Metastatic triple negative breast cancer with NTRK gene fusion on tissue but not on ctDNA molecular profile.

A woman presented to medical oncology with almost 4 years of untreated, slowly progressing, triple negative metastatic breast cancer to the lung. About 15 years prior, she was diagnosed with invasive ductal carcinoma of the right breast with ipsilateral chest wall recurrence 6 years later. Comprehensive molecular profiling of a metastatic lesion detected a hotspot ETV6-NTRK3 fusion, which was not present on circulating tumour DNA or molecular profile performed 4 years prior. A second look pathological examination demonstrated tumour characteristics consistent with secretory breast carcinoma. Identification of ETV6--NKRT3 fusion allowed for treatment with larotrectinib, a tyrosine kinase inhibitor specifically indicated for secretory breast carcinoma. After 3 months, she experienced a partial response.