NTRK-rearranged soft tissue sarcoma that originated from the head and neck with a robust response to the TRK inhibitor larotrectinib.

Targeted therapy for NTRK fusion gene-positive solid tumors has been approved as tumor-agnostic, but the frequency of such tumors is extremely low (~ 1%-2%). The same is true of soft-tissue sarcoma (STS), but STS includes some histologic types with high rates of NTRK-fusion positivity such as infantile fibrosarcoma. NTRK-rearranged spindle cell neoplasm is newly defined in the current WHO classification (5th edition), and there is thus relatively high motivation to search for NTRK-fusion in STS patients depending on the background. We report the case of a 31-year-old Japanese female with no remarkable history and an NTRK fusion-positive STS with a head and neck primary tumor who was successfully treated with targeted therapy. She was referred to our hospital due to a maxillary gingival tumor that had rapidly increased over the past several months. At her referral, the patient's previous pathological review revealed (by FISH) that the NTRK fusion gene was positive. We performed a pathological review and close examination with a view to introducing a TRK inhibitor. The patient was refractory to standard chemotherapy and radiotherapy. Concurrent comprehensive genome profiling (FoundationOne® CDx) confirmed the LMNA-NTRK1 fusion, and treatment with the TRK inhibitor larotrectinib was started. With larotrectinib treatment, the tumor shrank in size at an early stage, and the response has been maintained for > 2 years.