Proton Pencil Beam Scanning Plans Research Articles

Selecting Optimal Proton Pencil Beam Scanning Plan Parameters to Reduce Dose Discrepancy between Discrete Spot Plan and Continuous Scanning: A Proof-of-Concept Study.

Pencil beam scanning delivered with continuous scanning has several advantages over conventional discrete spot scanning. Such advantages include improved beam delivery efficiency and reduced beam delivery time. However, a move dose is delivered between consecutive spots with continuous scanning, and current treatment planning systems do not take this into account. Therefore, continuous scanning and discrete spot plans have an inherent dose discrepancy. Using the operating parameters of the state-of-the-art particle therapy system, we conducted a proof-of-concept study in which we systematically generated 28 plans for cubic targets with different combinations of plan parameters and simulated the dose discrepancies between continuous scanning and a planned one. A nomograph to guide the selection of plan parameters was developed to reduce the dose discrepancy. The effectiveness of the nomograph was evaluated with two clinical cases (one prostate and one liver). Plans with parameters guided by the nomograph decreased dose discrepancy than those used standard plan parameters. Specifically, the 2%/2 mm gamma passing rate increased from 96.3% to 100% for the prostate case and from 97.8% to 99.7% for the liver case. The CTV DVH root mean square error decreased from 2.2% to 0.2% for the prostate case and from 1.8% to 0.9% for the liver case. The decreased dose discrepancy may allow the relaxing of the delivery constraint for some cases, leading to greater benefits in continuous scanning. Further investigation is warranted.

Treatment Planning of Bulky Tumors Using Pencil Beam Scanning Proton GRID Therapy.

To compare spatially fractionated radiation therapy (GRID) treatment planning techniques using proton pencil-beam-scanning (PBS) and photon therapy. PBS and volumetric modulated arc therapy (VMAT) GRID plans were retrospectively generated for 5 patients with bulky tumors. GRID targets were arranged along the long axis of the gross tumor, spaced 2 and 3 cm apart, and treated with a prescription of 18 Gy. PBS plans used 2- to 3-beam multiple-field optimization with robustness evaluation. Dosimetric parameters including peak-to-edge ratio (PEDR), ratio of dose to 90% of the valley to dose to 10% of the peak VPDR(D90/D10), and volume of normal tissue receiving at least 5 Gy (V5) and 10 Gy (V10) were calculated. The peak-to-valley dose ratio (PVDR), VPDR(D90/D10), and organ-at-risk doses were prospectively assessed in 2 patients undergoing PBS-GRID with pretreatment quality assurance computed tomography (QACT) scans. PBS and VMAT GRID plans were generated for 5 patients with bulky tumors. Gross tumor volume values ranged from 826 to 1468 cm3. Peak-to-edge ratio for PBS was higher than for VMAT for both spacing scenarios (2-cm spacing, P = .02; 3-cm spacing, P = .01). VPDR(D90/D10) for PBS was higher than for VMAT (2-cm spacing, P = .004; 3-cm spacing, P = .002). Normal tissue V5 was lower for PBS than for VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Normal tissue mean dose was lower with PBS than with VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Two patients treated using PBS GRID and assessed with pretreatment QACT scans demonstrated robust PVDR, VPDR(D90/D10), and organs-at-risk doses. The PEDR was significantly higher for PBS than VMAT plans, indicating lower target edge dose. Normal tissue mean dose was significantly lower with PBS than VMAT. PBS GRID may result in lower normal tissue dose compared with VMAT plans, allowing for further dose escalation in patients with bulky disease.

Investigation of beam delivery time for synchrotron-based proton pencil beam scanning system with novel scanning mode

Objective. To investigate synchrotron-based proton pencil beam scanning (PBS) beam delivery time (BDT) using novel continuous scanning mode. Approach. A BDT calculation model was developed for the Hitachi particle therapy system. The model was validated against the measured BDT of 36 representative clinical proton PBS plans with discrete spot scanning (DSS) in the current Hitachi proton therapy system. BDTs were calculated with the next generation using Mayo Clinic Florida system operating parameters for conventional DSS, and novel dose driven continuous scanning (DDCS). BDTs of DDCS with and without Break Spots were investigated. Main results. For DDCS without Break Spots, the use of Stop Ratio to control the transit dose largely reduced the beam intensity and consequently, severely prolonged the BDT. DDCS with Break Spots was able to maintain a sufficiently high beam intensity while controlling transit dose. In DDCS with Break Spots, tradeoffs were made between beam intensity and number of Break Spots. Therefore, BDT decreased with increased beam intensity but reached a plateau for beam intensity larger than 10 MU s−1. Averaging over all clinical plans, BDT was reduced by 10% for DDCS with Break Spots compared to DSS. Significance. DDCS with Break Spots reduced BDT. DDCS has the potential to further reduce BDT under the ideal scenario which requests both stable beam intensity extraction and accurately modelling the transit dose. Further investigation is warranted.

FLASH Radiotherapy Using Single-Energy Proton PBS Transmission Beams for Hypofractionation Liver Cancer: Dose and Dose Rate Quantification.

PurposeThis work aims to study the dose and ultra-high-dose rate characteristics of transmission proton pencil beam scanning (PBS) FLASH radiotherapy (RT) for hypofractionation liver cancer based on the parameters of a commercially available proton system operating under FLASH mode.Methods and MaterialsAn in-house treatment planning software (TPS) was developed to perform intensity-modulated proton therapy (IMPT) FLASH-RT planning. Single-energy transmission proton PBS plans of 4.5 Gy × 15 fractions were optimized for seven consecutive hepatocellular carcinoma patients, using 2 and 5 fields combined with 1) the minimum MU/spot chosen between 100 and 400, and minimum spot time (MST) of 2 ms, and 2) the minimum MU/spot of 100, and MST of 0.5 ms, based upon considerations in target uniformities, OAR dose constraints, and OAR FLASH dose rate coverage. Then, the 3D average dose rate distribution was calculated. The dose metrics for the mean dose of Liver-GTV and other major OARs were characterized to evaluate the dose quality for the different combinations of field numbers and minimum spot times compared to that of conventional IMPT plans. Dose rate quality was evaluated using 40 Gy/s volume coverage (V40Gy/s).ResultsAll plans achieved favorable and comparable target uniformities, and target uniformity improved as the number of fields increased. For OARs, no significant dose differences were observed between plans of different field numbers and the same MST. For plans using shorter MST and the same field numbers, better sparing was generally observed in most OARs and was statistically significant for the chest wall. However, the FLASH dose rate coverage V40Gy/s was increased by 20% for 2-field plans compared to 5-field plans in most OARs with 2-ms MST, which was less evident in the 0.5-ms cases. For 2-field plans, dose metrics and V40Gy/s of select OARs have large variations due to the beam angle selection and variable distances to the targets. The transmission plans generally yielded inferior dosimetric quality to the conventional IMPT plans.ConclusionThis is the first attempt to assess liver FLASH treatment planning and demonstrates that it is challenging for hypofractionation with smaller fractional doses (4.5 Gy/fraction). Using fewer fields can allow higher minimum MU/spot, resulting in higher OAR FLASH dose rate coverages while achieving similar plan quality compared to plans with more fields. Shorter MST can result in better plan quality and comparable or even better FLASH dose rate coverage.

Pencil Beam Scanning Proton GRID Therapy of Bulky Tumors: A Feasibility Study

<h3>Purpose/Objective(s)</h3> To evaluate a novel technique for spatially fractionated (GRID) therapy of bulky tumors using proton pencil beam scanning (PBS) and to compare dosimetry to a photon arc (VMAT) technique. <h3>Materials/Methods</h3> PBS and VMAT GRID plans were generated for five patients with bulky tumors. GRID targets were arranged along the long axis of gross tumor, spaced at 2 cm and 3 cm apart, and treated with a prescription of 15 Gy in a single fraction. PBS plans utilized two-beam multiple-field optimization with robustness evaluation. Dosimetric parameters including peak-to-edge ratio (PEDR), ratio of dose to 90% of the valley to dose to 10% of the peak (VPDR (D90/D10)), and volume of normal tissue receiving at least 5 Gy (V5) and 10 Gy (V10) were calculated. <h3>Results</h3> Gross tumor volume values ranged from 826-1468 cc. PEDR for PBS was higher than for VMAT at 7.2 ± 2.51 versus 2.4 ± 0.61 (2-cm spacing, <b>P</b> = 0.02), and 8.4 ± 2.00 versus 2.8 ± 0.37 (3-cm spacing, <b>P</b> = 0.01), respectively. VPDR (D90/D10) for PBS was lower than for VMAT at 0.034 ± 0.031 versus 0.41 ± 0.12 (2-cm spacing, <b>P</b> = 0.004), and 0.008 ± 0.007 versus 0.33 ± 0.08 (3-cm spacing, <b>P</b> = 0.002), respectively. Normal tissue V5 was lower for PBS than for VMAT at 343.2 cc ± 176.4 cc versus 917.6 cc ± 459.0 cc (2-cm spacing, <b>P</b> = 0.03), and 250.3 cc ± 114.7 cc versus 520.7 cc ± 181.1 cc (3-cm spacing, <b>P</b> = 0.02), respectively. Normal tissue mean dose was lower with PBS than with VMAT at 0.20 Gy ± 0.15 Gy versus 0.92 Gy ± 0.41 Gy (2-cm spacing, <b>P =</b> 0.03), and 0.17 Gy ± 0.11 Gy versus 0.82 Gy ± 0.33 Gy (3-cm spacing, <b>P =</b> 0.02), respectively. <h3>Conclusion</h3> PBS GRID for bulky tumors may result in improved dosimetry when compared to a VMAT approach. This PBS technique may be useful in bulky tumors that have a deep-seated location, complex shape, or adjacent critical organs.

Strategies for Motion Robust Proton Therapy With Pencil Beam Scanning for Esophageal Cancer.

Proton therapy of esophageal cancer is superior to photon radiation therapy in terms of normal tissue sparing. However, respiratory motion and anatomical changes may compromise target dose coverage owing to density changes, geometric misses, and interplay effects. Here we investigate the combined effect on clinical target volume (CTV) coverage and compare proton therapy with intensity modulated radiation therapy (IMRT). This study includes 26 patients with esophageal cancer previously treated with IMRT planned on 4-dimensional computed tomography (4D-CT). For each patient, 7 proton pencil beam scanning (PBS) plans were created with different field configurations and optimization strategies. The effect of respiration was investigated by calculating the phase doses, 4D dose, and 4D dynamic dose (including interplay effects). The effect of anatomical changes was investigated by recalculating all plans on all phases of a 4D-CT surveillance scan. The most robust PBS plans were achieved using 2 posterior beams requiring coverage of planning target volume (PTV) and simultaneously using robust optimization (RO) of CTV (2PAPTVRO), resulting in only 1 patient showing V95%CTV <97% in 1 or more phases of the planning CT. For the least robust PBS plans obtained using lateral+posterior beams and CTV-RO, but not requiring PTV coverage (2LPRO), 10 patients showed underdosage. For IMRT, 2 patients showed underdosage. Interplay effects reduced V95%CTV significantly when delivering only 1 fraction, but the effects generally averaged out after 10 fractions. The effect of interplay was significantly larger for RO-only plans compared with plans optimized with RO combined with PTV coverage. Combining the effect of anatomical changes and respiration on the 4D-CT surveillance scan resulted in V95%CTV <97% for 3 2PAPTVRO, 16 2LPRO, and 8 IMRT patients. PBS using posterior beam angles was more robust to anatomical changes and respiration than IMRT. The effect of respiration was enhanced when anatomical changes were present. Single fraction interplay effects deteriorated the dose distribution but were averaged out after 10 fractions.

Target coverage and cardiopulmonary sparing with the updated ESTRO-ACROP contouring guidelines for postmastectomy radiation therapy after breast reconstruction: a treatment planning study using VMAT and proton PBS techniques

Background The European Society for Therapeutic Radiology and Oncology Advisory Committee in Radiation Oncology Practice (ESTRO-ACROP) recently released new contouring guidelines for postmastectomy radiation therapy (PMRT) after implant-based reconstruction (IBR). As compared to prior ESTRO guidelines, the new guidelines primarily redefined the chest wall (CW) target to exclude the breast prosthesis. In this study, we assessed the impact of these changes on treatment planning and dosimetric outcomes using volumetric-modulated arc therapy (VMAT) and proton pencil-beam scanning (PBS) therapy. Methods We performed a treatment planning study of 10 women with left-sided breast cancer who underwent PMRT after IBR. All target structures were delineated first using standard (ESTRO) breast contouring guidelines and then separately using the new (ESTRO-ACROP) guidelines. Standard organs-at-risk (OARs) and cardiac substructures were contoured. Four sets of plans were generated: (1) VMAT using standard ESTRO contours, (2) VMAT using new ESTRO-ACROP contours, (3) PBS using standard contours, and (4) PBS using new contours. Results VMAT plans using the new ESTRO-ACROP guidelines resulted in modest sparing of the left anterior descending coronary artery (LAD) (mean dose: 6.99 Gy standard ESTRO vs. 6.08 Gy new ESTRO-ACROP, p = 0.010) and ipsilateral lung (V20: 21.66% vs 19.45%, p = 0.017), but similar exposure to the heart (mean dose: 4.6 Gy vs. 4.3 Gy, p = 0.513), with a trend toward higher contralateral lung (V5: 31.0% vs 35.3%, p = 0.331) and CW doses (V5: 31.9% vs 35.4%, p = 0.599). PBS plans using the new guidelines resulted in further sparing of the heart (mean dose: 1.05 Gy(RBE) vs. 0.54 Gy(RBE), p < 0.001), nearly all cardiac substructures (LAD mean dose: 2.01 Gy(RBE) vs. 0.66 Gy(RBE), p < 0.001), and ipsilateral lung (V20: 16.22% vs 6.02%, p < 0.001). Conclusions PMRT after IBR using the new ESTRO-ACROP contouring guidelines with both VMAT and PBS therapy is associated with significant changes in exposure to several cardiopulmonary structures.

Dosimetric Comparison of Volumetric-Modulated Arc Therapy and Proton Therapy for Postmastectomy Radiation Using the New ESTRO-ACROP Breast Reconstruction Contouring Guidelines

In 2019, ESTRO-ACROP released new consensus guidelines for postmastectomy radiation therapy (PMRT) in the setting of breast reconstruction. In particular, the chest wall (CW) target was redefined to exclude the breast prosthesis and underlying tissue. In this study, we assess the impact of these changes on treatment planning and compare the dosimetric outcomes using both volumetric-modulated arc therapy (VMAT) and pencil-beam scanning (PBS) proton therapy. We performed a treatment planning study of 10 women with left sided breast cancer who received PMRT after implant-based breast reconstruction. All target structures and organs-at-risk (OARs) were delineated first using current RTOG breast contouring guidelines and then separately delineated using the updated ESTRO-ACROP consensus guidelines. Four sets of plans were then generated; 1) VMAT using RTOG contours, 2) VMAT using new ESTRO-ACROP contours, 3) PBS using RTOG contours, and 4) PBS using new ESTRO-ACROP contours. All plans prescribed a dose of 50.4 Gy or GyRBE to the CW and 45 Gy or GyRBE to the regional lymphatics, inclusive of the internal mammary nodes. Similar target coverage objectives and normal tissue (OAR) constraints were utilized, applying the concept of ALARA. Dosimetric goals between the RTOG and updated ESTRO-ACROP contoured plans and across treatment modalities were compared. Utilizing the updated ESTRO-ACROP contouring guidelines, VMAT and PBS plans were able to generate equivalent coverage for all targets. The mean CW D95 was 49.9 Gy for VMAT vs. 48.4 GyRBE with PBS, and the mean regional lymphatic D95 was 45.1Gy (VMAT) vs. 42.8 GyRBE (PBS). Despite the more limited CW volume, proton PBS plans still demonstrated superior OAR metrics compared to VMAT: ipsilateral lung V20 of 16.1% (VMAT) vs. 5.3% (PBS), mean heart doses of 4.6 Gy (VMAT) vs. 0.51 GyRBE (PBS), LAD 1cc dose of 7.43 Gy (VMAT) vs. 5.95 GyRBE (PBS). Compared to plans generated with RTOG guidelines, the updated guideline plans were able to achieve equivalent target coverage with similar, but modestly improved sparing of most OARs: ipsilateral lung V20 7.3% (RTOG PBS) vs. 5.3% (ESTRO PBS) and 24% (RTOG VMAT) vs. 20% (ESTRO VMAT), mean heart dose 0.49 GyRBE (RTOG PBS) vs. 0.51 GyRBE (ESTRO PBS) and 4.95 Gy (RTOG VMAT) vs. 4.82 Gy (ESTRO VMAT). Notably, contralateral lung doses with VMAT were higher using the updated ESTRO-ACROP guidelines compared to RTOG guidelines with a mean V10 of 19.6%. PMRT with reconstruction utilizing the updated ESTRO-ACROP consensus contouring guidelines is feasible with both VMAT and PBS proton therapy. Treatment plans utilizing the updated contouring guidelines provided equivalent target coverage compared to conventional guidelines, while modestly further sparing cardiopulmonary structures. Care must be taken with new contouring guidelines when utilizing VMAT to minimize undue contralateral lung exposure.

Automated Monte-Carlo re-calculation of proton therapy plans using Geant4/Gate: implementation and comparison to plan-specific quality assurance measurements.

Software re-calculation of proton pencil beam scanning plans provides a method of verifying treatment planning system (TPS) dose calculations prior to patient treatment. This study describes the implementation of AutoMC, a Geant4 v10.3.3/Gate v8.1 (Gate-RTion v1.0)-based Monte-Carlo (MC) system for automated plan re-calculation, and presents verification results for 153 patients (730 fields) planned within year one of the proton service at The Christie NHS Foundation Trust. A MC beam model for a Varian ProBeam delivery system with four range-shifter options (none, 2 cm, 3 cm, 5 cm) was derived from beam commissioning data and implemented in AutoMC. MC and TPS (Varian Eclipse v13.7) calculations of 730 fields in solid-water were compared to physical plan-specific quality assurance (PSQA) measurements acquired using a PTW Octavius 1500XDR array and PTW 31021 Semiflex 3D ion chamber. TPS and MC showed good agreement with array measurements, evaluated using γ analyses at 3%, 3 mm with a 10% lower dose threshold:>94% of fields calculated by the TPS and >99% of fields calculated by MC had γ ≤ 1 for>95% of measurement points within the plane. TPS and MC also showed good agreement with chamber measurements of absolute dose, with systematic differences of <1.5% for all range-shifter options. Reliable independent verification of the TPS dose calculation is a valuable complement to physical PSQA and may facilitate reduction of the physical PSQA workload alongside a thorough delivery system quality assurance programme. A Gate/Geant4-based MC system is thoroughly validated against an extensive physical PSQA dataset for 730 clinical fields, showing that clinical implementation of MC for PSQA is feasible.

Flat-panel imager energy-dependent proton radiography for a proton pencil-beam scanning system

In proton-based radiotherapy, proton radiography could allow for direct measurement of the water-equivalent path length (WEPL) in tissue, which can then be used to determine relative stopping power (RSP). Additionally, proton radiographs allow for imaging in the beam’s-eye-view. In this work, a proton radiography technique using a flat-panel imager and a pencil-beam scanning (PBS) system is demonstrated in phantom. Proton PBS plans were delivered on a Varian ProBeam system to a flat-panel imager. Each proton plan consisted of energy layers separated by 4.8 MeV, and a field size of 25 cm × 25 cm. All measured data is binned into a layer-by-layer delivery in post processing. To build a calibration curve correlating detector response to WEPL, the plans were delivered to slabs of solid water of increasing thickness. Pixel-by-pixel detector response in the time/energy domain is then fit as a function of WEPL. Tissue equivalent phantoms are imaged for evaluation of WEPL accuracy. A spatial resolution phantom and a head phantom are also imaged. For all experiments, the detector was run with an effective pixel size of 0.4 mm × 0.4 mm. The proposed method reconstructed RSP with mean errors of 2.65%, −0.14%, and 0.61% for lung, soft tissue, and bone, respectively. In a 40 mm thick spatial resolution phantom, a 2 mm deep pinhole with 1 mm diameter can be seen. The accuracy and spatial resolution of the method show that it could be implemented for patient position verification. Further development could lead to patient-specific verification of RSP to be used for treatment guidance.

Assessing Outcomes of Patients Treated With Re-Irradiation Utilizing Proton Pencil-Beam Scanning for Primary or Recurrent Malignancies of the Esophagus and Gastroesophageal Junction.

Re-irradiation (re-RT) for locoregionally recurrent esophageal and gastroesophageal junction (GEJ) cancer and de novo esophageal + GEJ cancer arising in-field after a course of prior radiation poses considerable treatment challenges given the sensitivity of surrounding organs at risk (OARs). Guidelines for treatment of this presentation are not well established. Pencil-beam scanning (PBS) proton therapy has the ability to decrease radiation dose to OARs relative to photon plans. We present the first published series to date of re-RT with PBS for esophageal + GEJ malignancies and hypothesize that re-RT with proton PBS will be feasible and improve the safety profile of re-RT for this cohort of patients. Consecutive esophageal + GEJ cancers treated with PBS re-RT within a single institution were analyzed. Comparative volumetric-modulated arc therapy photon plans weregenerated. A total of 17 patients were included for analysis. At a median follow-up of 11.6 months, 1-year local control was 75.3% and overall survival was 68.9%. There were five (27.8%) grade 3 or higher late toxicities. When matched for clinical target volume coverage, proton PBS plans delivered significantly lower doses to the spinal cord, lungs, liver, and heart (all p < 0.05); five volumetric-modulated arc therapy plans would have been undeliverable on the basis of physician-specified OAR constraints. Re-RT for de novo or recurrent malignancies of the esophagus + GEJ, when delivered with PBS proton therapy, yields high rates of local control with acceptable acute and late toxicities in a high-risk population and decreased radiation dose to OARs relative to comparative photon plans. This is the largest series of proton re-RT for esophageal malignancies and the first that exclusively used PBS.

Minimum-MU and sparse-energy-layer (MMSEL) constrained inverse optimization method for efficiently deliverable PBS plans

The deliverability of proton pencil beam scanning (PBS) plans is subject to the minimum monitor-unit (MU) constraint, while the delivery efficiency depends on the number of proton energy layers. This work develops an inverse optimization method for generating efficiently deliverable PBS plans. The proposed minimum-MU and sparse-energy-layer (MMSEL) constrained inverse optimization method utilizes iterative convex relaxations to handle the nonconvexity from minimum-MU constraint and dose-volume constraints, and regularizes group sparsity of proton spots to minimize the number of energy layers. The tradeoff between plan quality and delivery efficiency (in terms of the number of used energy layers) is controlled by the objective weighting of group sparsity regularization. MMSEL consists of two steps: first minimize for appropriate energy layers, and then with selected energy layers solve for the deliverable PBS plan. The solution algorithm for MMSEL is developed using alternating direction method of multipliers (ADMM). Range and setup uncertainties are modelled by robust optimization. MMSEL was validated using representative prostate, lung, and head-and-neck (HN) cases. The minimum-MU constraint was strictly enforced for all cases, so that all plans were deliverable. The number of energy layers was reduced by MMSEL to 78%, 76%, and 61% for prostate, lung and HN, respectively, while the similar plan quality was achieved. The number of energy layers was reduced by MMSEL to 54%, 57%, and 37% for prostate, lung and HN, respectively, while the plan quality was comprised and acceptable. MMSEL is proposed to strictly enforce minimum-MU constraint and minimize the number of energy layers during inverse optimization for efficiently deliverable PBS plans. In particular, the preliminary results suggest MMSEL potentially enables 25% to 40% reduction of energy layers while maintaining the similar plan quality.

Teaching treatment planning for protons with educational open-source software: experience with FoCa and matRad.

Open‐source, MATLAB‐based treatment planning systems FoCa and matRAD were used in a pilot project for training prospective medical physicists and postgraduate physics students in treatment planning and beam modeling techniques for proton therapy. In the four exercises designed, students learnt how proton pencil beams are modeled and how dose is calculated in three‐dimensional voxelized geometries, how pencil beam scanning plans (PBS) are constructed, the rationale behind the choice of spot spacing in patient plans, and the dosimetric differences between photon IMRT and proton PBS plans. Sixty students of two courses participated in the pilot project, with over 90% of satisfactory rating from student surveys. The pilot experience will certainly be continued.

Efficient Interplay Effect Mitigation for Proton Pencil Beam Scanning by Spot-Adapted Layered Repainting Evenly Spread out Over the Full Breathing Cycle

To develop and implement a practical repainting method for efficient interplay effect mitigation in proton pencil beam scanning (PBS). A new flexible repainting scheme with spot-adapted numbers of repainting evenly spread out over the whole breathing cycle (assumed to be 4 seconds) was developed. Twelve fields from 5 thoracic and upper abdominal PBS plans were delivered 3 times using the new repainting scheme to an ion chamber array on a motion stage. One time was static and 2 used 4-second, 3-cm peak-to-peak sinusoidal motion with delivery started at maximum inhalation and maximum exhalation. For comparison, all dose measurements were repeated with no repainting and with 8 repaintings. For each motion experiment, the 3%/3-mm gamma pass rate was calculated using the motion-convolved static dose as the reference. Simulations were first validated with the experiments and then used to extend the study to 0- to 5-cm motion magnitude, 2- to 6-second motion periods, patient-measured liver tumor motion, and 1- to 6-fraction treatments. The effect of the proposed method was evaluated for the 5 clinical cases using 4-dimensional (4D) dose reconstruction in the planning 4D computed tomography scan. The target homogeneity index, HI = (D2 - D98)/Dmean, of a single-fraction delivery is reported, where D2 and D98 is the dose delivered to 2% and 98% of the target, respectively, and Dmean is the mean dose. The gamma pass rates were 59.6% ± 9.7% with no repainting, 76.5% ± 10.8% with 8 repaintings, and 92.4% ± 3.8% with the new repainting scheme. Simulations reproduced the experimental gamma pass rates with a 1.3% root-mean-square error and demonstrated largely improved gamma pass rates with the new repainting scheme for all investigated motion scenarios. One- and two-fraction deliveries with the new repainting scheme had gamma pass rates similar to those of 3-4 and 6-fraction deliveries with 8 repaintings. The mean HI for the 5 clinical cases was 14.2% with no repainting, 13.7% with 8 repaintings, 12.0% with the new repainting scheme, and 11.6% for the 4D dose without interplay effects. A novel repainting strategy for efficient interplay effect mitigation was proposed, implemented, and shown to outperform conventional repainting in experiments, simulations, and dose reconstructions. This strategy could allow for safe and more optimal clinical delivery of thoracic and abdominal proton PBS and better facilitate hypofractionated and stereotactic treatments.

A study of the beam-specific interplay effect in proton pencil beam scanning delivery in lung cancer

Background: For lung tumors with large motion amplitudes, the use of proton pencil beam scanning (PBS) can produce large dose errors. In this study, we assess under what circumstances PBS can be used to treat lung cancer patients who exhibit large tumor motion, based on the quantification of tumor motion and the dose interplay.Material and methods: PBS plans were optimized on average 4DCT datasets using a beam-specific PTV method for 10 consecutive patients with locally advanced non-small-cell-lung-cancer (NSCLC) treated with proton therapy to 6660/180 cGy. End inhalation (CT0) and end exhalation (CT50) were selected as the two extreme scenarios to acquire the relative stopping power ratio difference (Δrsp) for a respiration cycle. The water equivalent difference (ΔWET) per radiological path was calculated from the surface of patient to the iCTV by integrating the Δrsp of each voxel. The magnitude of motion of voxels within the target follows a quasi-Gaussian distribution. A motion index (MI (>5mm WET)), defined as the percentage of target voxels with an absolute integral ΔWET larger than 5 mm, was adopted as a metric to characterize interplay. To simulate the treatment process, 4D dose was calculated by accumulating the spot dose on the corresponding respiration phase to the reference phase CT50 by deformable image registration based on spot timing and patient breathing phase.Results: The study indicated that the magnitude of target underdose in a single fraction plan is proportional to the MI (p < .001), with larger motion equating to greater dose degradation and standard deviations. The target homogeneity, minimum, maximum and mean dose in the 4D dose accumulations of 37 fractions varied as a function of MI.Conclusions: This study demonstrated that MI can predict the level of dose degradation, which potentially serves as a clinical decision tool to assess whether lung cancer patients are potentially suitable to receive PBS treatment.

Proton pencil beam scanning for mediastinal lymphoma: treatment planning and robustness assessment

ABSTRACTBackground: Modern radiotherapy (RT) for lymphoma is highly personalized. While advanced imaging is largely employed to define limited treatment volumes, the use of proton pencil beam scanning (PBS) for highly conformal lymphoma RT is still in its infancy. Here, we assess the dosimetric benefits and feasibility of PBS for mediastinal lymphoma (ML).Materials and methods: Ten patients were planned using PBS for involved-site RT. The initial plans were calculated on the average four-dimensional computed tomography (4D-CT). PBS plans were compared with 3D conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and proton double scattering (DS). In order to evaluate the feasibility of PBS and the plan robustness against inter- and intra-fractional uncertainties, the 4D dose was calculated on initial and verification CTs. The deviation of planned dose from delivered dose was measured. The same proton beamline was used for all patients, while another beamline with larger spots was employed for patients with large motion perpendicular to the beam.Results: PBS provided the lowest mean lung dose (MLD) and mean heart dose (MHD) for all patients in comparison with 3D-CRT, IMRT, and DS. For eight patients, internal target volume (ITV) D98% was degraded by <3%; and the MLD and MHD deviated by <10% of prescription over the course of treatment when the PBS field was painted twice in each session. For one patient with target motion perpendicular to the beam (>5 mm), the degradation of ITV D98% was 9%, which was effectively mitigated by employing large spots. One patient exhibited large dose degradation due to pericardial effusion, which required replanning across all modalities.Conclusions: This study demonstrates that PBS plans significantly reduce MLD and MHD relative to 3D-CRT, IMRT, and DS and identifies requirements for robust free-breathing ML PBS treatments, showing that PBS plan robustness can be maintained with repainting and/or large spots.

SU‐F‐T‐208: An Efficient Planning Approach to Posterior Fossa Tumor Bed Boosts Using Proton Pencil Beam Scanning in Fixed‐Beam Room

Purpose:A fixed‐beam room could be underutilized in a multi‐room proton center. We investigated the use of proton pencil beam scanning (PBS) on a fixed‐beam as an alternative for posterior fossa tumor bed (PF‐TB) boost treatments which were usually treating on a gantry with uniform scanning.Methods:Five patients were treated with craniospinal irradiation (CSI, 23.4 or 36.0 Gy(RBE)) followed by a PF‐TB boost to 54 Gy(RBE) with proton beams. Three PF‐TB boost plans were generated for each patient: (1) a uniform scanning (US) gantry plan with 4–7 posterior fields shaped with apertures and compensators (2) a PBS plan using bi‐lateral and vertex fields with a 3‐mm planning organ‐at‐risk volume (PRV) expansion around the brainstem and (3) PBS fields using same beam arrangement but replacing the PRV with robust optimization considering a 3‐mm setup uncertainty.Results:A concave 54‐Gy(RBE) isodose line surrounding the brainstem could be achieved using all three techniques. The mean V95% of the PTV was 99.7% (range: 97.6% to 100%) while the V100% of the PTV ranged from 56.3% to 93.1% depending on the involvement of the brainstem with the PTV. The mean doses received by 0.05 cm3 of the brainstem were effectively identical: 54.0 Gy(RBE), 53.4 Gy(RBE) and 53.3 Gy(RBE) for US, PBS optimized with PRV, and PBS optimized with robustness plans respectively. The cochlea mean dose increased by 23% of the prescribed boost dose in average from the bi‐lateral fields used in the PBS plan. Planning time for the PBS plan with PRV was 5–10 times less than the US plan and the robustly optimized PBS plan.Conclusion:We have demonstrated that a fixed‐beam with PBS can deliver a dose distribution comparable to a gantry plan using uniform scanning. Planning time can be reduced substantially using a PRV around the brainstem instead of robust optimization.

TH‐CD‐209‐08: Quantification of the Interplay Effect in Proton Pencil Beam Scanning Treatment of Lung

Purpose:To quantify the dose degradation caused by the interplay effect based on a beam specific motion analysis in proton pencil beam scanning (PBS) treatment of lung tumorsMethods:PBS plans were optimized on average CT using a beam‐specific PTV method for 10 consecutive patients with locally advanced non‐small‐cell‐lung‐cancer (NSCLC) treated with proton therapy to 6660/180 cGy. End inhalation (CT0) and end exhalation (CT50) were selected as the two extreme scenarios to acquire the relative stopping power ratio difference (Δrsp) for a respiration cycle. The water equivalent difference (ΔWET) per radiological path was calculated from the surface of patient to the iCTV by integrating the Δrsp of each voxel. The motion magnitude of each voxel within the target follows a quasi‐Gaussian distribution. A motion index (MI (&gt;5mm WET)), defined as the percentage of target voxels with an absolute integral ΔWET larger than 5 mm, was adopted as a metric to characterize interplay. To simulate the treatment process, 4D dose was calculated by accumulating the spot dose on the corresponding respiration phase to the reference phase CT50 by deformable image registration based on spot timing and patient breathing phase.Results:The study indicated that the magnitude of target underdose in a single fraction plan is proportional to the MI (p&lt;0.001), with larger motion equating to greater dose degradation and standard deviations. The target homogeneity, minimum, maximum and mean dose in the 4D dose accumulations of 37 fractions varied as a function of MI.Conclusion:The MI quantification metric can predict the level of dose degradation in PBS lung cancer treatment, which potentially serves as a clinical decision tool to assess whether patients are suitable to receive PBS treatment.

TU‐H‐BRC‐03: Evaluation of Very High‐Energy Electron (VHEE) Beams in Comparison to VMAT and PBS Treatment Plans

Purpose:The aim of this study was to evaluate the performance of very high‐energy electron (VHEE) beams in comparison to clinically delivered treatment plans generated with volumetric modulated arc therapy (VMAT) and proton pencil beam scanning (PBS) technology.Methods:Three clinical cases were selected (prostate, lung, and pediatric CNS). The VHEE plans were calculated in the Monte Carlo EGSnrc code and pencil beam doses were calculated using the DOSxyznrc MC code for 100 and 200 MeV beams. Treatment plans with VHEE, VMAT, and PBS were optimized in a research version of RayStation using an in house build script in order to minimize operator bias between the different techniques.Results:For the prostate cancer case, the PBS plan showed lower mean organ at risk (OAR) doses compared to the other modalities. An exception was the femoral heads, due to the lateral beam arrangements. The VMAT plan showed lower mean doses to the rectum and the bladder compared to the 100 MeV VHEE plan. The lung cancer case showed minor differences between the three modalities. However, the PBS plan showed a lower contralateral lung dose. The pediatric CNS case showed a better conformity and lower spinal cord dose for the 100 MeV VHEE plan. For all cases, the 200 MeV VHEE plans were found to be similar to or better than the 100 MeV VHEE plans.Conclusion:The present study showed that VHEE plans are similar or superior to VMAT plans with reduced mean OAR dose and increased target conformity for a variety of clinical cases. With increased VHEE energy, better conformity and even higher reductions in mean OAR doses can be achieved.Funding: DoD, Award#:W81XWH‐13‐1‐0165, Weston Havens Foundation, Bio‐X (Stanford University), the Office of the Dean of the Medical School, the Office of the Provost (Stanford University), and the Swedish Childhood Cancer Foundation. BL and PM are founders of TibaRay,Inc. BL and PM have received research grants from Varian and RaySearch Laboratory.

SU‐F‐T‐188: A Robust Treatment Planning Technique for Proton Pencil Beam Scanning Cranial Spinal Irradiation

Purpose:To propose a proton pencil beam scanning (PBS) cranial spinal irradiation (CSI) treatment planning technique robust against patient roll, isocenter offset and proton range uncertainty.Method:Proton PBS plans were created (Eclipse V11) for three previously treated CSI patients to 36 Gy (1.8 Gy/fractions). The target volume was separated into three regions: brain, upper spine and lower spine. One posterior‐anterior (PA) beam was used for each spine region, and two posterior‐oblique beams (15° apart from PA direction, denoted as 2PO_15) for the brain region. For comparison, another plan using one PA beam for the brain target (denoted as 1PA) was created. Using the same optimization objectives, 98% CTV was optimized to receive the prescription dose. To evaluate plan robustness against patient roll, the gantry angle was increased by 3° and dose was recalculated without changing the proton spot weights. On the re‐calculated plan, doses were then calculated using 12 scenarios that are combinations of isocenter shift (±3mm in X, Y, and Z directions) and proton range variation (±3.5%). The worst‐case‐scenario (WCS) brain CTV dosimetric metrics were compared to the nominal plan.Results:For both beam arrangements, the brain field(s) and upper‐spine field overlap in the T2–T5 region depending on patient anatomy. The maximum monitor unit per spot were 48.7%, 47.2%, and 40.0% higher for 1PA plans than 2PO_15 plans for the three patients. The 2PO_15 plans have better dose conformity. At the same level of CTV coverage, the 2PO_15 plans have lower maximum dose and higher minimum dose to the CTV. The 2PO_15 plans also showed lower WCS maximum dose to CTV, while the WCS minimum dose to CTV were comparable between the two techniques.Conclusion:Our method of using two posterior‐oblique beams for brain target provides improved dose conformity and homogeneity, and plan robustness including patient roll.