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The mesorectal fascia (MRF) is a thin fascial envelope surrounding the mesorectum (fat, vessels, lymph nodes around the rectum). In rectal cancer imaging, “MRF involvement” means the tumor, a metastatic node, or tumor deposit extends to or within ≤1 mm of the mesorectal fascia.
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Mesorectal fascia versus Peritoneum: The low rectum is totally covered by the mesorectal fascia (green line). In the mid rectum the mesorectum is covered by the mesorectal fascia on the posterior and lateral side, but on the anterior side it is covered by the visceral peritoneum (red line indicating the peritoneal reflection). In the high rectum the peritoneal lining extends from the anterior to the lateral side (yellow line) and the MRF only lines the dorsal part of the mesorectum.
Doenja Lambregts, Rhiannon van Loenhout, Frank Zijta, Max Lahaye, Regina Beets-Tan and Robin Smithuis, Radiology Departement of the Netherlands Cancer Institute in Amsterdam, the Medical Centre Haaglanden in the Hague and the Alrijne Hospital in Leiderdorp, the Netherlands The Radiology Assistant : Rectal Cancer MR staging 3.0. Published September 1, 2021. https://radiologyassistant.nl/abdomen/rectum/rectal-cancer-mr-staging-1
Progressive envelopment of the upper rectum by peritoneum. (A) Sagittal illustration shows the peritoneal cavity (pink overlay) and peritoneum (curvilinear white line). (B–F) Cross sections show the extraperitoneal or lower rectum, the APR, and the relationship of the MRF to the peritoneum in the upper rectum. The sigmoid mesocolon above the STO is also depicted. There are corresponding MR or CT images for each section. (B) Axial illustration and T2-weighted MR image below the APR show the extraperitoneal or lower rectum surrounded by MRF on the illustration (dark brown line) and MR image (arrowheads). (C) Axial illustration and T2-weighted MR image at the APR show the seagull appearance of the APR on the illustration (white line) and MR image (arrow). An anterior rectal tumor is near but not involving the APR. Arrowheads on the MR image = MRF. (D) Axial illustration and T2-weighted MR image above the APR. The illustration shows that peritoneum coverage anteriorly (curved white line) has increased, with a corresponding reduction in MRF (brown line). The MR image shows smaller MRF posteriorly (arrowheads). The peritoneum (blue arrows) is delineated by ascitic fluid, which allows visualization of the lateral confluence of the peritoneum and MRF (orange arrows). The uterus is seen anterior to the rectum, and incidental note is made of a left ovarian mass (white arrow). (E) Axial illustration and T2-weighted MR image of the upper rectum just proximal to the STO. The illustration shows peritoneum almost completely encircling the rectum, with a small amount of presacral MRF (brown line) remaining. The MR image shows the peritoneum outlined in a white line and presacral MRF (arrowheads) posteriorly. (F) Axial illustration and coronal contrast-enhanced CT image of the sigmoid colon and mesocolon. The illustration shows the sigmoid mesocolon with sigmoid arteries suspended in the peritoneal cavity (pink shading) and entirely encircled by peritoneum, except at the point of attachment of the mesocolon. The CT image shows ascitic fluid outlining the sigmoid colon and fat and arteries in the sigmoid mesocolon (arrow).
Kaur H, Gabriel H, Awiwi MO, et al. Anatomic Basis of Rectal Cancer Staging: Clarifying controversies and misconceptions. Radiographics. 2024;44(7). doi:10.1148/rg.230203
Imaging pearls: