This patient went on to have a biopsy which revealed the mass to be a plasmacytoma. 

Histology 

MICROSCOPIC DESCRIPTION:

The sections show a densely cellular malignant infiltrate, forming sheets and aggregates. It invades in-between bony trabeculae. The tumor cells are plasmacytoid in appearance. They have enlarged round nuclei, clumped chromatin, occasional perinuclear hofs and moderate amounts of eosinophilic cytoplasm. There is no amyloid in the background. The tumor cells are CD138 positive. Scattered cells are lambda light chain positive. Kappa light chain is negative. CD3 and CD20 are also negative. The features are those of multiple myeloma.

MRI of this patient is present as a separate case here.