Purpose: We present on the validation of small-collimated field dose calculations and delivery to enable the transition of a clinical proton radiosurgery (PSRS) program from a passive scattering (PS) to pencil beam scanning (PBS) delivery technique.
Methods: 250 PSRS PS fields treated in the previous two-years at our institution were used to establish base parameters including range, modulation, aperture diameter and field aspect ratio. Previous Monte Carlo simulations of the treatment planning system’s (TPS) small field halo algorithm and limited measurements paved the way to a more comprehensive validation for PSRS. Micro-diamond detectors were used to measure central axis depth dose profiles of PBS fields satisfying parameter combinations described above. Output measurements were repeated in a solid water phantom to establish equivalence when applying a solid water to water ratio to account for differences in scatter within the mediums. The solid water setup is intended for use for subsequent patient specific QA. Film were calibrated for doses between 50-1,300 cGyRBE. An alignment template was incorporated into a phantom to ensure setup reproducibility. The phantom allows the micro-diamond detector and films measurements to be performed simultaneously. Dose profiles were assessed using a hybrid 2/3D gamma-analysis.
Results: In-water dose measurement agreement was -1.7 ± 2.9 and 2.5 ± 5.1 %, at isocenter and 10 mm depth, respectively. Distal and proximal SOBP 90% depth agreement was 0.10 ± 0.66 and 0.74 ± 0.48 mm, respectively. PASS rates for isocentric profile gamma-analysis within 2% global dose, 1 mm distance to agreement and 10% threshold was 97.2 ± 4.8 %.
Conclusion: Measurements performed for small PBS collimated fields with a variety of range shifters and delivery parameters confirms the TPS can properly model small fields used for proton PSRS.
Not Applicable / None Entered.