Prepared by William R. Krueger, MA, BA, J.G. Fletcher, MD, and Thomas J. Sebo, MD, PhD of the Department of Radiology, Mayo Clinic, Rochester, MN.

CASE SUMMARY

A 70-year-old man presented to an outside institution with a vague sensation of rectal fullness. Digital rectal examination revealed a rounded, firm, and mobile mass along the left side of the anorectum, perceived as lying just superior to the dentate line. A barium enema was performed (figure 1), followed by pelvic magnetic resonance imaging (MRI) (figures 2 and 3).

DIAGNOSIS

Low-grade anorectal stromal sarcoma

IMAGING FINDINGS

Double-contrast barium enema demonstrated an intramural mass along the posterolateral aspect of the rectum with intact overlying mucosa (figure 1). Subsequently, magnetic resonance imaging of the anorectum was performed with a torso phased-array coil, before and after the administration of intravenous gadolinium. Fast-spin echo T2-weighted images revealed a 6 x 5 x 5-cm mass, located between the internal and external anal sphincters, extending from the subcutaneous external anal sphincter to the caudal aspect of the rectum, where it was located within the circular muscle layer. The tumor itself was of homogeneous signal intensity on T2-weighted fast-spin echo images, and slightly hyperintense compared with both sphincters. It contained a central region of cystic degeneration or necrosis (figure 3). T1-weighted images demonstrated no invasion of the perirectal fat or surrounding lymphadenopathy. The MR findings were felt to be consistent with an intramural gastrointestinal stromal tumor confined entirely to the wall of the anorectum.

The patient subsequently underwent a limited resection via a left parasacral incision. Pathologically, the lesion was well-circumscribed and located within the anorectal wall. Microscopic evaluation demonstrated a spindle cell neoplasm composed of acidophilic cells with an architectural growth pattern of intersecting fascicles. Mild to moderate nuclear atypia characterized by variability in size and shape was present. Mitoses were present but the overall mitotic rate was low (<3 per 10 high-powered fields), and tumor cell necrosis was not identified. These microscopic features, in conjunction with the tumor size and location, supported the diagnosis of an intramural low-grade malignant gastrointestinal stromal sarcoma of the anorectum.

DISCUSSION

Mesenchymal tumors of the gastrointestinal tract were formerly referred to as leiomyomas, leiomyosarcomas, or schwannomas, given their spindle cell growth pattern. Recent pathologic analyses have demonstrated, however, that most mesenchymal tumors of the gastrointestinal tract have unique histological, immunophenotypic and molecular genetic features that distinguish them from typical smooth-muscle and neurogenic tumors. ^1,2 These tumors lack features of typical smooth-muscle tumors such as actin-containing attachment filaments, positive staining for desmin, and other ultrastructural and immunohistochemical characteristics. These tumors usually express the CD117 antigen (c-kit protein) of the interstitial cells of Cajal, which are the intrinsic pacemaker cells of the gastrointestinal tract, as well as the CD34 antigen expressed by hematopoietic progenitor cells. They consequently may represent tumors of a primitive mesenchymal precursor cell. With the exception of esophageal leiomyomas and rare colorectal leiomyomas, which possess the pathologic characteristics of true leiomyomas, nearly all intramural mesenchymal tumors of the gastrointestinal tract are gastrointestinal stromal tumors. ^1,2

Only 10% of gastrointestinal stromal tumors are found in the colorectum, ^1,2 and only 10% of these are found in the anus. ³ Gastrointestinal stromal sarcomas in the anus and rectum account for only 0.07% to 0.1% of malignant tumors in this region. ^4,5 The most common presenting symptom is rectal bleeding, followed by rectal pain or fullness. Constipation or a palpable mass may also be present. ^3,5

Gastrointestinal stromal tumors are classified according to their features of differentiation and according to their potential for malignancy. ⁶ Size >5 cm in diameter, mucosal ulceration, presence of tumor necrosis, mitoses, high nuclear-to-cytoplasmic ratio, infiltration of the mucosa overlying the tumor, or infiltration of adjacent organs all predict malignant behavior, ⁶ but such criteria may be unreliable in the colon and rectum, where few data are available. ² While gastric stromal tumors tend to be benign, small bowel and colorectal tumors carry a greater risk of malignancy. Rectal stromal tumors tend to recur locally following resection, whether benign or malignant. ³ Consequently, abdominoperineal resection has traditionally been the treatment of choice for these tumors. ⁵ Recently, other therapies such as transanal excision accompanied by brachytherapy, or local excisions of small confined tumors have been advocated in order to preserve continence. ^7,8 Like other malignant stromal sarcomas, these anorectal tumors tend to metastasize to the liver and lungs, with lymph node metastases being relatively uncommon. ^2,3

Although little has been written regarding the MRI features of stromal sarcomas in the anus and rectum, several recent publications have discussed radiologic findings of these tumors.

Lee et al, ⁹ in an evaluation of the radiologic features of 12 leiomyomatous tumors of the colon and rectum, reported that 8 of 12 tumors showed varying degrees of internal necrosis, and all but 1 had at least a portion of the tumor that demonstrated exocolic growth. Calcification and heterogeneous enhancement were also more common in malignant tumors. One rectal tumor in Lee's series was imaged with MR and appeared as a large, lobulated, heterogeneously enhancing mass posterior to the bladder, but it was without a clear organ of origin, similar to other reports. ^10,11

In our case, preoperative imaging demonstrated clearly that the tumor lay entirely between the internal and external anal sphincters in the anus and was confined within the muscularis propria in the rectum, suggesting the tumor's intramural origin. While MR could not evaluate for mucosal ulceration overlying the tumor, preservation of the anorectal mucosa on the patient's barium enema (figure 1) suggested a less malignant stromal neoplasm. ⁶ Additionally, the crescent-shaped region of cystic degeneration or necrosis in this tumor supported the diagnosis of stromal sarcoma, and is described in the sonographic literature. ^12,13 Importantly, for our patient, the confined nature of the tumor on MR allowed for a sphincter-preserving left parasacral excision.

The imaging characteristics of this tumor favored anorectal stromal sarcoma over other intramural tumors of the anorectum. Rectal carcinoids can also occur in the anorectum but are usually <2 cm in diameter. ¹⁴ A tumor of the size reported here would be expected to demonstrate metastatic lymphadenopathy or hepatic metastases. Lymphoid polyps are also smaller than the tumor demonstrated here. ¹⁵ Intramural hemangiomas should display a more intense T2-weighted signal within the lesion, and are often accompanied by a serpiginous pattern of signal abnormality in the perirectal fat, representing engorged feeding vessels. ¹⁶

In conclusion, anorectal stromal sarcomas are uncommon mesenchymal tumors of the anorectum. The exclusion of metastatic disease and delineation of a confined tumor may allow for sphincter-preserving surgical excision. Intramural location, large size, positive contrast enhancement, lobulated borders, and an internal region of cystic degeneration or necrosis may suggest the preoperative diagnosis at cross-sectional imaging.