Expression of dose in neutron therapy

Abstract

A pragmatic approach in neutron dosimetry is to consider the energy spectrum to consist of a neutron and a gamma component. The relationship between the two components of dose in neutron radiotherapy has been investigated for energies currently in clinical use. Changes in the neutron component itself are not dealt with. Because the two components are given simultaneously there is some interaction between them so that the gamma fraction is more effective than if the neutron and gamma doses were separated in time. This interaction has been accounted for using a well-proven extension of the linear-quadratic (LQ) equation. Using values of the LQ parameters α and β measured recently in vivo, we have modelled the total effect from the neutron and gamma dose-contributions in terms of an equivalent neutron dose. This allows a comparison of different methods of expressing the measured physical dose with the biologically effective dose. The current practice in neutron therapy is to give the total (neutron plus gamma) dose, quoting also the gamma contamination. In all cases within the range of energies used for therapy, the total dose will give an overestimate of the biologically effective dose by approximately 4% for each 5% of gamma contamination. Expression of the neutron dose only (ignoring the gamma component) will give an underestimate of the biologically effective dose by approximately 1.5% for each 5 % of gamma contamination, i.e. the error is approximately three times less for neutron dose than for total dose. However, if the biologically effective dose, in terms of an equivalent neutron dose, is given by neutron dose +0.25 gamma dose, the error is reduced to less than 1% for all relevant neutron energies, dose levels and fractions of contaminating gamma rays. The same is true for both early and late responding tissues. We, therefore, strongly recommend that the use of total neutron plus gamma dose should be discontinued in favour either of neutron dose or better still would be the adoption of a “standard” expression of therapy dose as the neutron dose +0.25 gamma dose.