A study of ion chamber response compared with water calorimetry in a clinical 85 MEV proton beam

As clinical proton therapy knew an increasing importance during the last years, the demand for accurate dosimetry of proton beams enhanced. Since the reference dosimetry quantity for proton dosimetry is absorbed dose and the reference material recently became water, a water calorimeter is very important as reference dosimetry instrument for proton beams. Using the water calorimeter constructed in Gent, we compared the absorbed dose based on it with ion chamber dosimetry in the 85 MeV proton beam of Louvain-la-Neuve. To first asses a potential impact of the high LET components in the modulated proton beam on the heat defect in water, a relative response study of the water calorimeter was performed using 3 different chemical systems: pure water, H2 saturated water and a 1 mM formate solution saturated with O2. By direct comparison to model calculations of the heat defect for these chemical systems, we were able to understand the measured calorimeter response as a function of accumulated dose. The absorbed dose resulting from this study was then compared with ionisation chamber dosimetry applying the ECHED protocol. The ratio of the calorimetric to the ionometric determined dose amounted to 0.974 ± 0.009. Measurements performed with ten ionisation chambers of five different types, yielded doses deviating no more than 1.1% from each other. They also showed a small influence of ion chamber wall thickness on the response, indicating that next to the contribution of the (Wair/e)p value for protons prescribed in the ECHED protocol, a small part of the above-mentioned discrepancy between the two dosimetry methods could possibly be attributed to chamber dependent parameters