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 p hantom. A depth scaling factor (C-pl) was determined from the ratio of dept hs 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 (R-80(wat)/R-80(med)) and (R -50(wat)/R-50(med)), or (R-100(wat)/R-100(med)) for 78 MeV unmodulated prot on beams. The depth scaling factor for this commercially available solid wa ter phantom is 1.023. An effective point of measurement for a cylindrical i onization chamber was found to be slightly greater than the 50% of the cavi ty radius proposed by the AAPM-TG25 dosimetry protocol for electron beams a nd amounts to 62.5% of the cavity radius of cylindrical ionization chambers . The ion collection efficiency, P-ion, and the polarity correction factor, P-pol, for all the ionization chambers studied are within 1% and 0.4% of u nity, respectively. Absolute doses measured with a parallel plate ionizatio n chamber in water and in the solid water phantom show that the doses measu red in the solid water phantom are 1.4% +/- 0.5% lower than in water. The d ose rate dependent response of the beam line monitor chamber was also inves tigated. Agreement between all the chambers was within 1.5% at the dose rat es studied but the results showed that all four ionization chambers are les s dose rate dependent than the monitor chamber. (C) 2000 American Associati on of Physicists in Medicine. [S0094-2405(00)00612-X].