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 .
12.1 What Is the Lymphatic Anatomy of the Rectum and Mesorectum?
12.2 Why Do We Want to Know the Location of Nodes?
The location of nodes and of metastatic nodes in particular is of importance for surgical planning and for radiation planning in more advanced tumours in those who have (chemo)radiotherapy . For the surgeon it is important to know how distal his resection should be to include all the possible metastatic nodes. Additionally, the surgeon needs to know the presence of nodes in the vicinity of the mesorectal fascia and possible nodes outside the surgical margin, e.g. obturator areas or proximal to the origin of the superior rectal artery. Presence of nodal spread through the inguinal and external iliac chain also alters the treatment strategy and should be noted in order to avoid recurrences. Radiotherapists can use information about the location of metastatic nodes to adequately delineate suspicious nodes . Furthermore, knowledge about the distribution of nodes relative to the tumour can guide the radiotherapist in assessing what part of the mesorectum and extramesorectal regions should be included in the radiation fields .
Knowledge about the distribution of (metastatic) nodes after chemoradiotherapy has become relevant only recently; now organ-sparing treatment has been shown to be feasible in good responders to chemoradiotherapy [5, 6]. When organ-preserving treatment is considered, the nodes should be sterilized, as the nodes remain in the patient. Knowledge about the mostly affected areas can guide the radiologist where to look for residual metastatic nodes. Moreover, during follow-up for organ-preserving treatment, the radiologist can assess these high-risk areas with special attention to identify potential nodal recurrences.
Furthermore, the location of involved nodes predicts the risk for recurrence and metastasis. Lateral lymph node spread is a poor prognostic factor , but proximal lymph node involvement (more proximal than at the mesorectal tumour level) has also been reported to lead to worse prognosis [8, 9]. In addition, patients with metastatic proximal nodes have been reported to have a significantly higher rate of synchronous metastases at the time of surgery than patients without proximal lymph node involvement . Therefore, the presence of proximal or lateral nodal involvement can guide decision-making with regard to adjuvant chemotherapy  and follow-up.
12.3 Where Are the Metastatic Nodes Located in Primary Rectal Cancer?
Yao et al. performed a study in which 60 TME specimens were analysed after dividing the specimen into a superior, middle and inferior portion, according to distance of the anal verge . Additionally the specimen was divided into four areas radially: anterior, lateral right, lateral left and posterior. They sliced the specimen into 2 mm sections and then evaluated the nodes by fat clearance technique. In the upper rectum, most benign nodes were found in the lateral and posterior areas (total 91%). Few nodes were found in the anterior compartment (9%), and these nodes were generally small (<5 mm). Metastatic lymph nodes followed the same distribution. All of the metastatic nodes in the anterior compartment were <5 mm. The authors reported that both in middle and lower rectal cancer, distal metastatic nodes can be found and therefore stressed the need to use an adequate distal resection margin caudal to the tumour in middle rectal cancer .
In the study by Engelen et al., nodal distribution and size in primary rectal cancer were assessed with MRI . They performed a lesion-by-lesion matching of nodes at MRI with histopathology. Engelen et al. showed that metastatic nodes follow the same distribution as benign nodes draining the rectum. Nodes are mostly located lateral or dorsal to the tumour, at the same level or higher than the tumour. Nodes below the tumour level were reported to be less likely malignant, i.e. in only 2 out of 33 nodes distal to the tumour (6%) . Engelen et al. also reported on nodes outside the mesorectum. Metastatic extramesorectal nodes were only found in patients with distal rectal cancer and were located at the root of the middle rectal artery at the level of the internal iliac artery and in the obturator areas .
12.4 Where Are the Metastatic Nodes Located After Chemoradiotherapy?
Chemoradiotherapy leads to downsizing and downstaging of nodes. Several reports showed that the number and size of nodes but also the incidence of metastatic nodes decrease significantly due to chemoradiotherapy [13, 14]. Given the interest for organ-preserving treatment, as mentioned above, it is important to know where to look for residual metastatic nodes, in order to avoid nodal recurrences. Heijnen et al. performed an MRI-based study in patients with a good response to chemoradiotherapy to evaluate the distribution of lymph nodes in this specific patient group . Lesion-by-lesion matching with histopathology was performed in 61/95 patients. In the remaining 34 patients who underwent organ-preserving treatment, definite proof of ycN0 status was determined by at least 2-year disease-free follow-up. In total 11 metastatic nodes were identified in 880 nodes. Even in patients with complete tumour response (ypT0), metastatic nodes were found in 5.6% of the cases . None of the metastatic nodes were located distal to the tumour level. Fifty-five percent of the metastatic nodes were located at the tumour level and the remainder proximal to the tumour level. Most metastatic nodes were located in the ipsilateral quadrant as the tumour . Median distance of metastatic nodes from tumour to node was 9 mm. Non-metastatic nodes were found distal to the tumour level in 5.4% of the cases. Most of the non-metastatic nodes were located proximal to the tumour level (76%) . The authors also stated that given the size decrease due to the chemoradiotherapy, remaining large nodes are more likely to be malignant, which can be of help to the radiologist during restaging of rectal cancer .
Koh et al. reported that 98% of all nodes in rectal cancer patients is located at or proximal to the tumour level . In a subsequent study of the same group, no nodes were found distal to the tumour level after chemoradiotherapy .
Knowledge of the nodal drainage and distribution of the rectum and rectal cancer is of clinical relevance for treatment planning and prognostication. The normal rectum drains via two pathways: inferiorly through the middle rectal artery and internal iliac chain and superiorly through the superior rectal artery and sigmoid mesentery. The very low anorectum drains through the inguinal and external iliac chain. Nodal metastasis follows the same drainage routes, depending on the tumour height. Metastatic nodes are almost always found at or proximal to the tumour level. If nodes are found distal to the tumour level, this usually occurs in middle and lower rectal cancer. Lateral spread is less common, but is associated with poor prognosis as is the proximal spread. A similar distribution is found after chemoradiotherapy. Metastatic nodes are usually found ipsilateral to the tumour, otherwise they are found lateral or posteriorly to the tumour. Anteriorly, few nodes are found, and these nodes tend to be small (<5 mm) even when they are metastatic. These results can guide tumour delineation for radiation, surgical planning, decision-making regarding adjuvant therapy and follow-up in rectal cancer.
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