Dose and LET distributions due to neutrons and photons emitted from stopped negative pions.

Computer calculations are made of the dose and LET distributions due to neutrons and photons produced when negative pions are stopped in a phantom. When negative pions are stopped in a material they undergo nuclear capture, resulting in the disintegration of the nucleus and the emission of short range charged particles and longer range neutrons and photons. The uncharged radiation constitutes a potentially large source of dose outside the treatment volume. A simple phantom consisting of a 0-25 m cube of either tissue or bone-equivalent material is set up with a 0-05 m cube in the centre to represent the treatment volume. Neutrons and photons are started in this central volume and transported across the phantom using Monte Carlo transport codes. Several different initial energy spectra for the neutrons are used, taken from experimental and theoretical data. These different spectra are found to give significant differences in dose, though the distance to the 80% dose level is always about 0-015 m. Order of magnitude differences in some LET regions are also found. The dose deposited by neutrons in bone is about 24% less than in soft tissue, the photon dose being small compared with the neutron dose.