A High-Grade CD34-Positive Spindle Cell Sarcoma of the Palm Revealing an Unexpected TPM3::NTRK1 Fusion: A Diagnostic Pitfall With Major Therapeutic Implications

NTRK-rearranged spindle cell neoplasms are increasingly recognized soft-tissue sarcomas that can closely mimic other CD34-positive spindle cell tumors, particularly dermatofibrosarcoma protuberans (DFSP) and solitary fibrous tumor (SFT). Failure to recognize this entity has important diagnostic and therapeutic consequences. We report a 17-year-old male with a rapidly enlarging palmar mass who initially underwent tumor excision abroad, where the lesion was diagnosed as synovial sarcoma. Upon presentation to our center with rapid local recurrence, the original pathology was reviewed and reinterpreted as fibrosarcomatous dermatofibrosarcoma protuberans (FS-DFSP). The patient subsequently received multiagent chemotherapy in an attempt at limb preservation; however, the tumor continued to progress aggressively, necessitating a midforearm amputation. Review of the amputation specimen prompted further expert consultation, during which pan-TRK immunohistochemistry demonstrated diffuse cytoplasmic staining. Subsequent next-generation sequencing confirmed a TPM3::NTRK1 fusion, establishing the correct diagnosis. This case illustrates a major diagnostic pitfall in the evaluation of high-grade CD34-positive spindle cell sarcomas and underscores the importance of incorporating pan-TRK immunohistochemistry early in the diagnostic work-up. Recognition of this entity is critical, as it directly identifies patients eligible for highly effective TRK-inhibitor therapy.