Dose levels outside radiotherapy beams

Abstract

In an earlier paper (Greene et al, 1983) the results of measurements on the dose levels outside the main X-ray beams of three linear accelerators were described. These measurements were made in air and in a large phantom, and rough estimates were made of the dose contribution due to the radiation scattered from the treatment field, inside the phantom. This present note reports similar measurements on two more linear accelerators, and describes a method of measuring the contribution due to phantom scatter. Some measurements on the dose distribution outside electron fields are also reported. While consistent with the previously reported dose levels (Greene et al, 1983), the measurements now reported show clearly that the principal source of radiation outside the main beam is scatter from the collimator system rather than head leakage or scatter from the irradiated tissue in the treatment volume. Measurements were made by the methods described in the previous paper (Greene et al, 1983) along the lines OX and OX' of Fig. 1, at 110 cm from the X-ray target. Measurements were made in air with the X-ray beam blocked off (leakage measurements), in air as a function of field size, and in the phantom. The phantom was placed in the four alternative positions shown. In position ABCD, the dose distribution along OX included leakage radiation, radiation scattered from the beam defining system, and radiation scattered from the main beam in the phantom.