Age / Sex : 39 / M

Chief complaint: Left buttock mass

What is your impression?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Seung Hyun Lee, National Health Insurance Service Ilsan Hospital

Discussion

Answer: Intramuscular myxoma  

Findings: Well-defined intramuscular soft tissue mass involving the left gluteus maximus muscle. The mass demonstrates intermediate signal intensity on T1WI, high signal intensity on T2WI with perilesional high signal intensity on fluid-sensitive sequence (T2FS), with peripheral and septal enhancement.

Differential Diagnosis:

 neurogenic tumor, myxofibrosarcoma, lymphangioma, ganglion cyst

Discussion:

Clinical features

- Myxoma is a benign mesenchymal neoplasm that is composed of bland undifferentiated stellate cells with thin collagen fibers within a myxoid stroma. Myxoma is most frequently diagnosed in patients 40–70 years of age, with a 57% female predilection. Intramuscular myxomas are typically solitary. When multiple myxomas are present, they are almost always associated with monostotic or polyostotic fibrous dysplasia, known as Mazabraud syndrome. Most musculoskeletal myxomas are intramuscular (82%) in location, occurring most often in the thigh (51%), upper arm (9%), calf (7%), and buttock (7%). Management options include observation (because of the benign behavior) or surgical excision, which is typically curative without recurrence.

Imaging features

- US demonstrates a well-defined hypoechoic to near-anechoic mass with some internal echoes and increased through-transmission.

- CT typically shows a well-defined homogeneous soft-tissue mass with attenuation higher than that of water and less than that of surrounding muscle.

- MRI: Myxomas demonstrate homogeneous low (81%–100%) to intermediate (0%–19%) signal intensity on T1-weighted MR images. At T2-weighted MR imaging, all myxomas demonstrate high signal intensity. In 65%–89% of cases, a thin rim of fat is noted most prominently at the superior and inferior poles of the lesion, representing atrophy of the adjacent muscle associated with the slowly growing mass. Perilesional high signal intensity may be noted with the use of fluid-sensitive sequences in 79%–100% of myxomas, a finding caused by leakage of the myxomatous tissue into the surrounding muscle. Myxomas usually demonstrate mild (76%) to moderate (24%) contrast enhancement in a diffuse pattern (57%) or a thick peripheral and septal enhancement pattern (43%).  

References:

• Soft-tissue myxomatous lesions: review of salient imaging features with pathologic comparison. Radiographics 2014; 34:964-980.


• Intramuscular myxoma: characteristic MR imaging features. AJR Am J Roentgenol 2002; 178:1255-1259.