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

Abstract Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to water D w in carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factor k Q. In this study, the k Q values 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 factor P of each chamber was also determined through MC simulations. Approach. k Q for each chamber was calculated using MC code Geant4. The simulated k Q ratios 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 Q ratios were obtained from D w values of 60Co, 290- and 400 MeV u−1 carbon-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 and P) was acquired from D w and the absorbed dose to air calculated in the sensitive volume of each chamber. k Q values were then calculated from the simulated f Q and the literature-extracted W air and compared with previous publications. Main results. The calculated k Q ratios in the subjected chambers to the reference chamber agreed well with the measured k Q ratios. The k Q uncertainty was reduced from the current recommendation of approximately 3% to 1.7%. The P values were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers. Significance. The k Q values of carbon-ion beams were accurately calculated in MC simulations and the k Q ratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constant P of unity in carbon-ion beams, to recommendations that consider chamber-induced differences.