Microdosimetric studies on the Orsay proton synchrocyclotron at 73 and 200 MeV

A planar microdosemeter has been developed for making measurements on charged particle radiotherapy beams. Initially used for making measurements on the 62 MeV proton therapy beam at Clatterbridge, the detector has now been used on both the 73 MeV and 200 MeV proton beams at Orsay. Measurements on this facility were complicated by the fact that it is a synchrocyclotron, with 440 Hz modulation of the primary MHz-pulsed beam. As a result, extreme care had to be exercised to prevent recording multiple events. Analysis of the microdosimetric spectra shows that for both the 73 and 200 MeV beams there is a significant increase in the dose averaged mean lineal energy, ybarD, towards the end of the proton range. This will be associated with a rise in RBE. The implications of this increase are discussed. (author)