MONTE-CARLO STUDY OF FLUENCE PERTURBATION EFFECTS ON CAVITY DOSE-RESPONSE IN CLINICAL PROTON-BEAMS

Current protocols for clinical proton beam dosimetry have not implemen ted any chamber-dependent correction factors for absorbed dose determi nation. The present work initiates a Monte Carlo study of these factor s with emphasis on proton fluence perturbation effects and preliminary calculations of perturbation effects from secondary electrons. The pr oton Monte Carlo code PTRAN was modified to allow simulation of proton transport in non-homogeneous geometries of both unmodulated and modul ated beams. The dose to water derived from the dose calculated in an a ir cavity agrees well with results from analytical calculations assumi ng a displacement of the point of measurement. For unmodulated beams s mall differences, limited to 0.8%, could be partially attributed to pr oton multiple scattering. Effects of replacing water around the cavity with wall material are explained by the introduction of a water-equiv alent wall thickness. For modulated beams no significant perturbation effects arise. Secondary electron spectra are calculated analytically. Preliminary electron transport calculations with EGS4 show that wall perturbations of the order of 1% could result. Perturbation effects ca used by the energy transport of secondary particles from inelastic nuc lear interactions have not been studied here. Inclusion of inelastic n uclear energy transfers in the cavity dose, assuming total local absor ption, indicate that separate scaling of this contribution with the ra tio of total inelastic nuclear cross sections could be important.