Microdosimetric characterisation of a therapeutic proton beam used for conjunctival melanoma treatments

In this study we have investigated the radiation quality of proton beams used to treat conjunctival melanomas at the Biomedical Cyclotron in Nice. To quantify radiation quality we have used a mini tissue-equivalent proportional counter (TEPC). Microdosimetric spectra have been measured at different depths and lateral positions in a Plexiglas eye phantom. A weighting function, which was derived from evaluations of the early effects on the mouse intestine, was applied to the spectra to obtain the microdosimetric assessment of relative biological effectiveness (RBEμ). Data show that RBEμ varies significantly in the eye phantom, from 1.1 to 1.7. However, within the conjunctiva the RBE-weighted dose varies in a similar fashion to the absorbed, although it is up to 20% higher than the corresponding absorbed dose.