Internal Mammary Lymph Nodes Involvement in Patients with Breast Cancer: Anatomical Characteristics and Implication for Target Definition

Abstract

Internal mammary nodes (IMN) as one of the regional lymph nodes (RLN) in breast cancer, its importance to be included in regional nodes irradiation (RNI) has been assessed by important prospective trials. Optimal definition of target volume is critical in balancing target coverage and toxicity to normal tissues. The imaging and anatomical information for IMN metastasis are less described than other RLN in breast cancer. The current study aims to collect and analyze the imaging information of IMN involvement so as to provide information for optimize IMN delineation. Patients with IMN involvement were identified from single-center breast cancer database with imaging data. The image information including thoracic CT, breast MRI, and PET/CT. Anatomical characteristics from axial imaging including distribution of involved ribs, distance from the internal mammary vessels (IMV) were collected for each metastatic nodes. IMNs greater than 5 mm in short axis was defined as metastasis in this study. A total of 53 breast cancer patients with a total number of 63 metastatic IMN met the criteria for INM involvement were identified and located in this study. In 27 patients (50.9%), more than one intercostal spaces were involved, of which the second rib space was the most involved (26/27). The number of metastatic nodes in the 1st, 2nd, 3rd and 4th intercostal space were 26 (41.3%), 43 (68.3%), 21 (33.3%) and 4(6.3%) respectively. The maximal radial distance from center of metastatic nodes to the IMV in each patient were measured. The percentage of including IMN with a 4mm, 5mm, 6mm and 7mm medial/lateral distance to the IMV were 56.6% (30/53), 75.5% (40/53), 90.6% (48/53) and 96.2% (51/53) respectively. While in maximal dorsal/ventral distance, all extension of the IMN nodes were in dorsal direction, the percentage of including IMN with a 4mm, 5mm, 6mm and 7mm depth to the IMV were 85%(45/53), 90.6% (48/53), 94.3%(50/53) and 98.1%(52/53) respectively, one case had enlarged mass invading the fourth rib, in which the identification was difficult. In total, 28 of 53 patients had no documentation of IMN disease on imaging report regardless of the visible IMN mass on thoracic CT. The most common description was parasternal mass (50%, 14/28), chest-wall mass (7.1%, 2/28) and no description at all (32.1%, 9/28). All these 12 patients had thoracic CT as the only image evaluation. In 12 patients combined with breast MRI and one patient combined with PET/CT, none had miss diagnosis of IMN disease. In most of the patients indicated for IMN irradiation in the adjuvant setting, clinical target volume of IMN can be delineated with a 6mm medial/lateral distance and a 5mm depth to the IMV on the same axial CT image. This optimization in delineation might facilitate the balance of target coverage and normal tissue constraints. Multimodal image information may help to improve the sensitivity of diagnosis in IMN metastasis.