Changes in biological effectiveness with depth of the Medicyc neutron therapy beam

V79 cells were exposed to fast neutrons generated by 60 MeV p-->Be produced by the cyclotron Medicyc at four different depths: 1.3, 25.8, 72.2 and 116.8 mm. Survival was assessed by the in vitro colony method. Mean inactivation doses (MID) were significantly different among the four points. The ratio of MID was used to determine the relative efficiency of the neutron beam at these points. Compared to 25.8 mm depth, a 40% increase in biological effect was observed at the superficial point versus a 14 to 16% decrease in effect for the deeper points. This is ascribed to absorption of low energy neutrons near the surface and to beam hardening with depth. Taking in consideration the relative physical dose delivered, these findings suggest that skin-sparing may be markedly reduced and that the lower effectiveness with depth should be kept in mind when dealing with deep tumours.