Background: There is a discrepancy about the metastatic rate of internal mammary lymph nodes (IMNs) between\nclinical and pathologic findings. We aimed to investigate the metastatic rate of IMNs and to provide\nrecommendations on target volume delineation of IMNs for adjuvant radiotherapy in breast cancer patients.\nMethods: We retrospectively analyzed data from 114 breast cancer patients treated with surgery without adjuvant\nradiotherapy who developed local and/or regional lymph node recurrence/metastasis at our institute from January\n2015 to January 2019. Patients with widely lung or pleural metastases were excluded. We first analyzed the recurrence rate\nwith the chest wall, the metastatic rate of internal mammary/anterior mediastinal, ipsilateral axillary and supraclavicular\nlymph nodes, and then investigated the distribution of the IMNs.\nResults: Among the 114 included patients, the recurrence rate with the chest wall, metastatic rate of IMNs, IMNs/anterior\nmediastinal lymph nodes, ipsilateral axillary lymph nodes, and the ipsilateral supraclavicular lymph nodes was 43, 37.7, 59.6,\n12.3, and 22.8%, respectively. The metastatic IMNs were mainly located from the first to the second intercostal space.\nHowever, metastatic lymph nodes could also be observed above the upper edge of the first rib.\nConclusions: The metastatic rate is high in the IMNs and irradiation of the internal mammary lymphatic chain is required. It\nis suggested that the upper bound of the internal mammary lymphatic chain should be up to the subclavian vein with a\n5-mm margin, thus connecting to the caudal border of supraclavicular clinical target volume in breast cancer patients at\nhigh risk of recurrence.
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