Microdosimetric depth measurements in the Nice fast neutron therapeutic beam

In Nice biomedical cyclotron, proton beams of 65 MeV of energy are used since June 1991 to treat ocular tumours (mainly uveal melanomas): up today 550 patients have been treated. Another application of the facility is tumour treatment by using fast neutron beams. The neutron beams are produced by bombarding beryllium targets with 60 MeV protons. The treatment unit consist of a vertical beam completed with a multileaf collimator. The maximum aperture of the collimator at the treatment distance, 20 cm from the end of collimator, is 23x24.5 cm2. The depth dose characteristics are similar to those ones of a 8 MeV Linac. For 10x10 cm2 field the maximum dose is at 2 cm and the 50% isodose at 17 cm. The penumbra (80% - 20%) at 5 cm depth for this field is about 8 mm. Neutrons interact with biological targets giving rise to charge particle secondary spectra of protons, deuterons, alfa particles and light ions. Since neutron energy spectrum changes with depth, charge particle energy spectra is expected to change as well as their LET spectra. On the other words, the quality of the neutron beam is expected to change with the depth. Dosimetric measurements performed with ionisation chambers are not able to measure the variation of radiation quality. Microdosimetry is a well known scientific tool able to describe the radiation quality and to measure its variation. We have performed microdosimetric measurements in the Nice fast neutron beam at different depths with a spherical tissue-equivalent proportional counter which simulates 2 μm of thickness. Experimental data will be compared with the dose depth curve and with radiobiological experiment data. Results and comparisons will be discussed