How Can We Identify the Position of the Positive Nodes in the Different Pelvic Compartments by Imaging?
Knowledge of the normal rectal lymphatic drainage is necessary to understand the location of metastatic nodes. The lymphatic drainage of the rectum and the enveloping fat (the mesorectum) follows the venous drainage of the rectum. The drainage occurs along a superior and lateral pathway . The inferior part of the rectum (the distal 3 cm) drains through the lymphatic vessels along the middle rectal artery and subsequently into the internal iliac lymph nodes in the obturator areas (Fig. 12.1 shows the internal iliac lymph node chain and obturator area at MRI) . The lymphatic drainage of the superior part of the rectum follows the superior rectal artery (Fig. 12.2) in the mesorectum to the pararectal lymph nodes and then towards the mesenteric lymph nodes of the sigmoid mesentery, from which they drain along the inferior mesenteric and lumbar lymph nodes . The very low rectum (or the anorectum and thus relevant when low tumours invade the anal canal) can drain into the inguinal nodes and then along the external iliac chain . Reports have shown that there is some variety in the lymphatic anatomy, mostly occurring in the drainage towards the internal iliac chain . Additionally, it has been postulated that lateral drainage via the middle rectal artery to the internal iliac nodes might occur when the superior drainage is blocked, e.g. when tumour is obstructing the pathway . Moreover, one should keep in mind that even the presence of tumour in the rectum can lead to an increase in the number and size of nodes, even in the absence of nodal metastasis .
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