Experimental dosimetric validation of an open-source treatment planning system for IMPT dose delivery at a horizontal PBS research beamline

Objectives: This study investigates the ability of the open-source treatment planning system (TPS) matRad to generate accurate intensity-modulated proton therapy (IMPT) plans for a real-world horizontal proton pencil beam scanning (PBS) research beamline. Initial validation for IMPT dose delivery is addressed by comparing simulation data with measurements.

Materials and Methods: The PBS delivery was modelled with Monte Carlo (MC) simulations using TOPAS version 3.7. First, base data measurements were performed to develop a proton beam model (BM) for the OncoRay horizontal PBS research beamline. Data include spot sizes in air, depth-dose profiles and a monitor unit (MU) calibration. BM data was then integrated into matRad to build base-data for IMPT treatment planning.
For initial validation of dose delivery, simple 3D box plans were generated at three depths (target beam entrance at 10, 15 and 20cm) covering energies from 115 to 195MeV. Absolute and relative doses were measured using a Semiflex 31013 ionization chamber (PTW Dosimetry) and a scintillation detector (Lynx®, IBA Dosimetry), respectively.

Results: Simulations and dose measurements showed good agreement of relative dose profiles at depths of 14.75, 18.05 and 24.05cm for field sizes of 5 x 5, 10 x 10 and 15 x 15cm2. The overall absolute difference of measured and planned target doses was ~1% (see Table 1).

Conclusions: Accurate MC-based BM was calculated and integrated into the TPS toolkit matRad. Absolute and relative measurements were in agreement with planned IMPT data. Full validation will be completed next months, including absolute and relative dosimetry of patient plans.