Proton beam dosimetry for radiosurgery: implementation of the ICRU Report 59 at the Harvard Cyclotron Laboratory.

Recent proton dosimetry intercomparisons have demonstrated that the adoption of a common protocol, e.g. ICRU Report 59, can lead to improved consistency in absorbed dose determinations. We compared absorbed dose values, measured in the 160 MeV proton radiosurgery beamline at the Harvard Cyclotron Laboratory, based on ionization chamber methods with those from a Faraday cup technique. The Faraday cup method is based on a proton fluence determination that allows the estimation of absorbed dose with the CEMA approximation, under which the dose is equal to the fluence times the mean mass stopping power. The ionization chamber technique employs an air-kerma calibration coefficient for 60Co radiation and a calculated correction in order to take into account the differences in response to 60Co and proton beam radiations. The absorbed dose to water, based on a diode measurement calibrated with a Faraday cup technique, is approximately 2% higher than was obtained from an ionization chamber measurement. At the Bragg peak depth, the techniques agree to within their respective uncertainties, which are both approximately 4% (1 standard deviation). The ionization chamber technique exhibited superior reproducibility and was adopted in our standard clinical practice for radiosurgery.