Dose levels outside radiotherapy beams.

The specification of a radiotherapy generator calls for the leakage radiation through the housing of the source to be 0.1% (the ICRP has recently (1982) changed this figure to 0.2% of the dose rate inside the useful beam). When a patient is irradiated, the dose level outside the useful beam is not only determined by this leakage radiation, but also by scattered radiation from the beam-defining system and from the treatment volume of the patient. Measurements have been made on a 300 kV X-ray unit, and on two different 4 MV and one 8 MV linear accelerators to determine the levels of leakage radiation and scattered radiation in air, and in a phantom comparable in size to the trunk of the patient. Similar measurements have also been made for a 15 MeV neutron generator. It is shown that for all these generators the dose delivered outside the radiation beam is determined mainly by the two scattering processes mentioned, and it is argued that for linear accelerators it would be feasible to increase the permitted level of leakage radiation by a factor of 2 or 3 without a significant increase in the stray radiation delivered to the patient.