Surrounding muscle edema detected by MRI is valuable for diagnosis of intramuscular myxoma.

We recently experienced 4 cases of intramuscular myxoma and analyzed MRI findings, comparing them with histological ones. Results showed that all tumors were depicted with a homogeneous low signal intensity on T1-weighted images and a markedly high signal intensity on T2-weighted images, findings which are similar to those of cystic lesions like intramuscular ganglions. However, tumors were diffusely and finely enhanced on T1-weighted images with intravenous gadolinium administration. Three cases showed the characteristic fat rim and fat cap. A diffuse edematous lesion demonstrating intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images was also found in the adjacent muscle surrounding the tumor in 3 of 4 cases. In this lesion, tumor invasion, diffuse and severe muscle degeneration, blood and lymphatic congestion with exudates, and reactive adipose tissue formation were observed. The present results suggested that for the clinical diagnosis of intramuscular myxoma with MRI examination, the combination of three MRI signs such as homogeneous low signal intensity on T1-weighted mages, markedly high signal intensity on T2-weighted images, and an enhancement effect with contrast medium are important, but the fat rim or fat cap, and the surrounding muscle edema as detected by MRI are also characteristic and allow for a more firm diagnosis. We presume that the diffuse and severe muscle fiber degeneration induced by tumors may cause such specific surrounding muscle edema.