Anatomical Lymph Node Mapping in Normal Mesorectal Adipose Tissue

PURPOSE: In view of divergent outcomes of surgery for rectal cancer despite standard protocols, the aim of this study was to provide a basis for improving lymph node assessment by defining the number, shape, and distribution of all lymphatic structures in the mesorectum. METHODS: Cadavers from 6 males and 6 females who died from causes other than colorectal or neoplastic pathologies were studied. Rectum and mesorectum were excised en bloc. The adipose tissue was separated from the rectum and divided into 9 sections before fixing the specimen in paraffin, cutting into smaller portions, and staining with hematoxylin and eosin. Slides were analyzed with an optical microscope, and identified lymph nodes were counted in each section. RESULTS: The mean age of the deceased was 52.7 (range, 26-65) years. No evidence of previous history of neoplastic pathology or any type of premortal colorectal inflammatory process was found. A total of 412 lymph nodes were identified, with a mean of 34.3 (SD, 2.1; range, 31-37) lymph nodes per cadaver. The mean number of lymph nodes differed significantly across levels of the mesorectum, with 22.2 lymph nodes in the upper, 9.8 in the middle, and 2.3 in the lower sections; 266 (64.6%) of all lymph nodes were located in the upper third of the mesorectum. Distribution density was higher in the proximal posterior sections, with 197 lymph nodes (47.8%) in the upper 2 thirds of the posterior mesorectum. Node diameter was less than 5 mm in 330 (80%) of 412 nodes. CONCLUSIONS: Our study confirmed that more than 30 lymph node units normally exist in the mesorectal area. In view of previous studies demonstrating advantages of increasing the number of lymph nodes evaluated, staging of rectal cancer might be improved by counting more than 12 lymph nodes per specimen.