Fibro-osseous pseudotumor of the digit: a clinicopathological study of 17 cases.

Fibro-osseous pseudotumor (FP) of the digit is a rare, non-neoplastic heterotopic ossifying lesion involving the subcutaneous tissues of the digits. To date, there are only a few published series in the literature. Our study of 17 cases, retrieved from the authors' referral archives, shows that the condition chiefly affects young to middle-aged adults (median = 34 years), with a slight female predominance and involves the fingers (n = 8) and toes (n = 8). One identical lesion was identified on the forehead (n = 1). Lesional size ranges from 0.8 to 5.6 cm. Treatment was by surgical excision. Histologically, 14 cases show a fairly well-circumscribed dermal (n = 10) or dermal and subcutaneous (n = 7) lesion with surface ulceration (n = 7). The lesion is composed of fascicles of variably cellular, spindle-shaped cells [calponin (n = 14) and smooth muscle actin (SMA) (n = 11) positivity], with minimal to mild atypia (n = 5), dispersed in a myxoid stroma, focally reminiscent of nodular fasciitis. At least focal irregular trabeculae with osteoid formation and osteoblastic rimming are seen in all cases. The main differential diagnosis is an extraskeletal osteosarcoma; however, this afflicts an older age group, with prominent cytological atypia and atypical mitoses. Clinical follow-up (range: 18 months - 14 years, n = 12) reveals evidence of local recurrence in some cases (n = 2), but no evidence of metastases. In conclusion, we report an additional 17 cases of this rare lesion to increase awareness amongst dermatopathologists.