Low-grade anorectal stromal sarcoma
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.
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
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
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
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
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.
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