Over the years, concern has arisen that the scattered dose absorbed by the contralateral breast during radiotherapy may lead to subsequent radiation-related second malignancies. This study tested the efficacy of a mobile breast shield (MBS) in reducing the opposite breast dose during radiation therapy for breast cancer. The MBS, developed by the Cleveland Clinic, is composed of a 2.5-cm-thick plate of lead and is encased within a polystyrene coating. It was hypothesized that the MBS would be able to reduce external scatter to the contralateral breast, which includes leakage from the gantry head and scatter from the collimator. Measurements were performed on both breast cancer patients and on a Rando phantom. For all subjects, measurements of the opposite breast dose were taken both with and without the MBS using three p-type semi-conducting diodes that were arrayed across the papillary plane of the contralateral breast. Measurements were taken for both the medial and lateral fields; however, the MBS was only put in place while the medial beam was being delivered. Although this study has not yet been completed, some preliminary conclusions may be drawn from the phantom study as well as the patient data that has been accumulated thus far. From the present patient data, it appears that the MBS can reduce the opposite breast dose on the average of 41%, while the MBS reduced the opposite breast dose on the phantom on the average of 32%. The data also showed that the opposite breast dose was greatest closer to the medial edge, and that the MBS reduced the scattered dose least at this position. From the phantom study, the influence of the shield's positioning and of using a wedge on the effectiveness of the MBS were also examined. Even though this study seems to demonstrated that the MBS does in some degree reduce the dose to the contralateral breast, further long-term studies are needed to determine whether or not this reduction will have any significance on the induction of metachronous contralateral breast cancer.
This work was supported by the Simons Foundation.