Orthopaedic case of the month: A 30-year-old woman with a painful forearm mass.

A 30-year-old woman with a history of chronic congenital lymphedema and overgrowth of the right upper extremity presented with a visibly inflamed, reddened, nodular, palpable mass arising from the skin on the dorsal aspect of her right forearm of 9 months duration. The patient described the initial appearance as three small pimples on the skin of the forearm that coalesced and became larger. There were no fevers, weight loss, or night sweats. The patient was otherwise healthy. Eventually the mass became tender and painful, which caused her to seek medical attention. On presentation, there was an irregular, nodular, nontender, cutaneous mass that was approximately 3 9 3 cm surrounded by a 1-cm halo of erythema. Sensation and motor function were grossly intact. She had no history of malignancy or radiation treatments. Surgical history was significant for several surgical debulking procedures and liposuction of the right forearm and arm. The entire upper extremity from shoulder through the fingers was diffusely enlarged approximately four times the circumference of her contralateral normal upper extremity. It seemed to involve the skin and subcutaneous tissue. There were no palpable enlarged lymph nodes in the axilla or epitrochlear region although these areas were difficult to palpate because of the chronic lymphedema. Laboratory studies showed complete blood count, chemistries, coagulation analysis, erythrocyte sedimentation rate, and C-reactive protein were all normal. MRI was performed (Figs. 1–3). Based on the history, physical examination, and imaging studies, what is the differential diagnosis?