Spot Scanning Proton Beam Therapy Research Articles

Relative Biological Effectiveness Values of Spot-scanning Proton Beam Therapy at Shonan Kamakura General Hospital.

This study aimed to confirm the relative biological effectiveness (RBE) values of the proton beam therapy (PBT) system installed in Shonan Kamakura General Hospital. Clonogenic cell-survival assays were performed with a human salivary gland (HSG) cell line, a human tongue squamous-cell carcinoma cell line (SAS), and a human osteosarcoma cell line (MG-63). Cells were irradiated with proton beams and X-rays with different doses (1.8, 3.6, 5.5, and 7.3 Gy for proton beams, and 2, 4, 6, and 8 Gy for X-rays). Proton beam irradiation used spot-scanning methods and three different depths (at the proximal, center, and distal sides of the spread-out Bragg peak). RBE values were obtained from a comparison of the dose that resulted in a surviving fraction of 10% (D10). D10 of proton beams at the proximal, center, and distal sides and X-rays in HSG were 4.71, 4.71, 4.51, and 5.25 Gy, respectively; those in SAS were 5.08, 5.04, 5.01, and 5.59 Gy, respectively; and those in MG-63 were 5.36, 5.42, 5.12, and 6.06 Gy, respectively. The RBE10 values at the proximal, center, and distal sides in HSG were 1.11, 1.11, and 1.16 respectively; those in SAS were 1.10, 1.11, and 1.12, respectively; and those in MG-63 were 1.13, 1.12, and 1.18, respectively. RBE10 values of 1.10-1.18 were confirmed by in vitro experiments using the PBT system. These results are considered acceptable for clinical use in terms of therapeutic efficacy and safety.

Simulation study using the spots deletion technique in spot scanning proton beam therapy for prostate cancers.

The aim of the present study was to investigate the effects on the dose distribution and beam delivery time in spot scanning proton beam therapy (PBT) incorporating the spot deletion technique. A spot scanning plan was created for 30 patients with prostate cancer. The plan was then modified via two processes: Spots with lower weighting depositions were deleted (process A) and spots that were distant from the clinical target volume (CTV) were deleted (process B). The dose distribution to the organs at risk (OAR), the expanded CTV (exCTV), which was defined by a uniform expansion of the CTV by a radius of 5 mm, and the beam delivery time were compared among initial and modified plans. The V50 Gy [relative biological effectiveness (RBE)] to the rectum and bladder, and V60 Gy(RBE) to the urethral bulb, inhomogeneity index (INH) of the exCTV showed a difference (P=1.1x10-14, P=6.4x10-14, P=2.7x10-7, P=3.2x10-17), although only changes by process B were significant. Modified plan by process B showed the V50 Gy(RBE) to the rectum and bladder decreased by -2.4±1.6 and -2.3±1.4%, and the V60 Gy (RBE) to the urethral bulb decreased by -15.9±19.4%. The INH of the exCTV increased by 0.05±0.03%. On the other hand, modification of the initial plan by process A did not affect the dose of the OAR, exCTV or beam delivery time. In spot scanning PBT, modification of the initial radiotherapy plan by systemic deletion of spots distant from the CTV could result in a dose reduction to the OAR.

Quantitative analysis of treatments using real-time image gated spot-scanning with synchrotron-based proton beam therapy system log data.

A synchrotron‐based real‐time image gated spot‐scanning proton beam therapy (RGPT) system with inserted fiducial markers can irradiate a moving tumor with high accuracy. As gated treatments increase the beam delivery time, this study aimed to investigate the frequency of intra‐field adjustments corresponding to the baseline shift or drift and the beam delivery efficiency of a synchrotron‐based RGPT system. Data from 118 patients corresponding to 127 treatment plans and 2810 sessions between October 2016 and March 2019 were collected. We quantitatively analyzed the proton beam delivery time, the difference between the ideal beam delivery time based on a simulated synchrotron magnetic excitation pattern and the actual treatment beam delivery time, frequency corresponding to the baseline shift or drift, and the gating efficiency of the synchrotron‐based RGPT system according to the proton beam delivery machine log data. The mean actual beam delivery time was 7.1 min, and the simulated beam delivery time in an ideal environment with the same treatment plan was 2.9 min. The average difference between the actual and simulated beam delivery time per session was 4.3 min. The average frequency of intra‐field adjustments corresponding to baseline shift or drift and beam delivery efficiency were 21.7% and 61.8%, respectively. Based on our clinical experience with a synchrotron‐based RGPT system, we determined the frequency corresponding to baseline shift or drift and the beam delivery efficiency using the beam delivery machine log data. To maintain treatment accuracy within ± 2.0 mm, intra‐field adjustments corresponding to baseline shift or drift were required in approximately 20% of cases. Further improvements in beam delivery efficiency may be realized by shortening the beam delivery time.

A New Method to Reconstruct in 3D the Emission Position of the Prompt Gamma Rays following Proton Beam Irradiation

A new technique for range verification in proton beam therapy has been developed. It is based on the detection of the prompt γ rays that are emitted naturally during the delivery of the treatment. A spectrometer comprising 16 LaBr3(Ce) detectors in a symmetrical configuration is employed to record the prompt γ rays emitted along the proton path. An algorithm has been developed that takes as inputs the LaBr3(Ce) detector signals and reconstructs the maximum γ-ray intensity peak position, in full 3 dimensions. For a spectrometer radius of 8 cm, which could accommodate a paediatric head and neck case, the prompt γ-ray origin can be determined from the width of the detected peak with a σ of 4.17 mm for a 180 MeV proton beam impinging a water phantom. For spectrometer radii of 15 and 25 cm to accommodate larger volumes this value increases to 5.65 and 6.36 mm. For a 8 cm radius, with a 5 and 10 mm undershoot, the σ is 4.31 and 5.47 mm. These uncertainties are comparable to the range uncertainties incorporated in treatment planning. This work represents the first step towards a new accurate, real-time, 3D range verification device for spot-scanning proton beam therapy.

A new SOBP-formation method by superposing specially shaped Bragg curves formed by a mini-ridge filter for spot scanning in proton beam therapy

PurposeWe propose a new spread-out Bragg peak (SOBP) formation method for low-energy regions of spot-scanning proton therapy in order to reduce the required number of energy layers while maintaining high dose uniformity, while maintaining the distal falloff as sharp as possible. MethodsWe use only one specially shaped mini-ridge filter (MRF) to create new trapezoidal Bragg curves (TBCs) from very sharp pristine Bragg curves (PBCs) of low-energy proton beams. The TBC has three pre-designed dose regions of proximal, flat-top, and distal components. These components are designed to have nearly equal depth lengths and good linearity. Then, the required SOBP is formed by superposing the TBCs with the correct spacing and beam intensity weights. We then compare the performance of the TBC-based SOBPs with those formed by PBCs. ResultsThe dose uniformities of the SOBP formed by the proposed method are kept within the design tolerance, and are equivalent to those of conventional SOBPs. The sharpness of the distal falloff is reasonably kept by the deepest TBC. The required number of energy layers is significantly reduced compared with that of conventional PBC-based SOBP. ConclusionsThe proposed method enables shortening of the irradiation time of spot-scanning proton beam therapy in low-energy regions with a reduced number of energy layers. It can be realized by using only one specially shaped MRF, which can be easily installed at any facility.

Initial Clinical Outcomes of Real-Time-Image Gated Spot-Scanning Proton Beam Therapy for Hepatocellular Carcinomas

Initial Clinical Outcomes of Real-Time-Image Gated Spot-Scanning Proton Beam Therapy for Hepatocellular Carcinomas

An in silico planning study comparing doses and estimated risk of toxicity in 3D-CRT, IMRT and proton beam therapy of patients with thymic tumours

PurposeTo compare the dose distributions produced in patients (pts) treated for thymic tumours with spot-scanning proton beam therapy (PBT) implemented with single-field uniform dose (SFUD), intensity-modulated radiation therapy (IMRT) and three-dimensional conformal photon-beam based radiotherapy (3D-CRT). MethodsTwelve pts, treated with 3D-CRT, were included. Alternative IMRT and SFUD plans were constructed. The IMRT plans were created using a setup with beams incident from 5 to 6 different angles. For the SFUD plans, a field-specific planning target volume (PTV) was created for each patient and a clinical target volume (CTV)-based robust optimization was performed. A robustness evaluation was performed for the CTV for all SFUD plans. A dosimetric evaluation was conducted for the doses to the CTV and organs at risk (OARs) for all plans. The normal tissue complication probability (NTCP), for different endpoints, was calculated using the Lyman-Kutcher-Burman (LKB)-model and compared between plans. ResultsSFUD was associated with significantly lower mean doses to the oesophagus, the heart, the left anterior descending coronary artery (LAD), lungs and breasts compared to 3D-CRT and IMRT. The maximum dose given to the spinal cord was significantly lower with SFUD. The risks for pneumonitis, esophagitis and myelopathy were significantly reduced in the SFUD plans. ConclusionsThe present study showed dosimetric advantages of using scanned-beam PBT for the treatment of thymic tumours, as compared to 3D-CRT and IMRT, especially in regard to lower doses to the oesophagus and lungs. The risk of toxicity was reduced with SFUD.

Analysis of Beam Delivery Times and Dose Rates for the Treatment of Mobile Tumors Using Real Time Image Gated Spot-Scanning Proton Beam Therapy

Analysis of Beam Delivery Times and Dose Rates for the Treatment of Mobile Tumors Using Real Time Image Gated Spot-Scanning Proton Beam Therapy

Analysis of Threshold Doses for Radiation Induced Liver Parenchymal Changes on MRI after Real Time–Image Gated Spot-Scanning Proton Beam Therapy of Hepatocellular Carcinomas

Radiation induced liver parenchymal changes after radiation therapy is depicted as a focal area of low signal intensity (FLSI) around the irradiated liver tumor on images of gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance imaging (MRI). In 2014, we developed a real-time tumor-tracking function with two sets of fluoroscopic X-ray sources in the gantry and put it to use with spot-scanning proton beam therapy for moving tumors, we have called this real-time-image gated spot-scanning proton beam therapy (RGPT). The purpose of this study is to analyze the threshold dose (TD) for FLSI on Gd-EOB-DTPA-enhanced MRI after RGPT for hepatocellular carcinomas (HCC). We retrospectively reviewed patients with HCC treated by RGPT from December 2014 to October 2016 and followed this up with MRI using Gd-EOB-DTPA 3 months after the RGPT in our institution. We identified 10 patients with 11 lesions. Five patients had liver cirrhosis among the 10 patients. Before the RGPT, the median Child-Pugh score was 5 (range: 5-7) and nine patients were evaluated as Child -Pugh class A, with one as class B. The mean gross tumor volume was 33 ± 37 ml. Five patients received 76 Gy (relative biological effectiveness: RBE) in 20 fractions, three 72.6 Gy (RBE) in 22 fractions, and two patients with three lesions 66 Gy (RBE) in 10 fractions. We defined the FLSI as the low signal intensity area around the irradiated liver tumors on images in the hepatobiliary phase of Gd-EOB-DTPA-enhanced MRI 3 months after RGPT. The FLSI and the whole liver 3 months after RGPT were visually delineated. When we calculated the threshold dose (TD) for FLSI using a dose volume histogram, we adopted the non-FLSI liver volume (whole liver volume 3 months after RGPT minus the FLSI volume) to mitigate the impact of the shrinkage of the irradiated tumor and liver parenchyma within the FLSI. To analyze the TD among three treatment regimens, we used the equivalent dose in 2 Gy (RBE) fractions using a linear-quadratic model, assuming an α/β of 3. The mean liver volume before and after RGPT were 1278 ± 218 ml and 1218 ± 221 ml, respectively. The mean FLSI volume and non-FLSI liver volume were 143 ± 108 ml and 1061 ± 244 ml, respectively. The mean TD in 2Gy (RBE) fractions was 36.9 ± 11.0 Gy (RBE). The mean TD for patients with and without liver cirrhosis were 38.2 ± 10.8 Gy (RBE) and 34.9 ± 12.6 Gy (RBE), respectively (p = 0.673). The threshold dose for FLSI after RGPT on Gd-EOB-DTPA-enhanced MRI could be estimated in the RGPT planning for HCC. This would seem to be potentially useful in predicting liver damage before RGPT and prevent serious events in the following therapy.

An Organ Motion and Acute Toxicity Study of Image-Guided Spot-Scanning Proton Beam Therapy With An Internal Fiducial Marker for Pancreatic Cancers.

An Organ Motion and Acute Toxicity Study of Image-Guided Spot-Scanning Proton Beam Therapy With An Internal Fiducial Marker for Pancreatic Cancers.

Analysis of Durations and Dose Rates for Treatment of Moving Liver Tumors Using Real-Time Image Gated Spot Scanning Proton Beam Therapy

Analysis of Durations and Dose Rates for Treatment of Moving Liver Tumors Using Real-Time Image Gated Spot Scanning Proton Beam Therapy

NTCP modeling analysis of acute hematologic toxicity in whole pelvic radiation therapy for gynecologic malignancies – A dosimetric comparison of IMRT and spot-scanning proton therapy (SSPT)

PurposeThis treatment planning study was conducted to determine whether spot scanning proton beam therapy (SSPT) reduces the risk of grade ⩾3 hematologic toxicity (HT3+) compared with intensity modulated radiation therapy (IMRT) for postoperative whole pelvic radiation therapy (WPRT). Methods and materialsThe normal tissue complication probability (NTCP) of the risk of HT3+ was used as an in silico surrogate marker in this analysis. IMRT and SSPT plans were created for 13 gynecologic malignancy patients who had received hysterectomies. The IMRT plans were generated using the 7-fields step and shoot technique. The SSPT plans were generated using anterior-posterior field with single field optimization. Using the relative biological effectives (RBE) value of 1.0 for IMRT and 1.1 for SSPT, the prescribed dose was 45Gy(RBE) in 1.8Gy(RBE) per fractions for 95% of the planning target volume (PTV). The homogeneity index (HI) and the conformity index (CI) of the PTV were also compared. ResultsThe bone marrow (BM) and femoral head doses using SSPT were significantly lower than with IMRT. The NTCP modeling analysis showed that the risk of HT3+ using SSPT was significantly lower than with IMRT (NTCP=0.04±0.01 and 0.19±0.03, p=0.0002, respectively). There were no significant differences in the CI and HI of the PTV between IMRT and SSPT (CI=0.97±0.01 and 0.96±0.02, p=0.3177, and HI=1.24±0.11 and 1.27±0.05, p=0.8473, respectively). ConclusionThe SSPT achieves significant reductions in the dose to BM without compromising target coverage, compared with IMRT. The NTCP value for HT3+ in SSPT was significantly lower than in IMRT.

TU-FG-BRB-12: Real-Time Visualization of Discrete Spot Scanning Proton Therapy Beam for Quality Assurance

Purpose: With the growing adoption of proton beam therapy there is an increasing need for effective and user-friendly tools for performing quality assurance (QA) measurements. The speed and versatility of spot-scanning proton beam (PB) therapy systems present unique challenges for traditional QA tools. To address these challenges a proof-of-concept system was developed to visualize, in real-time, the delivery of individual spots from a spot-scanning PB in order to perform QA measurements. Methods: The PB is directed toward a custom phantom with planar faces coated with a radioluminescent phosphor (Gd2O2s:Tb). As the proton beam passes through the phantom visible light is emitted from the coating and collected by a nearby CMOS camera. The images are processed to determine the locations at which the beam impinges on each face of the phantom. By so doing, the location of each beam can be determined relative to the phantom. The cameras are also used to capture images of the laser alignment system. The phantom contains x-ray fiducials so that it can be easily located with kV imagers. Using this data several quality assurance parameters can be evaluated. Results: The proof-of-concept system was able to visualize discrete PB spots with energies ranging from 70 MeV to 220 MeV. Images were obtained with integration times ranging from 20 to 0.019 milliseconds. If not limited by data transmission, this would correspond to a frame rate of 52,000 fps. Such frame rates enabled visualization of individual spots in real time. Spot locations were found to be highly correlated (R2=0.99) with the nozzle-mounted spot position monitor indicating excellent spot positioning accuracy Conclusion: The system was shown to be capable of imaging individual spots for all clinical beam energies. Future development will focus on extending the image processing software to provide automated results for a variety of QA tests.

Reducing Dose Uncertainty for Spot-Scanning Proton Beam Therapy of Moving Tumors by Optimizing the Spot Delivery Sequence

To develop and validate a novel delivery strategy for reducing the respiratory motion-induced dose uncertainty of spot-scanning proton therapy. The spot delivery sequence was optimized to reduce dose uncertainty. The effectiveness of the delivery sequence optimization was evaluated using measurements and patient simulation. One hundred ninety-one 2-dimensional measurements using different delivery sequences of a single-layer uniform pattern were obtained with a detector array on a 1-dimensional moving platform. Intensity modulated proton therapy plans were generated for 10 lung cancer patients, and dose uncertainties for different delivery sequences were evaluated by simulation. Without delivery sequence optimization, the maximum absolute dose error can be up to 97.2% in a single measurement, whereas the optimized delivery sequence results in a maximum absolute dose error of ≤11.8%. In patient simulation, the optimized delivery sequence reduces the mean of fractional maximum absolute dose error compared with the regular delivery sequence by 3.3% to 10.6% (32.5-68.0% relative reduction) for different patients. Optimizing the delivery sequence can reduce dose uncertainty due to respiratory motion in spot-scanning proton therapy, assuming the 4-dimensional CT is a true representation of the patients' breathing patterns.

SU‐E‐T‐439: Fundamental Verification of Respiratory‐Gated Spot Scanning Proton Beam Therapy

Purpose:The spot‐scanning proton beam irradiation with respiratory gating technique provides quite well dose distribution and requires both dosimetric and geometric verification prior to clinical implementation. The purpose of this study is to evaluate the impact of gating irradiation as a fundamental verification.Methods:We evaluated field width, flatness, symmetry, and penumbra in the gated and non‐gated proton beams. The respiration motion was distinguished into 3 patterns: 10, 20, and 30 mm. We compared these contents between the gated and non‐gated beams. A 200 MeV proton beam from PROBEAT‐III unit (Hitachi Co.Ltd) was used in this study. Respiratory gating irradiation was performed by Quasar phantom (MODUS medical devices) with a combination of dedicated respiratory gating system (ANZAI Medical Corporation). For radiochromic film dosimetry, the calibration curve was created with Gafchromic EBT3 film (Ashland) on FilmQA Pro 2014 (Ashland) as film analysis software.Results:The film was calibrated at the middle of spread out Bragg peak in passive proton beam. The field width, flatness and penumbra in non‐gated proton irradiation with respiratory motion were larger than those of reference beam without respiratory motion: the maximum errors of the field width, flatness and penumbra in respiratory motion of 30 mm were 1.75% and 40.3% and 39.7%, respectively. The errors of flatness and penumbra in gating beam (motion: 30 mm, gating rate: 25%) were 0.0% and 2.91%, respectively. The results of symmetry in all proton beams with gating technique were within 0.6%.Conclusion:The field width, flatness, symmetry and penumbra were improved with the gating technique in proton beam. The spot scanning proton beam with gating technique is feasible for the motioned target.

WE‐EF‐303‐05: Development and Commissioning of Real‐Time Imaging Function for Respiratory‐Gated Spot‐Scanning Proton Beam Therapy

Purpose:To realize real‐time‐image gated proton beam therapy (RGPT) for treating mobile tumors.Methods:The rotating gantry of spot scanning proton beam therapy has been designed to equip two x‐ray fluoroscopy devices that enable real‐time imaging of the internal fiducial markers during respiration. Three‐dimensional position of the fiducial marker located near the tumor can be calculated from the fluoroscopic images obtained from orthogonal directions and therapeutic beam is gated only when the fiducial marker is within the predefined gating window. Image acquisition rate can be selected from discrete value ranging from 0.1 Hz to 30 Hz. In order to confirm the effectiveness of RGPT and apply it clinically, clinical commissioning was conducted. Commissioning tests were categorized to main three parts including geometric accuracy, temporal accuracy and dosimetric evaluation.Results:Developed real‐time imaging function has been installed and its basic performances have been confirmed. In the evaluation of geometric accuracy, coincidence of three‐dimensional treatment room coordinate system and imaging coordinate system was confirmed to be less than 1 mm. Fiducial markers (gold sphere and coil) were able to be tracked in simulated clinical condition using an anthropomorphic chest phantom. In the evaluation of temporal accuracy, latency from image acquisition to gate on/off signal was about 60 msec in typical case. In dosimetric evaluation, treatment beam characteristics including beam irradiation position and dose output were stable in gated irradiation. Homogeneity indices to the mobile target were 0.99 (static), 0.89 (w/o gating, motion is parallel to direction of scan), 0.75 (w/o gating, perpendicular), 0.98 (w/ gating, parallel) and 0.93 (w/ gating, perpendicular). Dose homogeneity to the mobile target can be maintained in RGPT.Conclusion:Real‐time imaging function utilizing x‐ray fluoroscopy has been developed and commissioned successfully in order to realize RGPT.Funding Support: This research was partially supported by Japan Society for the Promotion of Science (JSPS) through the FIRST Program. Conflict of Interest: Prof. Shirato has research fund from Hitachi Ltd, Mitsubishi Heavy Industries Ltd and Shimadzu Corporation.

SU-E-T-266: Development of Evaluation System of Optimal Synchrotron Controlling Parameter for Spot Scanning Proton Therapy with Multiple Gate Irradiations in One Operation Cycle

Purpose: We have developed a gated spot scanning proton beam therapy system with real-time tumor-tracking. This system has the ability of multiple-gated irradiation in a single synchrotron operation cycle controlling the wait-time for consecutive gate signals during a flat-top phase so that the decrease in irradiation efficiency induced by irregular variation of gate signal is reduced. Our previous studies have shown that a 200 ms wait-time is appropriate to increase the average irradiation efficiency, but the optimal wait-time can vary patient by patient and day by day. In this research, we have developed an evaluation system of the optimal wait-time in each irradiation based on the log data of the real-time-image gated proton beam therapy (RGPT) system. Methods: The developed system consists of logger for operation of RGPT system and software for evaluation of optimal wait-time. The logger records timing of gate on/off, timing and the dose of delivered beam spots, beam energy and timing of X-ray irradiation. The evaluation software calculates irradiation time in the case of different wait-time by simulating the multiple-gated irradiation operation using several timing information. Actual data preserved in the log data are used for gate on and off time, spot irradiation time, and time moving to the next spot. Design values are used for the acceleration and deceleration times. We applied this system to a patient treated with the RGPT system. Results: The evaluation system found the optimal wait-time of 390 ms that reduced the irradiation time by about 10 %. The irradiation time with actual wait-time used in treatment was reproduced with accuracy of 0.2 ms. Conclusion: For spot scanning proton therapy system with multiple-gated irradiation in one synchrotron operation cycle, an evaluation system of the optimal wait-time in each irradiation based on log data has been developed. Funding Support: Japan Society for the Promotion of Science (JSPS) through the FIRST Program.

SP-0301: An image-guided spot-scanning proton beam therapy gated to real-time tumor-tracking system

SP-0301: An image-guided spot-scanning proton beam therapy gated to real-time tumor-tracking system

SU-D-BRE-02: Development and Commissioning of A Gated Spot Scanning Proton Beam Therapy System with Real-Time Tumor-Tracking

Purpose: A novel Proton Beam Therapy system has been developed by integrating Real-Time Tumor-Tracking (RTRT) and discrete spot scanning techniques. The system dedicated for spot scanning delivers significant advantages for both clinical and economical points of view. The system has the ability to control dose distribution with spot scanning beams and to gate the beams from the synchrotron to irradiate moving tumors only when the actual positions of them are within the planned position. Methods: The newly designed system consists of a synchrotron, beam transport systems, a compact and rotating gantry system with robotic couch and two orthogonal sets of X-ray fluoroscopes. The fully compact design of the system has been realized by reducing the maximum energy of the beam to 220MeV, corresponding to 30g/cm2 range and the number of circulating protons per synchrotron operation cycle, due to higher beam utilization efficiency in spot scanning. To improve the irradiation efficiency in the integration of RTRT and spot scanning, a new control system has been developed to enable multiple gated irradiation per operation cycle according to the gating signals. After the completion of the equipment installation, beam tests and commissioning has been successfully performed. Results: The basic performances and beam characteristics through the synchrotron accelerator to iso-center have been confirmed and the performance test of the irradiation nozzle and whole system has been appropriately completed. CBCT image has been checked and sufficient quality was obtained. RTRT system has been demonstrated and realized accurate dose distributions for moving targets. Conclusion: The gated spot scanning Proton Beam Therapy system with Real-Time Tumor-Tracking has been developed, successfully installed and tested. The new system enables us to deliver higher dose to the moving target tumors while sparing surrounding normal tissues and to realize the compact design of the system and facility by maximizing the efficiency of proton beam utilization. This research is granted by the Japan Society for the Promotion of Science(JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology(FIRST Program)”, initiated by the Council for Science and Technology Policy(CSTP).

SU-E-T-133: Dosimetric Impact of Scan Orientation Relative to Target Motion During Spot Scanning Proton Therapy

Purpose: This study seeks to evaluate the dosimetric effects of intra-fraction motion during spot scanning proton beam therapy as a function of beam-scan orientation and target motion amplitude. Method: Multiple 4DCT scans were collected of a dynamic anthropomorphic phantom mimicking respiration amplitudes of 0 (static), 0.5, 1.0, and 1.5 cm. A spot-scanning treatment plan was developed on the maximum intensity projection image set, using an inverse-planning approach. Dynamic phantom motion was continuous throughout treatment plan delivery.The target nodule was designed to accommodate film and thermoluminescent dosimeters (TLD). Film and TLDs were uniquely labeled by location within the target. The phantom was localized on the treatment table using the clinically available orthogonal kV on-board imaging device. Film inserts provided data for dose uniformity; TLDs provided a 3% precision estimate of absolute dose. An inhouse script was developed to modify the delivery order of the beam spots, to orient the scanning direction parallel or perpendicular to target motion.TLD detector characterization and analysis was performed by the Imaging and Radiation Oncology Core group (IROC)-Houston. Film inserts, exhibiting a spatial resolution of 1mm, were analyzed to determine dose homogeneity within the radiation target. Results: Parallel scanning and target motions exhibited reduced target dose heterogeneity, relative to perpendicular scanning orientation. The average percent deviation in absolute dose for the motion deliveries relative to the static delivery was 4.9±1.1% for parallel scanning, and 11.7±3.5% (p<<0.05) for perpendicularly oriented scanning. Individual delivery dose deviations were not necessarily correlated to amplitude of motion for either scan orientation. Conclusions: Results demonstrate a quantifiable difference in dose heterogeneity as a function of scan orientation, more so than target amplitude. Comparison to the analyzed planar dose of a single field hint that multiple-field delivery alters intra-fraction beam-target motion synchronization and may mitigate heterogeneity, though further study is warranted.