Measurement of the peripheral dose for the tangential breast treatment technique with Co-60 gamma radiation and high energy X-rays.
Published data to estimate the dose outside the primary beam, i.e. the peripheral dose (PD), are only valid for perpendicularly incident beams. In radiotherapy of breast cancer patients often a tangential beam technique is used, for which the published PD data cannot be applied. Measurements were made in a water phantom for three sizes of a breast phantom, representing a small, medium, and large breast. Open and wedged beams were used. Photon energies were 60Co gamma radiation and 6- to 25-MV X-rays. The PD was also determined in a humanoid phantom. To check the validity of the phantom measurements, the PD was measured in 50 patients referred to this institute for radiotherapy of the breast. The mean of the ratios of the measured to the estimated PD was 1.12, with a standard deviation of 26%. The data set was used to estimate the fetal dose for a 'standard' patient, as a function of stage of pregnancy. The measurements have resulted in a data set, which enables calculation of the peripheral dose for the tangential beam technique. The accuracy is considered to be adequate for risk assessment.