Pencil-beam scanning (PBS) proton therapy (PT), particularly intensity modulated PT, represents the latest advanced PT technology for treating cancers, including thoracic malignancies. On the basis of virtual clinical studies, PBS-PT appears to have great potential in its ability to tightly tailor the dose to the target while sparing critical structures, thereby reducing treatment-related toxicities, particularly for tumors in areas with complicated anatomy. However, implementing PBS-PT for moving targets has several additional technical challenges compared with intensity modulated photon radiation therapy or passive scattering PT. Four-dimensional computed tomography-based motion management and robust optimization and evaluation are crucial for minimizing uncertainties associated with beam range and organ motion. Rigorous quality assurance is required to validate dose delivery both before and during the course of treatment. Active motion management (eg, breath hold), beam gating, rescanning, tracking, or adaptive planning may be needed for cases involving significant motion or changes in motion or anatomy over the course of treatment.

The purpose of this study was to assess the feasibility of proton pencil beam scanning (PBS) for the treatment of mediastinal lymphoma. A group of 7 patients of varying tumor size (100-800 cc) were planned using a PBS anterior field. We investigated 17 fractions of 1.8 Gy(RBE) to deliver 30.6 Gy(RBE) to the internal target volume (ITV). Spots with σ ranging from 4 mm to 8 mm were used for all patients, while larger spots (σ = 6-16 mm) were employed for patients with motion perpendicular to the beam (⩾5 mm), based on initial 4-dimensional computed tomography (4D CT) motion evaluation. We considered volumetric repainting such that the same field would be delivered twice in each fraction. The ratio of extreme inhalation amplitude and regular tidal inhalation amplitude (free-breathing variability) was quantified as an indicator of potential irregular breathing during the scanning. Four-dimensional dose was calculated on the 4D CT scans based on the respiratory trace and beam delivery sequence, implemented by partitioning the spots into separate plans on each 4D CT phase. Four starting phases (end of inhalation, end of exhalation, middle of inhalation and middle of exhalation) were sampled for each painting and 4 energy switching times (0.5 s, 1 s, 3 s and 5 s) were tested, which resulted in 896 dose distributions for the analyzed cohort. Plan robustness was measured for the target and critical structures in terms of the percent difference between 'delivered' dose (4D-evaluated) and planned dose (calculated on average CT). It was found that none of the patients exhibited highly variable or chaotic breathing patterns. For all patients, the ITV D98% was degraded by <2% (standard deviations ∼ 0.1%) when averaged over the whole treatment course. For six out of seven patients, the average degradation of ITV D98% per fraction was within 5% . For one patient with motion perpendicular to the beam (⩾5 mm), the degradation of ITV D98% per fraction was up to 15%, which was mitigated to 2% by employing larger spots and repainting. Deviation of mean lung dose was at most 0.2 Gy(RBE) (less than 1% of prescribed dose, 30.6 Gy(RBE)), while the deviation of heart maximum dose and cord maximum dose could exceed 5% of the prescribed dose. No significant difference in either target coverage or normal tissue dose was observed for different energy switching times compared via two-sided Wilcoxon signed-rank tests (p < 0.05). This feasibility study demonstrates that, for mediastinal lymphoma, the impact of the interplay effect on the PBS plan robustness is minimal when volumetric repainting and/or larger spots are employed.

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Purpose: We have investigated the potential use of cone beam CT (CBCT) for beam range verifications in proton therapy treatment, in addition to its primary role in geometric targeting. Specifically, we studied the intrinsic imaging variability of a CBCT and its effect on the water equivalent path length (WEPL) calculations, in the context of daily beam range verification/correction required for a recently proposed method of treating prostate using anterior fields. The current approach uses only lateral fields due to the lack of precise range control in patient. Materials and Methods: An anthropomorphic pelvic phantom was scanned using CBCT, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then with a random shift of one centimeter in lateral directions. The Xio treatment planning system was used to perform the analysis. The average Hounsfield unit (HU) numbers for the water column in the rectal balloon was used to perform a linear calibration of the stopping power ratio, independently for each scan, as supported by the planning system. A number of WEPL values vertically from the anterior skin surface to the anterior surface of the water balloon were calculated on slices covering the region of the prostate, in relevance to a prostate treatment using an anterior field. Results: The HU number in the water column varied significantly even within the same CBCT. The average value also varied from day to day for up to 20 units. However, when these average values are used to calibrate the stopping power ratio, the variations in WEPL values along the anterior beam path are mostly within 2 mm. Conclusions: In‐room CBCT can be used in proton therapy to make online verification of protons range in patients with 2mm accuracy.

Purpose The purpose of this study is to determine the effect of headscatter for IMRT fields. Methods and material In air measurements were made for two types of fields. For the first type of fields, a series of offset fields with field sizes ranging between 10 and 21.5cm were used. The offset changed between 0 and 10cm depending on the field size. The detector was always placed on the central axis (CAX). For the second type of fields, headscatter factors were measured for a series of 10×10cm2 fields composed of slits 0.3, 0.4, 0.6, 0.8 and 1.0cm in width. In‐air output ratio, Sc, for a series of clinical IMRT fields was also measured. Sc is defined as dose per MU measured in a water‐equivalent miniphantom between IMRT field and a 10×10 cm2 open field. The measurements are compared with calculation using a two‐source headscatter model1. Results Sc on CAX for the same open beam with different offset changed by up to 4% for the Siemens accelerators. For stop‐and‐shoot method, Sc for 10×10cm2 fields composed of slit fields of different widths changes with the slit width to within 8% and 6.4% for 6 and 15MV, respectively. The 8% uncertainty is completely due to delivery error and does not seem to correlate with the slit width. The two‐source model predicts Sc for all cases including IMRT fields to within 1%. Conclusion In‐air output ratio changes with field shaping by up to 4%, even when the point of measurement is within the radiation field. Thus, it is important to model the headscatter in order to predict Sc for IMRT fields. Our two‐source model can accurately predict the headscatter for points within radiation field.