A right-handed man in his 50s presented with a painless nodule that has been enlarging for 1 year destroying his nail. The lesion proved to be more complex than initially apparent. What's your diagnosis?

The patient, whose case was published in JAMA Dermatology and reported by lead study author Sophie Diong, MB, BCh, BAO, and colleagues at St. James's Hospital in Dublin, Ireland, presented with an enlarging, painless nodule on his right index finger that had been progressively destroying his nail. The patient had a history of hepatitis C but reported no preceding trauma to the area.

                                 Clinical image (left) and punch biopsy (right).

Physical examination revealed a 15 × 10-mm erythematous, well-circumscribed, lobulated firm nodule extending from the dorsal surface of the distal interphalangeal joint to the nail plate. The lesion was causing significant destruction of the adjacent nail plate and did not transilluminate. A radiograph showed erosion of the distal phalanx.

Punch biopsies from both the proximal and distal poles of the nodule revealed the diagnosis: tenosynovial giant cell tumor (TGCT), previously known as giant cell tumor of the tendon sheath. Histopathologic analysis showed acral skin with a well-circumscribed lobular dermal lesion containing xanthoma cells and hemosiderin deposition, with bland mononuclear cells forming osteoclast-like giant cells focally.

"TGCTs are a rare, locally aggressive, mesenchymal tumor arising from the joints, bursa, and tendon sheaths," the study authors wrote. While typically benign, these tumors can significantly impact the quality of life through their destructive potential. The case exemplified the tumor's ability to cause nail plate changes, which occur in up to 50% of cases.

The patient was referred to plastic surgery for complete resection. The researchers noted that meticulous dissection ensuring complete excision has been shown to reduce the risk of recurrence, which occurs in about 10% of nodular cases.

The case highlighted the importance of considering TGCT in the differential diagnosis of enlarging acral tumors, particularly when nail plate changes are present. Other key differentials included amelanotic melanoma, cellular digital fibroma, and plexiform fibrohistiocytic tumor.