 by multichannel was 17 cGy (s.d.=6.4) and 44 cGy (s.d.=15.2) in bladder and rectum respectively. Conclusion: The optimization solution in multichannel was to maintain D95 CTV coverage while reducing the dose to OAR’s. Dosimetric advantage in sparing critical organs by using a multichannel applicator in HDR Brachytherapy treatment of the vaginal cuff is so promising and has been implemented clinically.

SU-E-T-27: A Dosimetric Evaluation of Boney Anatomy Versus Fiducial Marker Alignment for the Treatment of Prostate Cancer Using Scanned Beam Proton Therapy.

Purpose: In prostate proton radiotherapy, three fiducial markers are used for patient daily alignment. However fiducial alignment can change beamline heterogeneity in proton therapy. The purpose of this study is to determine the difference in fiducial and boney anatomy alignment for patient treatment. Methods and materials: Prostate cancer patients who received proton treatment were included in this study. 3 fiducial markers were implanted before the initial CT. All the patients were re-CT’d every 2 weeks to check the fiducial marker position reproducibility as well as dosimetric consistence of target coverage. In geometry study, re-CT were fused to the initial CT base on the boney anatomy and the average fiducial marker displacement was measured the centers of the fiducials. Dosimetrically, the initial plan was recalculated directly to re-CT image set based on the boney alignment and fiducial alignment to determine the difference from daily treatment. Prostate coverage and hotspots were evaluated using the dose to 98% of the CTV (D98) and dose to 2% (D2), respectively. Results: The shift from the initial 6 patient CT image sets resulted in an average change in the fiducial location of 5.70 +/− 3 mm. Dosimetric comparison from a single patient revealed that differences from the planned dose resulted from both boney and fiducial alignment. Planned clinical treatment volume coverage resulted in a D98 of 70.44Gy and D2 of 70.84Gy compared to a D98 of 70.13Gy and D2 70.94Gy for boney alignment and a D98 of 70.08Gy and D2 71.18Gy for fiducial alignment respectively. Conclusion: This study demonstrates that with boney anatomy alignment there is little change to CTV coverage and only slightly worse CTV coverage and hotspot production with fiducial alignment. An increase patient cohort and further investigation is necessary to determine the whether boney alignment can help better control dose heterogeneity.

SU-E-T-110: An Investigation On Monitor Unit Threshold and Effects On IMPT Delivery in Proton Pencil Beam Planning System

Purpose: An approved proton pencil beam scanning (PBS) treatment plan might not be able to deliver because of existed extremely low monitor unit per beam spot. A dual hybrid plan with higher efficiency of higher spot monitor unit and the efficacy of less number of energy layers were searched and optimized. The range of monitor unit threshold setting was investigated and the plan quality was evaluated by target dose conformity. Methods: Certain limitations and requirements need to be checks and tested before a nominal proton PBS treatment plan can be delivered. The plan needs to be met the machine characterization, specification in record and verification to deliver the beams. Minimal threshold of monitor unit, e.g. 0.02, per spot was set to filter the low counts and plan was re-computed. Further MU threshold increment was tested in sequence without sacrificing the plan quality. The number of energy layer was also alternated due to elimination of low count layer(s). Results: Minimal MU/spot threshold, spot spacing in each energy layer and total number of energy layer and the MU weighting of beam spots of each beam were evaluated. Plan optimization between increases of the spot MU (efficiency) and less energy layers of delivery (efficacy) was adjusted. 5% weighting limit of total monitor unit per beam was feasible. Scarce spreading of beam spots was not discouraging as long as target dose conformity within 3% criteria. Conclusion: Each spot size is equivalent to the relative dose in the beam delivery system. The energy layer is associated with the depth of the targeting tumor. Our work is crucial to maintain the best possible quality plan. To keep integrity of all intrinsic elements such as spot size, spot number, layer number and the carried weighting of spots in each layer is important in this study.