[Analysis of the feasibility and prognostic value of circulating tumor DNA in detecting gene mutations in small cell lung cancer]

Objective: To investigate the feasibility of circulating tumor DNA (ctDNA) in detecting small cell lung cancer (SCLC) gene mutations and its prognostic value in chemotherapy and/or radiotherapy for SCLC patients. Methods: A total of 77 SCLC patients who were admitted to the Department of Thoracic Medical Oncology and the Department of Thoracic Radiation Oncology of Zhejiang Cancer Hospital from July 2016 to November 2019 were included. There were 66 males and 11 females, with a median age of 60 years. Among them, 42 cases were in limited stage (LS) and 35 cases were in extensive stage (ES). Next-generation sequencing (NGS) of patients' plasma ctDNA was performed before treatment. The differences of mutated genes and signaling pathways between LS and ES patients were analyzed and compared. Blood-based tumor mutation burden (bTMB) was calculated according to detected somatic cell mutations. Patients were divided into the high bTMB and the low bTMB groups according to the optimal threshold calculated by R software. Log-rank tests were used to compare progression-free survival (PFS) between the high bTMB and the low bTMB groups. Results: Among the 77 patients, 76 patients had gene mutations detected in their plasma, and the positive rate of ctDNA test was 98%. Among the 76 patients, the genes with the highest mutation frequency were TP53 (89%), RB1 (70%), LRP1B (34%), CREBBP (21%), MLL3 (21%), MLL2 (16%), NOTCH1 (13%), ROS1 (13%), BRCA2 (12%), and PTPRD (12%). The most common mutated genes in LS patients were TP53 (90%), RB1 (68%), LRP1B (24%), MLL2 (22%), and BRCA2 (17%); the most common mutated genes in ES patients were TP53 (89%), RB1 (71%), LRP1B (46%), CREBBP (31%), and MLL3 (29%). The mutation rates of NOTCH1 and CREBBP genes were significantly higher in ES patients (31.4% and 22.9%) than those in LS patients (11.9% and 4.8%) (both P<0.05). Signaling pathway analysis showed that there were more NOTCH pathway gene variations in ES patients. Among LS patients, patients in the high bTMB group (≥ 6.96 mutations/Mb) had a longer PFS than that in the low bTMB group (<6.96 mutations/Mb) (P=0.033); but no such difference was noted in ES patients. Conclusion: Plasma ctDNA sequencing detected SCLC gene mutation profiles similar to those reported in previous literature, thus ctDNA could be used as a tool to study SCLC genomics; the mutation spectra of ES-SCLC and LS-SCLC were different. bTMB has potential prognostic value in LS-SCLCs treated with chemoradiotherapy.