Small Field Proton Dosimetry and Experimental Sfrt Commissioning Measurements for Clinical Pbs Proton Sfrt Deliveries
Abstract
Purpose
In this work, we experimentally commissioned proton spatially-fractionated-radiotherapy (pSFRT) clinical treatments on a compact proton synchrocyclotron. This involved an initial assessment of proton small-field dosimetry calculation accuracies with comprehensive experimental measurements, followed by 2D measurements of mock pSFRT plans with and without simulated delivery errors and finally, 2D measurements of five pSFRT clinical plans.
Methods
Proton small-field (0.5×0.5cm2) measurements were comprehensively performed at maximum (226MeV) proton energy using small-field detectors. Mock proton pSFRT plans were created from multiple uniformly arranged 0.5×0.5cm2 fields spaced at 3.4cm center-to-center (c-t-c) over a 20×20cm2 total field size. 2D measurements were performed at five depths (2-30cm solid water) over three spot spacings (0.1-0.5cm) using MatriXX and Lynx. pSFRT delivery errors such as positional, valley and peak dose, and energy (range) errors were simulated and measured. Lynx detector response was cross calibrated using empirical polynomial fitting to convert relative counts to 2D absolute doses and five pSFRT clinical plans were measured. Local and global 2D gamma comparisons at 2%/2mm criteria were performed with 2% low-dose threshold.
Results
All small-field detectors responded within ±2.5% of treatment-planning-system (TPS) calculations with 0.5×0.5cm2 fields having a 1.4±0.9% (95% C.I.) statistically significant (p-value: 0.00137, t-statistic: 3.49) increase vs TPS calculations. Lynx 2D gamma analyses could detect ±0.2cm spot positional errors, ±5% valley dose errors, proton energy errors of -5MeV but not peak dose errors. Lynx absolute dose cross-calibration was performed using a three-parameter polynomial fitting. All clinical pSFRT gamma scores at 2%/2mm were ≥95% (99.7% C.I.).
Conclusion
Proton small-field dose measurements were highly accurate. Lynx is preferred over MatriXX for pre-clinical pSFRT measurements due to their enhanced error-detection sensitivities. An experimental protocol enabling absolute 2D dose measurements on Lynx was proposed. Clinical pSFRT measurements showed gamma scores at 2%/2mm being ≥95% for all patient pSFRT deliveries.