The effect of a dynamic wedge in the medial tangential field upon the contralateral breast dose.

The elevated incidence of breast cancer following irradiation of breast tissue has led to concern over the magnitude of the scattered radiation received by the uninvolved contralateral breast during radiation therapy for a primary breast lesion and the risk of an induced contralateral breast cancer. Some linear accelerators use a single dynamic (or universal) wedge that is mounted within the treatment head at an extended distance from the patient. Because of the combined effects of distance and shielding, the contralateral breast dose due to a medial tangent containing a dynamic wedge is expected to be less than that containing a conventional wedge. This paper presents contralateral breast dose (CBD) measurements performed on an anthropomorphic phantom with breast prostheses irradiated with 6 MV X rays from a linear accelerator equipped with a dynamic wedge. Doses were measured at 15 points within the contralateral breast prosthesis with thermoluminescent dosimeters. It was found that the contralateral breast dose per unit target breast dose decreases with the perpendicular distance from the posterior edge of the medial tangent to the dose measurement point and increases with effective wedge angle by factors ranging up to 2.8, in agreement with data presented earlier for a water phantom geometry. This dose elevation showed no statistically significant dependence (p less than 0.05) upon the perpendicular distance from the beam edge. Comparisons with data in the literature show that the contralateral breast dose increase by a dynamic wedge is typically only about half of that reported for a conventional wedge for the same wedge angle and distance from the beam.