Abstract

A 57-year-old woman presented with 3 weeks of edema, erythema, paronychia, and pain of the left thumb nail unit. This was refractory to oral antibiotics and incision and drainage. Radiographs showed soft tissue swelling and destruction of the distal phalanx of the thumb. Osteomyelitis was favored, and the possibility of malignancy was included in the radiologic interpretation. Histopathologic analysis of the distal phalanx, amputated through the distal interphalangeal joint, revealed diffuse purulence throughout the subcutaneous tissues and bone, which showed near-total destruction. The initial histopathologic interpretation at an outside hospital was concerned for nail unit squamous cell carcinoma. Histopathology showed acute and chronic osteomyelitis: bone marrow fibrosis, bone resorption, remodeling, and an intramedullary mixed inflammatory infiltrate. Extensive suppurative cellulitis dissected through the subcutaneous tissue. Fragments of a multilobulated, epidermal inclusion cyst were seen in the soft tissue, showing reactive squamous epithelium. Focally cystic, intramedullary squamous proliferations associated with extensive osteomyelitis, were seen infiltrating between and circumferentially investing bony trabeculae. A diagnosis of intraosseous pseudocarcinoma with osteomyelitis and cellulitis was established. Intraosseous pseudocarcinoma is a diagnosis that is rare in dermatopathology practices. But nonetheless, is important for dermatopathologists to be aware of to avoid a malignant misdiagnosis. The nail unit presentation places this diagnosis within the realm of dermatopathologists. Intraosseous pseudocarcinoma has been described primarily in mandibular osteomyelitis. The differential diagnosis of nail unit squamous cell carcinoma, and site-specific variants such as verrucous carcinoma, and onycholemmal carcinoma must be considered.

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