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1Department of Diagnostic Radiology, Singapore General Hospital, Outram Road, Singapore 169608 and 2Department of Pathology, University of Hong Kong, Hong Kong, China
Correspondence: Dr W C G Peh
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)? What is the diagnosis?
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). Excision of the mass confirmed the pre-operative diagnosis of intramuscular myxoma. The cut sections of the well defined intramuscular tumour had a gelatinous whitish appearance. Histological examination showed that the tumour was composed of stellate and spindle cells within abundant myxoid stroma, the typical features of an intramuscular myxoma. Myxomas are uncommon benign neoplasms that occur in a variety of locations, including the heart, bones, skin, subcutaneous and aponeurotic tissue, genitourinary tract and skeletal muscle. In 1871, Virchow used the term myxoma to describe a tumour that histologically resembled the mucinous substance of the umbilical cord. Myxoma was subsequently established as a true mesenchymal neoplasm composed of undifferentiated spindle- and stellate-shaped cells in a background of myxoid stroma. It is generally accepted that myxomas originate from primitive mesenchymal cells that differentiate into altered fibroblasts. These cells lose their capacity to produce collagen and instead produce excess hyaluronic acid and immature collagen fibres.
Intramuscular myxomas frequently occur in middle-aged patients (mean age 50–55 years). There is a slight female preponderance. Most intramuscular myxomas are solitary, painless masses that grow slowly, or are stable in size for prolonged periods of time prior to sudden enlargement. They tend to involve the muscles of the thigh, buttocks, upper arm, shoulder and, rarely, the head and neck region. Intramuscular myxomas tend not to recur or metastasize. On gross pathological inspection, the tumour is oval or spherical in shape, and when cut open it has a white or grey-white mucoid gelatinous surface. Small cyst-like spaces or traversing trabeculae may also be seen. Although it appears well encapsulated, the delicate fibrous capsule is usually incomplete and most lesions show infiltration of adjacent musculature. Microscopically, myxoma is composed of undifferentiated stellate cells and an irregular meshwork of reticulum fibres in a matrix of hyaluronic acid-containing mucoid [1].
The association of soft tissue myxoma with fibrous dysplasia, especially the polyostotic form, is a rare but well recognized entity, also known as Mazabraud syndrome. The diagnosis of fibrous dysplasia usually antedates the intramuscular myxoma by several years. The myxomas are usually multifocal, occur in close vicinity to the bone lesions and have a curious predilection for the right limb.
On ultrasound examination, intramuscular myxoma appears as a hypoechoic mass with small, fluid-filled clefts, cystic spaces and absent intratumoral flow on colour Doppler examination. Intramuscular myxoma is seen as a well circumscribed, homogeneous, low attenuation, non-enhancing intramuscular mass on CT. MRI criteria of intramuscular myxoma allows a pre-operative diagnosis to be made in most cases. The MRI findings are typically a wholly intramuscular location, sharply defined margins, signal hypointensity lower than that of skeletal muscle on T1 weighted images, marked signal hyperintensity exceeding that of fat on T2 weighted images and heterogeneous enhancement. The T2 signal hyperintensity resembles that of fluid, while the pattern of enhancement has been described as being septum-like [2–4]. Although routine iv gadolinium is not warranted for every musculoskeletal mass, it is helpful in establishing the diagnosis of intramuscular myxoma. On unenhanced MRI, this lesion may be indistinguishable from a cystic lesion such as intramuscular ganglion.
Some malignant soft tissue tumours, namely liposarcomas, chondrosarcomas and malignant histiocytomas, may undergo myxomatous degeneration and simulate intramuscular myxoma. Soft tissue sarcomas have abundant mitotic activity, rich vascularity and specific cellular elements such as lipoblasts or chondroblasts. Differentiating features from myxomas on MRI include signal heterogeneity on T2 weighted images, septation, ill defined margins and invasion of adjacent structures [5]. Features that help to differentiate myxoid liposarcoma from myxoma are intense enhancement of 65–100% of the tumour volume and high signal intensity from fatty foci on T1 weighted MR images [6].
Treatment for intramuscular myxoma is surgical excision. Pre-operative diagnosis of intramuscular myxoma facilitates the avoidance of a biopsy, permits primary myectomy and assists the pathologist in the diagnosis of a tumour that frequently has overlapping histological features with other soft tissue tumours [4].
Received for publication November 13, 2000. Accepted for publication November 27, 2000.
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This article has been cited by other articles:
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  J. S. Wu and M. G. Hochman Soft-Tissue Tumors and Tumorlike Lesions: A Systematic Imaging Approach Radiology, November 1, 2009; 253(2): 297 - 316. [Abstract] [Full Text] [PDF]
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