Leiomyomas of the rectum are rare, benign smooth muscle tumors that are uniformly positive for smooth muscle actin and desmin, and they are not kit-positive in contrast to that of gastrointestinal stromal tumors (GISTs). In the rectum, leiomyomas are encountered more frequently than lipomas. They originate from either the muscularis mucosa or muscularis propria and they are usually located in the distal rectum. Most leiomyomas arising from the muscularis mucosa appear as small, sessile, asymptomatic polyps. Leiomyomas from the muscularis propria may show a larger size and symptoms such as bleeding, discomfort or stenosis. The MRI findings of rectal leiomyomas have not been well described, but it is reported that they are isointense to mildly hyperintense on T2-weighted images (T2WI) when compared to that of muscle (3) (Fig. 1).

Lipoma is a rare, benign, slow-growing mesenchymal tumor. It is the second most common benign tumor in the rectum after leiomyomas, whereas it has been reported to be the most frequently encountered submucosal tumor in the colon. Rectal lipomas may cause rectal bleeding, pain or tenesmus. Liposarcomatous degeneration has not been reported and if the patient remains asymptomatic, then treatment is usually not necessary (1).

On imaging studies, most lipomas present as a broad-based mass with a smooth, well-demarcated margin. Although submucosal in origin, lipomas occasionally evolve into pedunculated lesions (1). MRI readily demonstrates the fatty nature of the mass as being isointense compared to the subcutaneous fat on T1WIs, and low signal intensity on T1WI with fat suppression (Fig. 2), which confirms the diagnosis of lipoma.

Based on the WHO classification of 2000, neuroendocrine tumors are categorized into three groups; well-differentiated neuroendocrine tumor (NET) (benign behavior or uncertain malignant potential; synonymous to carcinoids in gastroenteric NETs), well-differentiated neuroendocrine carcinoma (low-grade malignancy; synonymous to malignant carcinoid) and poorly-differentiated neuroendocrine carcinomas (high-grade malignancy; usually small cell neuroendocrine carcinomas) (4).

Rectal NETs are much more frequently observed than colon NETs, and the former usually shows a lower risk of metastasis and so they have a better prognosis. Tumor size and microinvasiveness are probably the two most important prognostic factors upon which treatment decisions may be based (< 1 cm: endoscopic or trans-anal resection; 1-2 cm without muscular invasion or lymph node metastasis: wide excision; 2 cm or greater, muscular invasion or lymph node metastasis: radical surgery). Metastasis are only likely in tumors that are 2 cm and larger and they invade the muscularis mucosa (4, 5).

Typically, rectal carcinoid is seen on MRI as a single discrete superficial submucosal mass smaller than 1 cm with contrast enhancement. Carcinoid tumors develop from the muscularis mucosa or the submucosal layer (6), so a superficial location or an intraluminal growing pattern may be related with the superficial layer of origin. The signal intensity of the lesion is usually isointense on T1WI and isointense to hyperintense on T2WI in comparison to muscles, and carcinoid tumors are homogeneously enhanced by contrast injection (3) (Fig. 3). Most poorly-differentiated neuroendocrine carcinomas are small cell carcinomas, and these are high-grade malignancy. Early dissemination and rapid clinical deterioration are common characteristics of this disease entity, which is one of the most aggressive tumors arising at the rectum (7). The usual imaging finding is similar to that of adenocarcinoma; an ulcerative tumor with or without metastases (8) (Fig. 4). If a lesion is discovered to be a neuroendocrine carcinoma, then it should be treated as an adenocarcinoma (9).

Gastrointestinal stromal tumors harbor kit mutations that cause the overexpression of kit protein (CD 117) and these tumor show differentiation toward (or they are derive from) interstitial cells of Cajal. The malignant potential (very low, low, intermediate and high risk) depends on the mitotic rate and the size of the tumor (10) (Fig. 5). The sites most commonly involved by GIST are the stomach (60-70%) and small intestine (20-25%), with about 5% of all GISTs arising at the rectum (11). It is generally agreed that complete surgical resection with negative tumor margins is the principal curative procedure for primary, non-metastatic tumors.

Gastrointestinal stromal tumors are usually seen as an eccentric mural mass with welldefined tumor margins. Calcification, necrosis and ulceration may be observed in the intermediate and high-risk tumors (12) (Fig. 6). The MRI features described for rectal GISTs are well-defined masses with low signal intensity on T1WI, they are hyperintense or isointense with hyperintense areas on T2WI and they exhibit marked heterogeneous enhancement after gadolinium administration (12).

Schwannomas are uncommon stromal tumors that are considered to arise from neuron-supporting, site-specific, modified Schwann cells in the mesenteric plexus, and they may develop in any anatomic compartment. Nerve fibers stretch around the mass and the tumor is usually encapsulated (13).

Among the case reports of schwannoma in the digestive tract, the most frequent sites are the esophagus and stomach, followed by the small intestine and the appendix. Schwannoma of the rectum is extremely rare (13, 14) (Fig. 7). The imaging diagnosis is often confusing, and differentiating them from other rectal submucosal tumors such as leiomyomas or GISTs is difficult even based on conventional pathological techniques. Immunohistochemical studies should be done for making a more accurate diagnosis.

Although schwannoma is considered benign, the risk of local recurrence and even distant metastasis cannot be ignored. The possibility of malignant transformation is correlated with the tumor size and mitotic activity (13). The treatment for benign schwannoma is surgical excision (13).

Primary rectal lymphoma involving the rectum is a rare type of gastrointestinal lymphoma. The diagnosis of primary colorectal lymphomas requires that there is a lymphoproliferative disorder confined to the colorectum and regional lymph nodes without involvement in the abdominal organs and bone marrow, and neither retroperitoneal nor medial lymphadenopathy is present (15).

Gastrointestinal lymphoma has usual features such as a bulky mass or diffuse infiltration. Preservation of the extramural fat planes is more likely to be seen in lymphoma than in adenocarcinoma in the presence of a bulky tumor. In addition, luminal obstruction is rare even when there is extensive lymphomatous infiltration. However, gastrointestinal tract lymphoma may have diverse radiologic manifestations and it may mimic adenocarcinoma. The MRI features of lymphoma usually show homogeneous intermediate signal intensity on T1WI and high signal intensity on T2WI (Fig. 8). Mild to moderate enhancement is seen after the intravenous administration of gadolinium-based contrast material (16). Treatment often involves a multimodality approach that combines surgery and chemotherapy, and radiotherapy is used in selected cases (17).

Malignant melanoma is a highly malignant tumor that rarely arises at the anorectum. It has been reported that anorectal malignant melanoma makes up 2.3% of all non-cutaneous-origin melanomas and 0.1% of all rectal malignancies. Since melanosis is often found in non-cutaneous melanoma patients, the origin of anorectal malignant melanoma might be the melanocytes in the squamous epithelium of the anal canal (18).

Malignant melanoma usually presents as a bulky intraluminal fungating mass in the distal rectum. It focally expands and obscures the lumen without causing obstruction and there is perirectal infiltration and frequently enlarged lymph nodes (19) (Fig. 9). Melanoma cells carry melanin pigments that contain free radicals, and this makes the cells paramagnetic, which consequently shortens the T1 relaxation time and increases the T2 relaxation time on MRI. This accounts for the high-in-low intensity seen on T1WI and the mixed intensity seen on T2WI (18, 20). Based on these MRI features, a few reports have stated that MRI is useful for the characterization of malignant melanoma (20). However, approximately 10% to 29% of anorectal melanomas are known to be the amelanotic type, and the MRI findings resemble rectal cancer (18, 20). Sometimes metastatic lymph nodes might contain pooled hemorrhage, and this presents as a high-intensity signal on MRI (20).

Endometriosis is defined as the presence of functioning endometrial glands and stroma outside the uterine cavity. This entity should be considered in the differential diagnosis if an extramucosal rectal mass is seen in premenopausal women.

The typical MRI features are high signal intensity on both T1WI and T2WI, and persisting high signal intensity is observed on fat suppressed T1WI (Fig. 10). T2WI can display a gradual variation of signal intensity and this phenomenon is described as "shading," and it is due to chronic bleeding with accumulated iron and protein. However, the disease can also present as either fibrous masses that appear to show speculation and retraction and hypointense signals on both T1WI and T2WI or fibrous masses with punctuate foci of hyperintense signals on T1WI and T2WI (21).

Developmental disorders should be considered in the differential diagnosis of retrorectal masses that have major cystic components. These lesions may have similar imaging features. Rough differentiation is possible by the fact that epidermoid cyst, dermoid cyst and rectal cystic duplication are more often unilocular, whereas the tailgut cysts tend to be multilocular. Demonstration of fat components is suggestive of a dermoid cyst. Rectal duplications often have communications with the rectal lumen and they are anterior to the rectum (22). However, the role of MRI for the detection of calcification is limited, and this should be supplemented with evaluation by other modalities, and especially when assessment of calcification is crucial for narrowing the possibility of dermoid cyst or teratoma.

Rectal involvement may occur via contiguous tumor spread from adjacent organs such as ovary or prostate (Fig. 15). Direct invasion of the rectum by an extra-rectal neoplasm may sometimes mimic rectal intramural submucosal tumors, and even on MRI and optical colonoscopy.