Yields and quality of photoneutron fields in the vicinity of high energy therapy machines

Dosimetric limits for the controlled usage of therapy machines are becoming increasingly strictly specified, both with regard to the primary beam and to unwanted leakage and scattered radiations which contribute to the general levels within the treatment room. Proposals have been made that the equivalent dose of the unwanted radiation be limited to 0.1% of the treatment dose in the plane of the patient. For leakage through the tube housing the proposed limits are: 0.1% for X rays and 0.5% for neutrons (as a fraction of the treatment dose). For assessing the equivalent dose of photoneutrons, measurements have been made of the neutron yields in the vicinity of high energy (6 and 10 MV) therapy machines. The γ and neutron spectra and their relative contributions to dose have been determined using specially designed cylindrical proportional counters operated in microdosimetric mode. Neutron fluence yields were obtained by activation of indium wire placed in a moderator to optimise the sensitivity. The optimum thickness of moderator was determined by direct absorption techniques. Equivalent dose rates of between 0.104 Sv.h-1 and 0.028 Sv.h-1, depending on location, were calculated. Results obtained directly from a sievert meter are given for comparison. Equivalent doses calculated from the observed frequency distribution in a tissue-equivalent microdosemeter are in agreement. It is concluded that the neutron component of dose can represent a significant hazard that should be monitored for the protection of patients and staff, especially for higher photon energies. (Author)