Intramuscular myxomas are a rare benign type of soft tissue myxoma most commonly seen in middle-aged women. On imaging, they are often seen in large muscles from the thighs, buttocks, or shoulder girdle, and present as well-defined cystic-like lesions with a surrounding rim of fat.
Intramuscular myxomas typically tend to occur in adults around 40 and 60 years 6. There is a recognised female predilection (70%). The reported incidence is ~0.10 per 100,000 6.
Intramuscular myxomas are benign soft-tissue tumours of mesenchymal origin that present as a deeply seated mass confined to skeletal muscle. It is classically described as hypocellular and hypovascular and is composed of cytologically bland stellate and bipolar fibroblasts separated by abundant extracellular myxoid matrix 2.
Typically seen as a well-circumscribed intramuscular mass. Some may have partially ill-defined borders.
They are typically hypoechoic to the surrounding soft tissue and often show a heterogeneous echotexture. Posterior acoustic enhancement may be observed in a significant portion cases.
Some lesions may say show a bright cap sign (i.e. peripheral rim of echogenicity) +/- a bright cap sign (i.e. increased echogenicity adjacent to one of the poles). 8
MRI features include:
- intratumoural cysts may be present in a small proportion of cases
- pseudocapsule, perilesional/rim of fat and perilesional oedema may be present
- T1: hypointense to muscle
- T2: hyperintense
T1 C+: four different patterns have been described
- peripheral enhancement
- peripheral and patchy internal enhancement
- peripheral and linear internal enhancement
- heterogeneous internal enhancement 10
On PET-CT FDG uptake may be present 5.
Treatment and prognosis
Intramuscular myxomas are benign. Surgical excision is virtually always curative. Recurrence (even after incomplete resection) is extremely uncommon.
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