Clinical History
A 67-year-old-woman reporting a solitary painless mass in the right inner thigh (just above the knee), with evolution and slow growth for one year.
Imaging Findings
US: Solid lesion located in the inner right thigh, with oval morphology and well defined contours, showing a heterogeneous echogenicity with hypoechoic areas within it suggestive of necrosis or cystic degeneration. The tumour shows minimal vascularisation in colour Doppler study.
MRI: Intramuscular lesion located in right vastus medialis, with oval morphology and well defined contours, showing low signal intensity on T1-weighted sequences and high on T2-weighted sequences without fat component seen inside. The tumour has a minimum internal enhancement and peritumoral oedema discrete. Neither remodelling, destruction or infiltration of adjacent bone were observed, nor spread to other muscle groups of the thigh.
Discussion
Intramuscular myxomas are considered to be mesenchymal tumours arising from fibroblasts. A traumatic factor in the genesis is unlikely, since a history of trauma is only present in less than 25% of cases. Occurring almost exclusively in individuals between the fifth and seventh decades, intramuscular myxoma is a tumour of adult life. The tumour is rare in young persons and virtually nonexistent in children. Female patients outnumber male patients by a narrow margin. [1-3]
An association exists between multiple intramuscular myxomas and fibrous dysplasia of bone, and is referred to as Mazabraud’s syndrome. A relationship between myxoma of the soft tissues and McCune-Albright syndrome has been mentioned in the literature. [1-4]
In the majority of cases the only sign is a solitary, painless mass that is firm and often fluctuant on palpation. At the time of diagnosis most lesions measure 5–10 cm in diameter. The areas most frequently involved are the large muscles of the thigh, shoulder, buttocks, and upper arm. [1, 2]
As intramuscular myxomas do not contain calcifications, conventional radiographs are of little value in the diagnosis: they may be normal or reveal a nonspecific soft tissue mass. [1-4]
On ultrasound, myxomas are usually well defined and hypoechoic. There may be fluid clefts or cystic areas. Often there is a peripheral rim of increased echogenicity (termed the “bright rim” sign) and a triangular hyperechoic area adjacent to at least one of the poles of the mass (termed the “bright cap sign”), corresponding to muscle atrophy around the mass. [4]
On CT the myxoma presents as a sharply demarcated mass within skeletal muscle. The attenuation of the lesion is intermediate between that of water and muscle. Owing to the paucity of vascular structures within the lesion, the myxoma presents as a poorly vascularised soft tissue mass surrounded by well-vascularised muscle on angiograms. [1, 2]
On MRI intramuscular myxoma presents as a well-circumscribed, homogeneous intramuscular mass. Signal intensity is low on T1-weighted images and very high on T2 weighted images, quite similar to the signal characteristics of fluid. In some cases of myxoma the presence of fat has been demonstrated. As a result, the appearance on MRI of these myxomas may be indistinguishable from that of liposarcoma. Following administration of gadolinium chelates, inhomogeneous enhancement is observed. The areas of low signal intensity on the contrast enhanced images represent cystic areas on histologic examination. [1-5]
Differential Diagnosis List
Intramuscular myxoma
Myxoid liposarcoma
Myxofibrosarcoma
Sinovial cyst
Peripheral nerve sheath tumour
Myxoid chondrosarcoma
