Intramuscular Myxoma

Intramuscular myxomas are benign proliferations of fibroblasts with abundant myxoid stroma. MRI findings correlate with the appearance of these tumors at pathology and can suggest the diagnosis. Patients are typically women older than 40, who present with a painless mass.

Pathologically, intramuscular myxomas are gelatinous masses without hemorrhage or necrosis. This is reflected as predominantly high signal intensity on T2-weighted images and low signal intensity on T1-weighted images.

Fibrous bands may be seen within the tumors on pathology. These appear as mildly enhancing internal linear strands (pink arrow) and may lead to a heterogeneous appearance on MRI.

Increased vascularity is not a feature of these tumors, a feature reflected in the mild or absent internal enhancement seen on post-contrast images. Areas of cellularity in the tumor present as patchy internal enhancement.

Intratumoral cysts can be seen in some cases (not seen here).

Myxomas tend to extend into the adjacent muscle, dividing it into small groups of fibers, a process that can lead to focal muscle atrophy. The surrounding muscle may also be edematous and T2-hyperintense (blue arrow).

Peripherally, these tumors have a pseudocapsule that represents condensation of fibrous tissue. The pseduocapsule is about 1 mm – 2 mm in thickness and is discontinuous at the junction of the tumor with the muscle. The thin pseudocapsule enhances on post-contrast images.

Finally, fat cells around the pseudocapsule present as areas of T1 hyperintensity around the tumor (white arrow).

Radiographic findings can be normal or reveal a nonspecific soft tissue mass. Calcifications can rarely be seen. Scintigraphic studies may show mild uptake due to the tumor's moderate vascularity. Ultrasound may reveal a multicystic hypoechoic or anechoic mass with well-defined margins. Similarly, CT reveals a well-defined mass with intermediate homogeneous attenuation.

While MR findings of a cystic intramuscular mass with peripheral and variable internal enhancement suggest the diagnosis of an intramuscular myxoma, a perilesional fat ring and the presence of edema in adjacent muscle are the most distinctive features of these tumors.

In some cases, however, biopsy may be necessary to exclude a malignant tumor with myxoid components, such as, myxoid peripheral nerve sheath tumors, extraskeletal myxoid chondrosarcoma, myxoid liposarcoma, myxofibrosarcoma, myxoid leiomyosarcoma, low-grade fibromyxoid sarcoma, and cellular myxoma.

References

Luna A, Martinez S, Bossen E. Magnetic resonance imaging of intramuscular myxoma with histological comparison and a review of the literature. Skeletal Radiol. 2005 Jan;34(1):19-28.