Neurotrophic Tyrosine Receptor Kinase–Rearranged Spindle Cell Neoplasm Presenting on the Breast of a 44-Year-Old Woman

Neurotrophic tyrosine receptor kinase–rearranged spindle cell neoplasms (NTRK–RSCN) are rare soft tissue sarcomas distinguished by various NTRK gene fusions. These tumors display a range of histologic features and tumor characteristics, with NTRK fusion genes serving as critical oncogenic drivers. We present the case of a 44-year-old woman 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 identified an EML4–NTRK3 rearrangement. Given that NTRK–RSCN tends 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 after re-excision. Next-generation sequencing played a crucial role in reaching the correct diagnosis in this case. This is the first reported case of a cutaneous EML4–NTRK3 fusion spindle cell neoplasm located on the cutaneous breast.