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

. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to waterin carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factor. In this study, thevalues 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 factorof each chamber was also determined through MC simulations..for each chamber was calculated using MC code Geant4. The simulatedratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba,ratios were obtained fromvalues ofCo, 290- and 400 MeV ucarbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations,(the product of the water-to-air stopping power ratio and) was acquired fromand the absorbed dose to air calculated in the sensitive volume of each chamber.values were then calculated from the simulatedand the literature-extractedand compared with previous publications.. The calculatedratios in the subjected chambers to the reference chamber agreed well with the measuredratios. Theuncertainty was reduced from the current recommendation of approximately 3% to 1.7%. Thevalues were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers.. Thevalues of carbon-ion beams were accurately calculated in MC simulations and theratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constantof unity in carbon-ion beams, to recommendations that consider chamber-induced differences.