The impact of dose-specification policies upon nominal radiation dose received by breast tissue in the conservation treatment of breast cancer.

Abstract

In the context of breast conservation treatment, absorbed dose is influenced by (1) prescribed nominal dose, and (2) dose-specification characteristics employed. Breast doses are generally specified either at tangent isocenter, varying anatomical points within the breast, or at isodoses varying from 90% to 100%. Boost doses are generally specified at 80-100%. An idealized axial slice of breast tissue at central axis is presented. Assuming varying dose-specification characteristics, absorbed doses are normalized and compared to those received by nominal prescriptions of 46 Gy to the breast and 20 Gy to the boost volume, both specified at 100%. Absorbed doses vary from the normalized total of 66 Gy (with specification of breast and boost at 100%) in gradations up to a maximum of 76.11 Gy (when breast dose is specified at the 90% isodose and boost dose at 80%), a 13.3% difference. The impact of dose specification is largely ignored in the breast irradiation literature and unappreciated in clinical practice. Its impact, however, is illustrated as dwarfing modest nominal dose escalations commonly recommended and prescribed among margin compromised patients. Progress in delineation of a dose-response relationship for treatment of breast cancer requires consensus as to dose specification. Arguments are offered that ICRU Report 50 dose-specification standards, as verified for reproducibility by the EORTC (22881/10882) trial group, constitutes the best data source currently available from which dose-specification consensus may be reached (1, 2). Dose to PTV(1) (whole breast plus 1- to 2-cm margin) should be specified at the tangent beam intersection on the central plane or, where such point is irrelevant, at two-thirds distance from dorsal beam edge to skin along the perpendicular breast bisector. Where irradiated via electrons, dose to the boost PTV(2) (lumpectomy cavity plus 1- to 3-cm margins) should be specified at 90%. Electron energy sufficient to provide 85% isodose coverage to all aspects of PTV(2) is recommended.