Incidental Radiation Dose to Presumed Uninvolved Internal Mammary Lymph Nodes in Breast Cancer Patients Receiving Post-mastectomy Irradiation Adjuvant Therapy

Abstract

Routine prophylactic treatment of uninvolved internal mammary nodes (IMN) in breast cancer patients requiring post-mastectomy radiation therapy (PMRT) is controversial. While randomized trials demonstrating survival benefit of PMRT often included IMNs, doing so increases the normal tissue doses, especially the heart dose, which is associated with an increase in radiation induced morbidity/mortality. The purpose of this study was to measure the incidental radiation dose to the internal mammary lymph nodes (IMN) not included in the planning target volume (PTV) in women with breast cancer receiving PMRT irradiation. This study was a retrospective, IRB approved protocol using CT-based 3D conformal (3D CRT) treatment plans. Thirty-three consecutive patients were included in the analysis: 17 left side and 16 right side patients. All patients received standard 3D CRT chest wall tangent fields and matched supraclavicular (SCV) fields. All plans were prescribed a total dose of 50 Gy in 25 fractions to the chest wall and 46 Gy in 23 fractions to the SCV field. The IMN chain was not intentionally included in the target volume. For purposes of this study, IMN vessels were contoured following the RTOG atlas, utilizing original CT simulation images. A 7 mm PTV expansion was added to the IMN contours. Pinnacle treatment planning algorithm was used to quantify the radiation dose volume histograms. The percent volume of IMN PTV receiving 100% of the prescribed dose (50Gy) for all patients was 24% (range, 0-73%, SD 23.9). The percent volume of IMN PTV receiving 95% of the prescribed dose (45 Gy) for all patients was 34% (range, 0.6-92%, SD 27.3). The percent volume of IMN PTV receiving 90% of the prescribed dose (40 Gy) for all patients was 39.5% (range, 1-97%, SD 28.8). The mean IMN dose for all patients was 23.4 Gy (range, 3.5-52.9, SD 15.8). The IMN absolute dose range for all patients was 5.09- 51.05 Gy. The volume of lung receiving 20 Gy (V20) for left sided and right sided were 13.8%, (range, 7.23-18.5%, SD 2.8%) and 15.5% (range, 8.9-28.4%, SD 5.1%), respectively. The volume of heart receiving 30 Gy (V30) for left sided and right sided were 4.2% (range, 0.05-13.3%, SD 3.8%) and 0.0003% (range, 0-0.4%, SD 0.001%) respectively. The results of this study suggest that the incidental dose to the IMN nodes does not achieve therapeutic levels in post mastectomy breast cancer patients treated with standard 3D CRT chest wall irradiation. If risk factors for microscopic involvement are present, IMNs must be specifically included in target volumes in order to be adequately treated.