15O and 11C production in neutron radiotherapy patients.

In order to establish the feasibility of performing blood flow measurements following therapeutic neutron irradiation by determining increased 15O disappearance rate from a volume of tissue, knowledge of the probabilities of 15O, 11C and 13N production in tissue by a p(42)Be neutron-beam irradiation is required. Isotope production probability per unit dose (defined as the isotope-yield coefficient) is determined from the measured production of 15O, 13N and 11C in irradiated H2O. Measured isotope-yield coefficients average 1.20 x 10(-16), 2.85 x 10(-18) and 5.56 x 10(-18) Gy-1 per 16O nucleus, respectively, at depths between 1 and 15 cm in a water phantom. The isotope yield coefficients for 11C production from 12C (polyethylene) average 1.04 x 10(-16) Gy-1 per 12C nucleus at depths between 1 and 15 cm in a water phantom. From these yield coefficients, the creation of 9700 Bq 15O and 450 Bq 11C per cubic centimetre of human soft tissue (chemical composition of soft tissue of Reference Man) is calculated assuming a 0.65 Gy neutron irradiation given at a dose rate of 40 cGy min-1 and assuming no perfusion of the activity. These results agree with those calculated using published neutron cross sections and neutron energy spectra, indicating that (n, 2n) reactions are the predominant activation mechanisms.