Incidental radiation to uninvolved internal mammary lymph nodes in breast cancer

Routine treatment of clinically uninvolved internal mammary nodes in breast cancer patients requiring post-mastectomy radiation therapy is controversial. The purpose of this study was to measure the incidental radiation dose to the internal mammary lymph nodes not included in the planning target volume (PTV) in women with breast cancer receiving post-mastectomy radiation therapy. This retrospective protocol utilized CT-based 3D conformal treatment plans. Fifty consecutive patients were included in the analysis: 25 left breast and 25 right breast patients. 3D conformal treatment plans chest wall tangent fields and matched supraclavicular were used. All plans were prescribed to a total dose of 50 Gy in 25 fractions to the chest wall and 46 Gy in 23 fractions to the supraclavicular field. The internal mammary node chain was intentionally not included in the target volume. For purposes of this study, internal mammary vessels were contoured following the Radiation Therapy Oncologist Group atlas with a 7-mm PTV expansion, utilizing original CT simulation images. The internal mammary nodes were contoured in between the first 3 and first 5 intercostal spaces for comparison. Percent volume of internal mammary node PTV receiving 95 % of the prescribed dose (47.5 Gy) with 7-mm expansion and first 5 intercostal spaces for all patients was 25.2 % (range 0.04-97.6 %, standard deviation (SD) 23.5). The mean internal mammary node dose for all patients was 24.98 Gy (range 3.54-55.93 Gy, SD 16). Results of this study suggest the incidental dose to the internal mammary nodes does not achieve clinically significant therapeutic levels in post-mastectomy breast cancer patients treated with standard 3D conformal radiation therapy chest wall irradiation. If risk factors for microscopic involvement are present, internal mammary nodes must be specifically included in target volumes in order to be adequately treated.