From UpToDate: For patients with SCLC, systemic chemotherapy is an important component of treatment, because SCLC is disseminated at presentation in almost all patients. For those with limited-stage disease, thoracic radiation therapy is used in combination with chemotherapy. Prophylactic cranial irradiation is often used to decrease the incidence of brain metastases and prolong survival. Prophylactic cranial irradiation and thoracic radiation may also be beneficial in those with a complete or partial response to initial systemic chemotherapy.

Even though most small cell lung cancer shows up in late stages, and only 2% of people diagnosed are alive after 2 years, people still frequently get treatment. Usually this is both chemo and radiation, but the chemo comes first to shrink the cancer so you don't have to radiate as large an area. Chemo + radiation is also sometimes used as paliation. (https://www.ncbi.nlm.nih.gov/books/NBK482458/)