Trends in Dose to the Contralateral Breast from Breast Cancer Radiotherapy in the United States

Contralateral breast (CB) cancer is the most common subsequent cancer among breast cancer survivors, and radiotherapy has been linked to CB cancer risk. The purpose of this work was to evaluate doses to subregions of the contralateral breast from historical breast cancer treatments carried out in the United States between 1990 and 2012. We extracted treatment data from radiation therapy summaries for 2,442 radiotherapy patients during that period. We estimated CB doses for five breast regions: the upper inner quadrant (UIQ), lower inner quadrant, upper outer quadrant, lower outer quadrant (LOQ), and nipple, using extracted data and out-of-beam CB dose measurements. The mean treatment dose was approximately 5,000 cGy for tangential fields, which comprised 84% of the photon fields, and this remained constant throughout our study period. Most of the dose to the contralateral breast was from the tangential fields, and it varied by contralateral breast region. The UIQ of the contralateral breast received the highest median dose which decreased by 23% from 185 cGy in 1990-1994 to 143 cGy in 2005 and later (P < 0.0001). The LOQ dose received the lowest dose, which also decreased by 24% from 74 to 56 cGy (P < 0.0001). This decrease was due to the reduction in the utilization of physical wedges and an increase in the field-in-field technique, particularly after 2005. We observed a significant reduction in CB doses from breast radiotherapy in the United States between 1990 and 2010, which can be attributed to the impact of advanced radiotherapy techniques.