The patient went on to have a biopsy.

Histology

Features of note include a dense spindle cell proliferation, vaguely fascicular, moderately pleomorphic, mitotically active, with prominent small interspersed myxoid areas. Less than 50% of the biopsy is myxoid. Necrosis is present, involving less than 50% of the tissue. Vimentin stain is positive, while keratin, S-100, caldesmon, actin, CD34, and BCL-2 are negative. MIB-1 stain shows a high proliferation index. The findings are consistent with malignant fibrous histiocytoma, precise subclassification and grading depending on the relative proportions of the myxoid component and degree of necrosis respectively in the tumor as a whole.

Final diagnosis: malignant fibrous histiocytoma (aka primary undifferentiated pleomorphic sarcoma)