Skin-sparing effect in fast neutron radiation therapy

The skin sparing effect in super voltage and telegamma radiotherapy is well known. In the case of fast neutron radiotherapy a similar effect is expected, though to a smaller extent, since the ranges of the secondary charged particles in tissue are smaller. For 14 MeV neutrons, for example, the maximum ranges in tissue of the highest energy secondaries produced in tissue are 2.2 mm (proton), 0.0054 mm (heavy recoil, say, oxygen), 0.134 mm (alpha particle from (n, α0) event in oxygen). Thus, most of the build up of dose is within a millimetre or two. Further, the skin on the incident field is also exposed to the charged secondaries arising from the (n, alpha) and elastic scattering events taking place in the overlying air.