Fibro-osseous pseudotumor of the digit: a challenging case involving the distal interphalangeal joint and flexor tendon.

Fibro-osseous pseudotumor of the digit (FOPD) is a rare benign lesion with a favorable prognosis. It was once regarded as an early-stage lesion within the spectrum of reactive osseous lesions, that include bizarre parosteal osteochondromatous proliferation and acquired exostosis such as turret exostosis. However, this lesion has been reclassified as a soft tissue tumor following the discovery of a shared COL1A1-USP6 gene rearrangement in both FOPD and myositis ossificans. Despite the low recurrence rate and the absence of reported malignant transformation, the lesion's rarity, nonspecific clinical presentation, and overlapping histopathological features can lead to misdiagnosis as malignancy, particularly osteosarcoma, occasionally resulting in unnecessary amputation. Therefore, accurate preoperative imaging diagnosis is essential. Here, we present the detailed imaging findings of a surgically confirmed FOPD in a 22-year-old male and review the relevant literature. The lesion appeared as a subcutaneous soft tissue mass along the distal phalanx, with intralesional ossification and no continuity with the underlying bone, consistent with typical FOPD imaging characteristics. However, atypical features such as distal interphalangeal joint involvement, adjacent cortical erosion, and flexor tendon adhesion complicated radiologic interpretation. Awareness of both classic and atypical imaging features is crucial for distinguishing FOPD from malignancy or other benign mimickers, and for guiding appropriate clinical management.