Experimental determination of the absorbed dose to water in a scanned proton beam using a water calorimeter and an ionization chamber

The absorbed dose to water is the reference physical quantity for the energy absorbed in tissue when exposed to beams of ionizing radiation in radiotherapy. The SI unit of absorbed dose to water is the gray (Gy = 1 J/kg). Ionization chambers are used as the dosimeters of choice in the clinical environment because they show a high reproducibility and are easy to use. However, ionization chambers have to be calibrated in order to convert the measured electrical charge into absorbed dose to water. In addition, protocols require these conversion factors to be SI traceable to a primary standard of absorbed dose to water. We present experimental results where the ionization chamber used for the dosimetry for the scanned proton beam facility at PSI is compared with the direct determination of absorbed dose to water from the METAS primary standard water calorimeter. The agreement of 3.2% of the dose values measured by the two techniques are within their respective statistical uncertainties.