An unusual case of nodular fasciitis presenting as an intra-tendinous mass.

Nodular fasciitis is a benign soft tissue pseudotumour typically occurring in the upper extremities, head and neck, thigh and trunk. It is most commonly seen in subcutaneous locations but also can be present in intramuscular and intermuscular (fascial) locations. Its occurrence in the hand is rare, and while it can occur in close proximity to tendons, its presentation as an intra-tendinous mass has not been previously described. We present a unique and rare case of nodular fasciitis arising within the flexor digitorum profundus (FDP) tendon of the hand in a 16-year-old female. The patient presented with a painful swelling in the volar aspect of the base of her left middle finger, with progressive flexion deformity of the finger. Ultrasound and magnetic resonance imaging revealed a mass within the FDP tendon of the middle finger. An ultrasound-guided biopsy revealed a diagnosis of nodular fasciitis. Given the self-limiting nature of the condition, she was managed conservatively with close clinical and imaging follow-up. This case highlights the importance of considering nodular fasciitis in the differential diagnosis of an intra-tendinous lesion in the hand, even though it is a rare occurrence in this location. The clinical presentation, diagnostic workup, and management of this unique case are discussed, emphasising the potential for its misdiagnosis as a malignancy which can have important implications in management.