| Summary: | Abstract Background Despite increasing clinical data suggest that internal mammary node (IMN) irradiation would improve local-regional control and overall survival in breast cancer, its role remains controversial due to increased risk of cardiac and pulmonary toxicity. The current study aims to determine the high risk areas of IMN metastases by collecting and analyzing the axial imaging of IMN involvement, in order to optimize IMN delineation for breast cancer. Methods Breast cancer patients with IMN involvement were retrospectively identified from single-center database. All available imaging modalities including thoracic CT, breast MRI, ultrasound and PET/CT were used to diagnose IMN metastases. Anatomical characteristics from axial imaging, including distribution of involved ribs and distance from the internal mammary vessels (IMV), were collected for each metastatic IMN. What’s more,the natural infiltration tendency of IMNs from IMV was calculated in this study. Results In total, 83 metastatic IMNs from 70 breast cancer patients (initial diagnosed:34 and recurrence: 36) were located from axial CT image in this study. The second intercostal space was the most likely involved in patients with single(n = 35, 53.0%) and multiple intercostal space (n = 31, 47.0%) involvement. The percentage of including IMN with a 5 mm, 6 mm and 7 mm medial/lateral distance to the IMV were 75.9% (63/83), 89.2.6% (74/83) and 92.3% (77/83) respectively. While in maximal dorsal/ventral distance, nearly 95% of the nodes were encompassed into 6 mm depth to the IMV. Over 65% of IMN adenopathy (32/49,65.3%) were found to have a growth direction close to the sternum. By retrospective reviewing diagnostic reports, MRI demonstrated a high diagnostic performance in diagnosis of IMN disease (90.3%, 28/31), while CT had a higher misdiagnosis rate (22/63, 34.9%). The diagnostic efficiency of IMN could be improved if different methods were combined. Conclusions For patients with indications of prophylactic IMN irradiation, a 7 mm medial and 6 mm dorsal distance to the IMV on axial CT would be optimal to cover the clinical volume of IMN; and it would be reasonable to extend clinical tumor volume (CTV) coverage towards sternum for patients with evident IMN disease. Multi-imaging modalities are recommended to improve the diagnostic specificity and sensitivity of IMN metastases.
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