What degree of accuracy is required and can be achieved in photon and neutron therapy?

In this paper an attempt is made to formulate criteria for the accuracy in the delivery of absorbed dose to a patient during photon or neutron therapy. These requirements are mainly based on the relative steepness of dose-effect curves for local tumour control and normal tissue damage. A review of these dose-effect curves after photon irradiation shows a great variety in steepness; the curves for normal tissue complications in general may be steeper than those for local tumour control. From these data a standard requirement for the combined uncertainty of type A (random) and type B (systematic), given as one relative standard deviation, in the absorbed dose delivery of 3.5% is proposed, even though it is known that in many cases larger values are acceptable and in a few special cases an even smaller value should be aimed at. From the available radiobiological and clinical data it can be concluded that no statistically significant difference can be observed in the relative steepness of dose-effect curves after photon or neutron irradiation. Similar limits will thus be requested in neutron therapy. The uncertainties in the various steps involved in the delivery of an absorbed dose to a point in a patient have been analysed for a treatment with two parallel-opposed beams. The results of this analysis showed that even for these simple treatment conditions, the required accuracy in the delivery of the absorbed dose cannot completely be obtained in photon therapy, and not nearly in neutron therapy. The uncertainties in physical, radiobiological and clinical approaches for weighting of the biological effectiveness of neutron radiation have been compared. The uncertainty in the RBE ratio will replace the type B uncertainty in the absorbed dose during patient treatment if the same dosimetry protocol is applied during biological and clinical procedures.