Depth ionization curves for an unmodulated proton beam measured with different ionization chambers.

Differences in depth dose curves for a 78 MeV unmodulated proton beam were measured with four commercially available ionization chambers. Measurements were performed both in water and in a commercially available solid water phantom. A depth scaling factor (Cpl) was determined from the ratio of depths distal to the Bragg peak where the dose is reduced to 80% of the maximum dose in water and in the solid water phantom. This scaling factor provides good agreement between the ionization curves at all depths in water and in this solid water phantom. There is no significant difference in the value of the depth scaling factor between the ratios (R80wat/R80med) and (R50wat/R50med), or (R100wat/R100med) for 78 MeV unmodulated proton beams. The depth scaling factor for this commercially available solid water phantom is 1.023. An effective point of measurement for a cylindrical ionization chamber was found to be slightly greater than the 50% of the cavity radius proposed by the AAPM-TG25 dosimetry protocol for electron beams and amounts to 62.5% of the cavity radius of cylindrical ionization chambers. The ion collection efficiency, Pion, and the polarity correction factor, Ppol, for all the ionization chambers studied are within 1% and 0.4% of unity, respectively. Absolute doses measured with a parallel plate ionization chamber in water and in the solid water phantom show that the doses measured in the solid water phantom are 1.4% +/- 0.5% lower than in water. The dose rate dependent response of the beam line monitor chamber was also investigated. Agreement between all the chambers was within 1.5% at the dose rates studied but the results showed that all four ionization chambers are less dose rate dependent than the monitor chamber.