Utilize empirical models of measured relative dose output factor (rDOF) and transverse penumbra (TP) to evaluate dosimetric uncertainties of in-air spot modelling for spot-scanning carbon-ion and proton radiotherapy

Abstract

ABSTRACT To evaluate the dosimetric uncertainties of symmetric Gaussian modelled in-air spot size/shape in our Syngo treatment planning system, we performed measurements of rDOF and TP width at a shadow depth of 2.7 mm. They are related to the effects of low dose halo and asymmetric spot shape. Empirically, a two-component Gaussian analytical model predicted the trend of rDOF and TP as a function of field size. From the result, the in-air spot size was described by a single Gaussian function for carbon-ion and high energy proton. The weights and widths for the second component of double Gaussian increased significantly as the beam energy decreases for protons. The TP fitting agreed with the rDOF fit for the scanned field (SF) of larger than 78 mm. For proton, the TP for SF of 12.0 mm for energies of 153.36 MeV/u and197.23 MeV/u, SF of 60.0 mm for energy of 48.08 MeV/u were fit better by single Gaussian component. The width of TP showed no field size dependence for each energy. Obtained Gaussian width by fitting TP was smaller than by fitting rDOF for carbon-ion. The empirical model allowed us to investigate the effects of beam emittance and scattering. They could provide reference values for clinical use and quality assurance.