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An otherwise healthy 57-year-old woman presented with a painless, palpable, firm, slightly movable mass on the medial aspect of the left thigh. Magnetic resonance imaging revealed a well-defined tumor with low signal intensity relative to the surrounding skeletal muscle on T1-weighted images and hyperintense signal relative to muscle on T2-weighted images. Received was a 8.0 x 6.8 x 4.4 cm portion of skeletal muscle containing a well-circumscribed 4.7 x 3.8 x 2.5 cm tumor with a uniform yellow-white cut surface.
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2006, Case 36 and is an Intramuscular myxoma.
Criteria for Diagnosis and Comments:
The diagnosis is intramuscular myxoma. This benign neoplasm was first described by Dr. Franz Enzinger in 1965. It has been described in patients aged 20 to 89 years, although most are between 40 and 70 years. Approximately two-thirds of the patients are female. Almost half the tumors are located in the thigh and range in size from 2.0 to 15.0 cm (average 5.6 cm).
Grossly, intramuscular myxomas are oval or spherical in shape and appear well-encapsulated. The cut surface is mostly solid, somewhat lobulated, and greasy. These tumors consist of three main histologic components: interstitial mucin, sparse spindle-shaped stromal cells, and fine fibrillary reticulin fibers that vary in number and distribution. Immunohistochemically, the cells are anti-vimentin immunoreactive but fail to stain for S-100 protein. The mucoid material is composed of hyaluronic acid and stains positively with alcian blue, mucicarmine, and colloidal iron.
Intramuscular myxoma is most commonly a solitary lesion. When they occur in association with fibrous dysplasia (Mazabraud's syndrome), they may be either multiple or solitary. Multiple tumors are seen in McCune-Albright syndrome and exist concurrently with polyostotic fibrous dysplasia, café-au-lait spots, and endocrine hyperfunction (most commonly precocious puberty). Rarer associations of intramuscular myxoma have been reported with the development of osteogenic sarcoma in the setting of Mazabraud's syndrome.
Extraskeletal myxoid chondrosarcoma affects adult men greater than women and is also typically found in the thigh. Local recurrence and late metastasis is characteristic. The lesions are well-circumscribed and are composed of cords and nests of cells within a myxoid stroma consisting predominantly of chondroitin sulfate. Cartilage is present in less than ten percent of the cases. Recently, chromosomal rearrangements involving 9q22, such as t(9:22)(q22;q12) have been described. These result in NR4A3 fusion genes that are present in greater than 90% of these tumors.
Low-grade fibromyxoid sarcoma is a rare malignant neoplasm that is typically situated in the deep soft tissues of adults, but may be seen in the trunk or retroperitoneum. Histologically, it consists of cellular areas with spindle-shaped cells that alternate with dense fibrous tissue and hypocellular myxoid nodules. They appear reasonably circumscribed but possess micro-infiltrative margins. Mitoses are rare and often there is swirling configuration of the tumor cells around small arcing blood vessels. By cytogenetic and molecular genetic analyses these lesions are now known to have a t(7;16) (q32-34;p11) balanced translocation which in the vast majority of cases results in a FUS/CREB3L2 fusion gene. In a subset of cases a variant FUS/CREB3L1 has been recognized.
Myxoid liposarcoma occurs most commonly in the extremities, specifically the thigh. It is more cellular than intramuscular myxoma with spindle-shaped to plump stellate cells, some of which may show mild cytologic atypia. A plexiform capillary pattern is characteristic, and well-differentiated lipoblasts help to identify this entity. Mitoses are rare in this well circumscribed/pseudo-encapsulated lesion. Soft tissue metastases may evolve.
Myxofibrosarcoma is a locally aggressive tumor and occurs predominantly in older adults. Approximately 70% are situated in the subcutis of the extremities. Histologically, low grade lesions which are in the differential diagnosis of intramuscular myxoma, are hypocellular, consisting of spindle cells within an abundant myxoid background. In comparison to intramuscular myxoma, the nuclei are usually more hyperchromatic and exhibit more cytologic atypia. Curvilinear vessels are a characteristic feature of this poorly demarcated neoplasm.
Nerve sheath myxoma tends to affect the extremities of men and women over a broad age range. It characteristically forms superficial, multilobular masses with myxoid stroma. The lesional cells are spindled, stellate, or epithelioid Schwann cells. The epithelioid Schwann cells typically have a corded arrangement or are grouped into syncytial-like aggregates. Fibrous bands surround the lobules and outer border of the lesion. This lesion is usually S100 protein and GFAP immunoreactive.