GI – Anorectal Staging

“The proximal rectum is covered by peritoneum anteriorly and laterally, and the mid rectum is only partially covered anteriorly. The distal rectum is entirely extraperitoneal and is surrounded by perirectal fat, mesorectal lymph nodes, and vessels, all of which are encased by the mesorectal fascia. The mesorectal fascia tapers inferiorly and fuses with the anal sphincter.”

“The anatomic rectosigmoid junction is often defined at the sacral promontory or S3 to aid in treatment planning.”

“High-resolution T2-weighted imaging is the key sequence in the magnetic resonance (MR) imaging evaluation of primary rectal cancer. This sequence generally consists of thin-section (3-mm) axial images obtained orthogonal to the tumor plane, with an in-plane resolution of 0.5–0.8 mm.”

“This technique allows differentiation between rectal tumors confined within the rectal wall (stage T2 tumors) and those that extend beyond the muscularis propria (stage T3 tumors). Most important, the depth of invasion outside the muscularis propria can be assessed with a high degree of accuracy. In addition, high-resolution T2-weighted images allow the morphologic assessment of pelvic nodes, thereby improving accuracy in the characterization of nodes as benign or malignant, since size criteria have proved to be of limited value.”

“Even with TME (total mesorectal excision), however, the presence of a tumor or malignant node within 1 mm of the CRM (circumferential reference margin) remains an important predisposing factor for local recurrence.”

“These studies also revealed that radiation therapy yields little survival benefit and results in significant morbidity when used to treat stage T1–T2 or favorable-risk early stage T3 tumors (<5 mm invasion outside the muscularis propria) in contrast to more advanced stage T3 tumors (>5 mm invasion outside the muscularis propria)”

“Recent studies have shown that high-resolution MR imaging is a reliable and reproducible technique with high specificity (92%) for predicting a negative CRM, the relationship of the tumor to the CRM, and the depth of tumor invasion outside the muscularis propria.”

“On T2-weighted images, stage T1 tumors are confined to the submucosa, which manifests as a hyperintense layer; stage T2 tumors extend into, but not beyond, the muscularis propria, which manifests as a hypointense layer; and stage T3 tumors extend beyond the muscularis propria into the mesorectal fat.”

“A distance greater than 1 mm between the tumor and the CRM at histopathologic examination has been shown to correlate with a decrease in local recurrence. At our institution, we consider a measured distance of 1 mm or less on high-resolution T2-weighted images to be indicative of CRM involvement. It is critical to remember that this measured distance is the distance to the mesorectal fascia from either (a) the tumor margin, (b) a tumor deposit in the mesorectum, (c) tumor thrombus within a vessel, or (d) a malignant node.”

“If a malignant node or tumor deposit abuts (ie, is less than 1 mm from) the mesorectal fascia, this information is important to the surgeon, who must stay well clear of the tumor at that margin. In the United States, nodes outside the mesorectal fascia along the pelvic sidewall are not routinely resected. However, if involvement of these nodes can be established preoperatively, it is important to modify the treatment approach to avoid recurrence in untreated nodes. Involved extramesorectal lymph nodes can be targeted with a widened field for preoperative radiation therapy and extended surgical resection.”

“It is well established that nodal size is of limited value in assessing for the presence of metastasis. The most frequently used size criterion for distinguishing malignant from nonmalignant nodes (ie, 5 mm) has a sensitivity of 68% and a specificity of about 78%. The limited accuracy of nodal size is likely related to the fact that 30%–50% of metastases in rectal cancer occur in nodes that are less than 5 mm. Recently, it was reported that nodal margins and internal nodal characteristics are the most reliable indicators of malignancy. Features that are suggestive of malignancy include irregular or spiculated nodal margins and heterogeneous signal intensity.”

“The most significant landmark of the anal canal is the dentate line, which lies 2.5–3 cm proximal to the anal verge and is visible macroscopically but not on MRI. Its position can be estimated either by measuring 2.5 cm above the anal verge or by dividing the anal canal into thirds so that the dentate line lies at the junction of the middle and upper thirds.”

“At initial staging, most tumors are T1 or T2, lie within 1.5 cm of the anal verge, have a circumferential extent of less than 50%, and are located predominantly along the anterior aspect of the anal canal.”

“The sphincter complex is the most commonly infiltrated structure, followed by the rectum. It should be emphasized that direct invasion of the rectal wall, perianal skin, subcutaneous tissues, or sphincter muscles does not signify a T4 tumor. Invasion of organs such as the vagina, urethra, prostate, or bladder is required to diagnose T4 disease.”

“MRI is especially useful in staging large tumors, particularly when the craniocaudal dimension is the largest dimension or when tumors protrude beyond the anal verge, because endoscopy is inadequate in these cases.”

“Nodal drainage depends on which side of the dentate line the anal cancer has its epicenter. Anal margin and anal canal tumors inferior to the dentate line spread to the inguinal and femoral lymph nodes, whereas anal canal tumors superior to the dentate line drain into the perirectal, internal iliac, and retroperitoneal nodes.

“Nodal staging evaluation relies on the distance from the primary tumour rather than on the number of involved nodes. MRI is highly helpful to assess lymph node metastatic involvement, although the mere size criterion is far from accurate and associated with both false-positive and false-negative results. Short-axis threshold values of 8 mm, 5 mm and 10 mm have been suggested for pelvic, perirectal and inguinal lymph nodes, respectively. Additional helpful features to increase specificity include loss of the normal bean-shaped morphology and fatty hilum, internal T1 and T2 signal heterogeneity with central necrosis, and inhomogeneous enhancement.”