Ossifying Panniculitis Presenting as a Painful Nodule on the Abdomen.

Myositis ossificans is a benign ossifying nodule that can affect any type of tissue but is most commonly found in muscle. Occurrence in the soft tissue of the subcutis is less common and has been referred to as panniculitis ossificans. In this case report, we describe a 46-year-old woman who presented with a 1.7 × 2.0 cm lesion on the abdomen that had been present for several months, was painful, and easily irritated by clothing. Initial clinical assessment based on history and physical examination was an epidermal cyst and removal by excision was performed. Initial histologic findings showed a spindle cell proliferation in the subcutis along with bone formation. Osteoblasts were noted to be rimming the bone and giant cells were present. The spindle cells were SMA positive and partially factor XIIIa positive while S100 protein staining was negative, confirming the diagnosis of panniculitis ossificans.