Targeted Therapy Using the Selective Tropomyosin Kinase Receptor Inhibitor Larotrectinib in an Infant with Infantile Fibrosarcoma with a TPM3–NTRK1 Gene Fusion with Lung and Central Nervous System Metastases: Case Report

This case report describes the outcomes of tropomyosin receptor kinase (TRK) inhibitor treatment in an infant with an infantile fibrosarcoma (IFS) with a TPM3–NTRK1 gene fusion. The IFS on the left foot was refractory to chemotherapy and was partially resected. After 5 months, there was local recurrence and further surgery was performed. The patient developed pulmonary and central nervous system metastases. Pan-TRK antibody staining was positive and genomic profiling with next-generation sequencing confirmed a TPM3–NTRK1 gene fusion. The patient started treatment with larotrectinib in February 2019, with a durable response, including a clear reduction in brain metastases. Research on novel gene fusions and molecular testing to direct the use of targeted therapy should be encouraged, especially in refractory solid tumors.