Ferrous sulphate gel dosimetry and MRI for proton beam dose measurements

Ferrous sulphate gel dosimetry has the potential for measurement of absorbe d dose distributions in proton therapy. The chemical properties of the gel are altered according to the radiation dose and these changes can be evalua ted in three dimensions using MRI. The purpose of this work was to investig ate the properties of a ferrous gel used a with clinical proton beams. The gel was irradiated with both monoenergetic and range-modulated proton b eams. It was then evaluated using MRI. The depth dose by means of the 1/T-1 distribution was studied and compared with data from a plane-parallel plat e ionization chamber. 1/T-1 was shown to be proportional to the dose at a mean proton energy of a pproximately 90 MeV. The dose response was no different from that obtained using photon beams. However, on normalization at the entrance, the relative 1/T-1 at the Bragg peak was 15-20% lower than the corresponding ionization chamber data for the monoenergetic proton beam. Better agreement was found for the modulated beam, but with significant differences close to the dist al edge of the 1/T-1 distribution, The change in sensitivity with depth was explained by means of a linear energy transfer dependence. This property w as further studied using Monte Carlo methods.