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Diagnosis Please |
1 From the Department of Radiology, Yale New Haven Hospital, 20 York St, New Haven, CT 06511. Received December 15, 2002; revision requested February 27, 2003; revision received May 5; accepted June 23. Address correspondence to S.T.S.
Index terms: Diagnosis Please • Lipoma and lipomatosis, 44.319 • Pelvis, abnormalities, 44.319
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IMAGING FINDINGS |
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DISCUSSION |
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AAM is a rare benign soft-tissue tumor that contains myxoid and vascular components, as described by Steeper and Rosai in 1983 (2). Rare instances of AAM in men have been reported (3), but the majority of these tumors involve the pelvis—particularly the perineum—in women of childbearing age (4). AAM is a compliant tumor, with a propensity to grow to large sizes within the pelvis, particularly within the perineum. The tumor is locally infiltrative but tends to displace adjacent organs without invading them. AAM metastases have not been reported (1); nevertheless, AAM is regarded as an aggressive neoplasm because of its propensity to recur locally if it is not completely excised (3). Indeed, complete surgical excision has been reported (3) to be technically difficult secondary to the bulky and locally infiltrative nature of this tumor.
The typical imaging characteristics of AAM, including those seen at ultrasonography (US), CT, MR imaging, and angiography, led to a description of AAM as a hypervascular tumor that demonstrates indolent growth and surrounds the structures of the pelvic floor without invading adjacent organs.
CT scanning of AAM (5,6) typically demonstrates a well-defined moderately enhanced mass within the pelvis or perineum that is iso- or hypoattenuated relative to muscle.
On T1-weighted MR images, the tumor is isointense when compared with muscle (6); however, after administration of contrast material, there is moderate enhancement of the tumor. On T2-weighted MR images, there is high signal intensity within the tumor. Outwater et al (1) describe the tumor as having distinctive internal architecture composed of a swirled or layered appearance on contrast-enhanced CT scans and T2-weighted MR images. This effect is attributed to enhancement of the fibrovascular stroma that develops in AAM tumors as they protrude through the pelvic diaphragm. MR imaging shows a lack of high fat content, which is a key feature in differentiating AAM from the main differential considerations, myxoid liposarcoma and infiltrating angiolipoma.
Catalano (5) described the US findings of a single AAM that demonstrated a homogeneous and hypoechoic internal structure. In another report, US was used to depict AAM, and primarily cystic masses were noted (1). Studies of AAM with angiography all demonstrate a hypervascular mass with multiple feeding vessels (1).
At gross pathologic examination, AAM is a large bulky mass, partially or completely encapsulated, with a grossly gelatinous appearance. Measurements range from 3 to 60 cm, and fingerlike projections are commonly reported (2). Histologically, AAM (2) consists of spindle or stellate cells separated by a myxoid stroma with abundant fibroblasts, myofibroblasts, and variably sized vessels. Mitotic activity has been shown to be rare in most cases (3,7).
Surgical excision is the primary treatment for AAM. Although the tumor is histologically benign (2), locally aggressive behavior and high recurrence rates require complete excision, if possible. The appearance of AAM on MR images is valuable in the identification of characteristic features of this tumor, evaluation of tumor extent, and planning of surgery (6). Recurrence rates of 36%–72% (8) have been reported. Chan et al (9) propose that incomplete resection is acceptable because AAM is a benign tumor and because, in some cases, complete resection has high surgical morbidity and can threaten fertility in a patient population that is composed of mainly women who are in their child bearing years. Radiation therapy or chemotherapy is unlikely to be helpful, as patients with AAM demonstrate low mitotic activity (8,9). Long-term follow-up with CT and, preferably, MR imaging is recommended, since recurrences have been reported several years after first excision (8).
The differential diagnosis includes myxoma, which is another benign mesenchymal neoplasm that most frequently occurs in older patients (2). Myxoma resembles AAM microscopically, but it lacks the vascular component and does not enhance, as does AAM. While myxoma is mainly intramuscular (10), AAM may abut the pelvic or perineal musculature but does not invade it. In rare instances, non-Hodgkin lymphoma may be seen in this area.
Infiltrating angiolipoma is also a benign soft-tissue neoplasm similar to AAM in that it demonstrates no mitotic activity, requires wide surgical excision, and does not metastasize (11). Infiltrating angiolipoma differs from AAM; however, in that it is usually found in the thigh region. Regarding imaging, infiltrating angiolipoma—as the name implies—infiltrates and replaces involved muscle and subcutaneous tissue (12). Angiography demonstrates a hypervascular lesion similar to AAM; but it also demonstrates a poorly marginated lesion with large draining veins. Characteristic CT findings of infiltrating angiolipoma are a largely intramuscular tumor with a heterogeneous appearance secondary to gross fat and blood vessels within infiltrated muscle. Histologically, this tumor is composed of mature lipocytes with angiomatous proliferation.
Myxoid lipoma is a morphologic variant of lipoma and is thought by some to be the benign counterpart to liposarcoma (2). As with AAM, myxoid lipoma is predominantly myxoid in nature, but it can contain mature fat with a transition zone between myxoid and fat components. Myxoid liposarcoma, which is a malignant tumor, most commonly occurs in lower extremities within the intramuscular fat planes in middle-aged individuals who present with a painless slow-growing mass (13). There is homogenous enhancement after administration of contrast material, whereas AAM is more heterogeneously enhanced.
In summary, the most likely diagnosis in this young woman with a pelvic mass is AAM, since the constellation of imaging findings demonstrates a heterogeneously enhancing soft-tissue mass that displaces but does not invade adjacent pelvic organs.
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FOOTNOTES |
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Authors stated no financial relationship to disclose.
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This article has been cited by other articles:
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  M. K. Demir, H. Genchellac, H. Ozdemir, R. Sinha, and R. Verma Perineal Angiomyxomas: Can a Differential Diagnosis Be Made with Imaging Studies? Radiology, November 1, 2007; 245(2): 612 - 613. [Full Text] [PDF]
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