Intramuscular plasmacytoma

In multiple myeloma, secondary infiltration of adjacent muscles from bone lesions is common. However, plasmacytoma directly arising within the skeletal musculature is rare. Imaging findings of this rare entity have been described only sporadically. The purpose of this study was to identify the clinical signs and radiological features of intramuscular plasmacytoma (IP).

Eleven patients with IP were retrospectively identified in the pathological and radiological databases of our institution. Computed tomography (CT) was performed in nine patients and magnetic resonance imaging (MRI) in four cases.

IP presented clinically with local pain in four patients. In one case with involvement of the rectus lateralis muscle of the eye, the patient showed a painless bulbus proptosis. In another patient, IP manifested as a massive bilateral forearm swelling with compartment syndrome. In four patients, IP was identified incidentally on computed tomography during staging examination. On imaging, two patterns of IP were found: intramuscular mass (n = 5) or diffuse muscle infiltration (n = 6). On CT with contrast, IP showed a moderate enhancement. With MRI on T1-weighted images, IP was isointense in comparison to the unaffected musculature, whereas on T2-weighted images, IP showed high signal intensity. After intravenous administration of contrast medium, a slight-to-moderate inhomogeneous enhancement was seen in all cases.

IP should be considered in the differential diagnosis of muscle tumors. It manifests with two radiological patterns, either as intramuscular mass or as diffuse muscle infiltration.