Even if an initial response was obtained, the prognosis remains poor due to small-cell lung cancer (SCLC’s) aggressiveness and the high likelihood of progression or relapse. As a result, the population had an urgent unmet clinical need. In the multicenter, phase 3, randomized, placebo-controlled, double-blind KEYLYNK-013 research, patients with previously untreated limited-stage SCLC were given Concurrent chemoradiotherapy (CCRT) followed by pembrolizumab with or without olaparib. Eligible patients aged more than or equal to 18 years and older with newly diagnosed, pathologically confirmed, limited-stage (i.e., stage I-III) SCLC will be randomized 1:1:1 to CCRT plus pembrolizumab (Groups A and B) or CCRT plus placebo (Group C). Participants will receive pembrolizumab plus placebo (Group A), pembrolizumab plus olaparib (Group B), or placebo in the absence of illness progression (Group C). The two key objectives were overall survival, and progression-free survival following RECIST version 1.1 by blinded independent central review were the 2 key objectives. Enrollment began in December 2020 and was still occurring at over 150 locations. In participants with limited-stage SCLC, KEYLYNK-013 would give valuable information on the efficacy and safety of pembrolizumab plus CCRT and pembrolizumab with or without olaparib post CCRT.