The purpose of this research study is to determine if certain markers in your blood are increased over the duration of your chemotherapy, especially as you get sicker, such as developing a severe infection (sepsis) or severe respiratory failure (acute respiratory distress syndrome).

The goal of this observational study is to describe the safety and effectiveness of larotrectinib in patients with locally advanced or metastatic TRK fusion cancer for whom a decision to treat with larotrectinib has been made before enrollment. This study will enroll participants with any tumors harboring the NTRK gene fusion including Soft Tissue Sarcomas, Central Nervous System, Melanoma, and Thyroid cancers.