A novel hybrid 3D dose reconstruction approach for pre-treatment verification of intensity modulated proton therapy plans

Aim. A novel hybrid three-dimensional (3D) dose reconstruction method, based on planar dose measured at a single shallower depth, was developed for use as patient-specific quality assurance (PSQA) of intensity modulated proton therapy (IMPT) plans. The accuracy, robustness and sensitivity of the presented method were validated for multiple IMPT plans of varying complexities. Methods and Materials. An in-house MATLAB program was developed to reconstruct 3D dose distribution from the planar dose (GyRBE) measured at 3 g cm−2 depth in water or solid phantom using a MatriXX PT ion chamber array. The presented method was validated extensively for 11 single-field optimization (SFO) and multi-field optimization (MFO) plans on Proteus Plus. A total of 47 reconstructed planar doses at different depths were compared against the corresponding RayStation treatment planning system (TPS) and MatriXX PT measurement using a gamma passing rate (γ%) evaluated for 3%/3 mm. The robustness of the reconstruction method with respect to depth, energy layers, field dimensions and complexities in the spot intensity map (SIM) were analysed and compared against the standard PSQA. The sensitivity of the reconstruction method was tested for plans with intentional errors. Results. The presented reconstruction method showed excellent agreement (mean γ% > 98%) and robustness with both TPS-calculated and measured dose planes at all depths (2.97-30 g cm−2), energy layers (82.1-225.5 MeV), field dimensions, target volume (17.7-1000 cm3) and SIMs from both SFO and MFO plans. In comparison to the overall mean SD γ% from standard PSQA, the reconstruction method showed reductions in mean γ% within 1% for both standard cubes and clinical plans. The reconstruction method was sensitive enough to detect intentional spot positional errors in a selected energy layer of a plan. Conclusion. The presented hybrid reconstruction method is sufficiently accurate, robust and sensitive to estimate planar dose at any user-defined depth. It simplifies the measurement setup and eliminates multiple depth measurements.