gms | German Medical Science

The 73 MeV proton beam line dedicated to ophthalmologic treatments at the ICPO was modelled at IRSN with the MCNPX Monte Carlo code. The ability of the model to adequately reproduce the measured proton absorbed dose distributions in a water phantom was shown in a previous work [1]. Three experimental validations of the model, relating secondary radiations produced during treatment, were performed. They concerned (i) the neutron ambient dose equivalents in the therapy room (ii) neutron spectra and (iii) neutron and photon absorbed doses inside an anthropomorphic phantom. The model was then used to estimate the secondary doses received by the organs of the patient. Absorbed doses and equivalent doses in organs due to secondary neutrons were estimated using a mathematical phantom. The neutron spectra were also calculated in the organs.

For the validation parts measurements were made under clinical treatment conditions. First, the ambient dose equivalents due to neutrons were measured at several points around the isocentre using neutron probes and tissue equivalent proportional counters. Second, the neutron spectrometry was performed at 2 points with a Bonner spheres system specifically developed for high energy neutron fields. Third, the doses deposited by secondary neutrons and photons were measured at 8 points inside a Rando-Alderson phantom placed to reproduce an ophthalmologic treatment. Thermoluminescent detectors (⁷LiF) and track etched detector (CR-39) were used to estimate the photon and neutron dose components, respectively. For the estimation of organ doses, a mathematical phantom was implemented and neutron absorbed and equivalent doses were calculated in the 14 organs defined in the ICRP Publication #60, as well as in the non-treated eye.

For the 3 sets of validation measurements a good agreement between experimental and calculated data is obtained. For a typical ophthalmologic treatment (60 Gy eq. ⁶⁰Co) neutron equivalent doses range from 0,65 mSv to around 13,5 mSv for the ovaries and the non-treated eye, respectively. Results relating to the validation process of the model will be discussed as well as the secondary organs doses which will be compared to available data from the literature.