Beyond adenocarcinoma: MRI of uncommon rectal neoplasms and mimickers.

The most common histological type of rectal neoplasms is adenocarcinoma; however, other histological types may occur within the rectum. Their characteristics are different, including risk factors, clinical manifestations, imaging findings, staging, treatment and prognosis. Familiarity with the MRI appearance of these uncommon rectal neoplasms is useful for abdominal radiologists ( Table 6 ).

In female patients, deep bowel invasive pelvic endometriosis can mimic a rectal mass clinically and on imaging [ 61 ]. The mass can be extrinsic or present with nodular thickening and most often involves the rectosigmoid colon. It can cause obstruction, and patients may present with bleeding and rectal pain [ 61 ]. The symptoms may be cyclic, related to the menstrual cycle. Other features of deep pelvic endometriosis on MRI may provide clues that it is in fact not a conventional rectal mass, including the presence of an ovarian endometrioma, T2 hypointense thickening or nodularity of the pelvic ligaments, thickening of the torus uterine or a “kissing” morphology of the ovaries. An intermediate signal on T2-weighted images with foci of T1-hyperintensity is a classic finding. Restricted diffusion can be seen as well but does not distinguish deep pelvic endometriosis from other neoplasms [ 61 , 62 ]. When deep pelvic endometrial implants invade through the serosa into the muscular layer of the rectum or colon, it forms a classic “mushroom cap sign,” wherein low signal endometrial implants in the rectal wall are covered by a higher signal thickened mucosa that forms a “cap” [ 63 ] ( Figure 14 ). Other primary malignancies in the pelvis can invade the rectum locally, and it can be difficult to ascertain the primary lesion. Other pelvic organs such as the uterus, ovaries and prostate may have aggressive biology with local rectal invasion.

Solitary rectal ulcer syndrome (SRUS) is a rare condition predominantly seen in young women, in which the lamina propria is replaced by fibrous tissue. Clinically, it can present with hematochezia and rectal prolapse, and on endoscopy it may appear as erythematous or ulcerated mucosa, sometimes with a polypoid or circumferential mass. Because of the clinical and endoscopic features, it is often initially thought to be a rectal cancer [ 51 , 52 ]. MRI features of SRUS have been described in the literature [ 51 , 52 ]. The MR appearance of SRUS may be variable, as either a polypoid or circumferential mass. Submucosal cysts within the mass have been described, but they are not consistently present [ 52 ]. Ultimately, histopathology is the reliable modality to make this rare diagnosis ( Figures 9 – 10 ). Colonic lipomas are the most common benign non-epithelial tumors in the GI tract [ 7 ]. In the rectum, however, lipomas are very rare [ 53 ]. They may be submucosal (90%) or subserosal (10%) [ 7 ]. Patients are usually asymptomatic, but when the lipoma is > 2 cm, bleeding or constipation can occur [ 7 , 53 , 54 ]. On MRI, lipomas demonstrate signal loss with fat suppression and minimal or no enhancement [ 7 ] ( Figure 11 ). If the patient is asymptomatic, no treatment is required [ 55 ]. Rectal hemangiomas are also rare, but in the colon the rectosigmoid region is the most common location [ 56 ]. Hemangiomas in the GI tract may be single or multiple and have syndrome associations (i.e., Maffucci, Klippel-Trénaunay) [ 22 ]. Recurrent, painless bleeding is common, and these lesions can be diagnosed at any age [ 57 , 22 , 58 ]. MRI may show a submucosal, pedunculated, polypoid or infiltrative lesion with very high T2-signal and adjacent serpiginous vessels [ 57 ]. Other clues that the lesion may be a hemangioma include the presence of pelvic phleboliths, often better seen on CT, and increased T2 signal intensity in the peri-rectal fat. Pre-operative embolization can be used to reduce the blood flow and reduce intraoperative bleeding if surgical resection is planned [ 59 ] ( Figure 12 ). Occasionally, infectious processes can mimic a colon or rectal mass. One such entity is basidiobolomycosis, a fungus that is present in arid climates. In the abdomen, it most commonly presents as focal bowel thickening and is often mistaken for a colonic mass or inflammatory bowel disease. It can also affect the liver and may be a cause of misdiagnosis. Biopsy should confirm the diagnosis [ 60 ] ( Figure 13 ).

Rectal cancer is the fourth most common cancer in the United States, with an overall incidence of 11.7 cancers per 100,000 persons for all ages and 44 cancers per 100,000 persons for those over 65 years [ 1 ]. There has been a rapid increase in the incidence of rectal cancer in younger patients, as those born in 1990 have four times the risk of developing rectal cancer compared to those born in 1950 [ 2 ]. By far, adenocarcinoma is the most common histologic type of rectal cancer. MRI is the established imaging modality for local staging of rectal adenocarcinoma and has important implications for patient management and outcomes [ 3 , 4 ]. In addition to adenocarcinoma, there are a variety of less common rectal neoplasms and non-neoplastic mimics. The practicing abdominal radiologist should be familiar with the imaging features of these rare rectal masses, as the management and prognosis of these masses can be significantly different compared to that of rectal adenocarcinoma. The aim of this article is to review some of the rare rectal tumors beyond adenocarcinoma, with a particular emphasis on the MRI imaging features, local staging, standard treatment, and prognostic implications.