https://www.healthline.com/health/small-cell-lung-cancer/what-it-means-when-your-small-cell-lung-cancer-becomes-extensive-stage

Because the cancer has spread, the main treatment for extensive stage SCLC is chemotherapy. Chemotherapy is a type of systemic therapy. It doesn’t target a specific tumor or area of the body. It seeks out and attacks cancer cells no matter where they are. It can shrink tumors and slow progression.