Abstract

Neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell neoplasms (NTRK-RSCN) are rare soft tissue sarcomas distinguished by various NTRK gene fusions. These tumors display a range of histological features and tumor characteristics, with NTRK fusion genes serving as critical oncogenic drivers. We present the case of a 44-year-old female who presented with a several year history of a slowly enlarging, 1.3 cm, erythematous nodule on the right medial breast. A punch biopsy revealed a dermal proliferation of bland, monomorphic spindled cells infiltrating the subcutaneous fat with admixed lymphoplasmacytic inflammation. Lesional cells were positive for S100, CD34, and Pan-TRK. Next-Generation Sequencing (NGS) of the biopsy identified an EML4:NTRK3 rearrangement. Given that NTRK-RSCN tend to be locally aggressive and there are rare reports of metastasis, complete surgical excision was recommended. The patient underwent wide local excision with positive margins that were subsequently cleared during a second surgical procedure. NGS was critical to the proper diagnosis in this case. Though Pan-TRK immunostaining is helpful, it is not entirely sensitive or specific and may stain positively in other entities on the histopathologic differential diagnosis including BCOR-positive sarcoma, undifferentiated sarcoma, and spindle cell rhabdomyoscarcoma. This case highlights the significance of precise molecular characterization in guiding appropriate management for these rare soft tissue sarcomas.