Measurement of the peripheral dose for the tangential breast treatment technique with Co-60 gamma radiation and high energy X-rays

Abstract

Background and purpose: 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.

Material and methods: 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.

Results: 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.

Conclusions: 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.