[Lymph nodes and rectal cancer]

Nodal invasion is a major prognostic factor of rectal cancer. Lymphatic extension of rectal cancer usually involves the mesorectal nodes then the inferior mesenteric chain but in 14% of patients, particularly with cancer of the lower rectum, metastasic nodes can be observed in the internal or lumboaortic chains situated beyond the usual territory of nodal dissection. On average, 30 nodes are examined on a proctectomy specimen, but with wide interindividual variation. The tumor can be adequately staged if at least 15 nodes are examined with little risk of not recognizing nodal metastasis. Metastatic nodes of rectal cancer are almost always small, more than 90% measuring less than 10 mm and 70% less than 5 mm. The number of invaded nodes and the total number of examined nodes are prognostic factors for survival. Hypofrationated preoperative radiotherapy does not alter the nodal status but a long radiotherapy protocol (45 Gy over 5 weeks) reduces significantly the total number of nodes examined without changing the number of metastasic nodes. Micrometastases (measuring less than 2 mm), identified by immunohistochemistry or gene amplification, can be detected in 25 to 70% of nodes considered metastasis-free at the usual microscopic examination. The prognostic value of these micrometastases remains to be established. The first node draining the tumor (sentinel node), which can be detected rapidly with dye infusion, appears to provide a good picture of the nodal status, the risk of finding an invaded node if the sentinel node is metastasis-free is less than 5%.