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Myxoma

Written by Sean V. McGarry, MD

Introduction
Myxoma is an uncommon tumor of soft tissue. There are several histological and anatomical varieties of myxoma, all with similar radiographic and pathological characteristics. Included among these varieties are intramuscular myxoma, juxta-articular myxoma, myxoma of the jaw, and cutaneous myxoma. This brief synopsis will focus primarily on intramuscular myxoma, the type most commonly encountered by musculoskeletal oncologists. Intramuscular myxoma is somewhat unique in that it can present as a deep (intramuscular) and large mass that can be similar in appearance to a sarcoma, but unlike sarcoma is readily curative with simple excision.

Epidemiology
Myxomas occur in a female-to-male ratio of 2:1. Most cases of myxoma present in the fifth, sixth, and seventh decades of life. Myxomas are uncommon in patients aged younger than 40 years and unheard of in children.¹

Sites of Predilection
Myxomas occur most commonly in a subfascial location in the proximal extremities. The most common location is the proximal thigh, followed by the upper arm. Myxomas are rare in the distal extremities or in the head and neck.

Clinical Presentation
The most common presentation of a myxoma is as a painless, enlarging mass. In a 2002 study by Silver et al, in a case series of 17 patients, only 6 (33%) of the patients presented with pain.² The patient will often describe a mass that slowly enlarges during the course of months to years. Systemic symptoms or symptoms in the distal extremity are uncommon.

Radiographic Findings
Plain Radiographs: Other than perhaps a soft tissue shadow suggesting a discreet mass, plain radiographs are not helpful in the diagnosis of myxoma. A 2002 study by Murphey et al found a soft tissue shadow in less than half (45%) of 20 cases studied.³

CT Scan: CT imaging of myxoma lesions demonstrates low attenuation (less than that of muscle). After the addition of intravenous (IV) contrast, enhancement is variable and dependent on the vascularity of the lesion. Myxomas are often difficult to distinguish from a cystic lesion on CT imaging.

Magnetic Resonance Imaging: Magnetic resonance imaging (MRI) is more helpful than CT for diagnosis of soft tissue masses. On T1-weighted imaging nearly all lesions show a low-to-intermediate signal intensity, most commonly homogeneous but variable, with some showing heterogeneity. Myxoma shows a markedly high signal intensity of T2-weighted imaging, again most commonly homogeneous but variable, with some showing heterogeneity. Imaging performed after the administration of IV contrast can show edema in adjacent tissues. In the same study by Murphey et al, edema was seen in 26 (79%) of 33 cases studied.³ Enhancement is almost always heterogeneous, and again dependent on the vascularity of the lesion.

Radiographic Differential Diagnosis
Myxoid sarcomas (myxoid MFH, myxoid liposarcoma, extra-skeletal myxoid chondrosarcoma) often have many of the same radiographic characteristics as myxoma, particularly a myxoma that shows more heterogeneity in signal intensity on MRI imaging. Often distinction between the two will require a biopsy. Physicians should note that fine needle aspirate is not particularly useful in diagnosing myxoid tumors, due to significant sampling error.

Schwannoma or neurolemmoma are also radiographically similar in appearance to myxomas. Myxomas that have a more homogeneous appearance are difficult to distinguish from nerve sheath tumors. However, oftentimes, on coronal or sagittal imaging, a tail can be seen on the lesion representing the nerve of origin in the case of a nerve sheath tumor.

Ganglion cysts can sometimes resemble a myxoma on imaging. The administration of IV contrast can differentiate between a completely cystic lesion (ganglion) and a vascular (even hypovascular) lesion such as a myxoma.

Histopathology
On gross examination, myxomas appear to be well circumscribed. They have a mucinous pearly white cut surface. Fibrous septa can be seen within the lesion. On microscopic examination the overall appearance is that of a myxoid matrix interspersed with a bland appearing spindle or stellate cellular component. There is very little cellular atypia and very rare mitosis. Overall the lesions are hypocellular, but may contain nests of hypercellular areas, in addition to acellular cystic areas. Fibrous septa give the overall architecture a multi-lobulated appearance. In general, the lesions are hypovascular.

Treatment
Once a lesion has been identified, a careful survey of the patient’s imaging studies is required. Lesions that are more homogenous in appearance are more likely to be benign. Lesions that are more heterogeneous in appearance raise more of a concern for a malignant diagnosis. Therefore, if the lesion is stable and asymptomatic (or found incidentally), it can be conservatively followed with imaging to prove stability. If the lesion is symptomatic or growing, and a diagnosis is certain, then simple excision is performed. As noted previously, fine needle aspirate is problematic due to sampling error. If there is any question regarding the diagnosis, the best plan is probably open incisional biopsy to be followed by excision of the mass if a benign diagnosis is proven.

Prognosis
Excision is almost always curative and recurrences are rare. In a 1998 series by Nielsen et al, there were no recurrences in the 32 patients for whom follow-up information was available.⁴

Myxomas are almost always solitary in presentation. However, very rarely they will present as 2 or more lesions in the same patient. Typically, myxomas will occur in the same area of the body and occur in association with monostotic or polyostotic fibrous dysplasia. This association is known as Mazabraud syndrome. Mazabraud syndrome occurs significantly more frequently in women than in men (70:30). The fibrous dysplasia is typically diagnosed when the patient is in their teens or twenties. The myxomas are diagnosed (not unlike in simple myxoma) when the patient is in fifth, sixth, and seventh decades. These patients are at an increased risk for osseous malignant transformation (ie, their benign fibrous dysplasia will degenerate into osteosarcoma or another primary sarcoma of bone).

References

Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors. 5th ed. New York, New York: Mosby; 2011.
Silver WP, Harrelson JM, Scully SP. Intramuscular myxoma: a clinicopathologic study of 17 patients. Clin Orthop Relat Res. 2002;403:191-197.
Murphey MD, McRae GA, Fanburg-Smith JC, et al. Imaging of soft-tissue myxoma with emphasis on CT and MR and comparison of radiologic and pathologic findings. Radiology. 2002;225(1):215-224.
Nielsen GP, O'Connell JX, Rosenberg AE. Intramuscular myxoma: a clinicopathologic study of 51 cases with emphasis on hypercellular and hypervascular variants. Am J Surg Pathol. 1998;22(10):1222-1227.