Age / Sex : 66 / M

Age / Sex : 66/M

Chief complaint : Palpable mass in left posterior arm.

Intramuscular myxoma

Discussion

Findings: Longitudinal US scan shows heterogenous mass with posterior acoustic enhancement and triangular bright cap on the top of the mass. MR signal intensity of the mass is very high signal intensity on T2WI like cyst, but the lesion is heterogenously enhanced after Gd enhancement. T1WI shows thin high signal intensity fat rind in the periphery of the mass. Muscle edema and fat deposition with striated pattern are noted in the proximal portion of the mass on MR T2WI, corresponding to “bright cap” on US image.

Differential Diagnosis: myxoid liposarcoma

Diagnosis: Intramuscular myxoma.

Discussion: The typical intramuscular myxoma is a well-defined ovoid lesion with fluid-like signal intensity, a peritumoral fat rind visible on T1-weighted MR images, and an increased signal in the adjacent muscle on T2-weighted or fluid-sensitive MR sequences. These features are the result of the infiltrative pattern of slow growth seen in intramuscular myxoma. Intramuscular myxoma is a paucicellular lesion composed of a mucoid basophilic matrix rich in mucopolysaccharide. Although lesions may appear well defined on imaging studies, these lesions have no capsule and will infiltrate the adjacent atrophic and edematous striated muscle. Mucoid matrix from the lesion may split the cells or become embedded in them. MR imaging planes along the long axis of the involved muscle yielded the greatest information regarding tumor border definition and differentiation of signal in the lesion and the adjacent muscle. The rind of adipose tissue is likely reactive fat, caused by muscle atrophy associated with the slowly growing mass. On US, bright rim sign (a peripheral rim of increased echogenecity, representing fat rind around the mass) and bright cap sign (a triangular hyperechoic area adjacent to at least one of the poles of the mass) are shown. Intramuscular myxoid tumors are differential diagnoses. However, they are generally more vascular and contain varying amounts of echogenic fat on US. Bright cap and bright rim sign were reported more common in intramuscular myxoma, compared with other malignant myxoid lesions such as liposarcoma.

References:

1.     Bancroft LW, Kransdorf MJ, Menke DM, O’Connor MI, Foster WC. Intramucular myxoma: Characteristic MR imaging features. AJR 2002;178(5):1255-9.

2.     Girish G, Jamadar DA, Landry D, Finlay K, Jacobson JA, Friedman L. Sonography of intramuscular myxomas: the bright rim and bright cap signs. J Ultrasound Med. 2006;25(7):865-9.