Monte Carlo-calculated beam quality and perturbation correction factors validated against experiments for Farmer and Markus type ionization chambers in therapeutic carbon-ion beams

Abstract

Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to waterD_win carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factork_Q. In this study, thek_Qvalues in four cylindrical chambers and two plane-parallel chambers were calculated using Monte Carlo (MC) simulations in the plateau region. The chamber-specific perturbation correction factorPof each chamber was also determined through MC simulations.Approach.k_Qfor each chamber was calculated using MC code Geant4. The simulatedk_Qratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba,k_Qratios were obtained fromD_wvalues of⁶⁰Co, 290- and 400 MeV u^-1carbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations,f_Q(the product of the water-to-air stopping power ratio andP) was acquired fromD_wand the absorbed dose to air calculated in the sensitive volume of each chamber.k_Qvalues were then calculated from the simulatedf_Qand the literature-extractedW_airand compared with previous publications.Main results. The calculatedk_Qratios in the subjected chambers to the reference chamber agreed well with the measuredk_Qratios. Thek_Quncertainty was reduced from the current recommendation of approximately 3% to 1.7%. ThePvalues were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers.Significance. Thek_Qvalues of carbon-ion beams were accurately calculated in MC simulations and thek_Qratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constantPof unity in carbon-ion beams, to recommendations that consider chamber-induced differences.

Keywords: Monte Carlo simulation; beam quality correction factor; carbon-ion therapy; ionization chamber; perturbation correction factor.