Objective:To explore the clinicopathological characteristics and the related influencing factors of efficacy and prognosis of elderly patients with malignant mesothelioma(MM)in Chinese population.Methods:We retrospectively analyzed the clinical data of 115 patients aged 65 years and above who were diagnosed with MM in Beijing Hospital, Peking University Cancer Hospital, and Cancer Hospital of Chinese Academy of Medical Sciences between November 2007 and July 2024, and the patients were grouped according to age(≥75 years in the older group and <75 years in the younger group), histological types and therapy regimens.Kaplan-Meier curve and Log-rank test were performed.Cox regression was used in prognostic analysis.Results:The positive expression rate of Calretinin in the Chinese elderly population with MM was consistent with previous reports, while the positive rates of Cytokeratin 5/6(CK5/6), WT-1, and D2-40 were much lower.The overall response ratio(ORR)for first-line treatment was 17.3%(9/52), and the disease control rate(DCR)was 92.3%(48/52).The ORR for second-line treatment was 7.7%(1/13)and the DCR was 76.9%(10/13).The ORR and DCR were higher in the first-line immunotherapy group than in the chemotherapy group, 50.0% vs.14.6%( P=0.134)and 100.0% vs.91.6%( P=1.000), respectively.The ORR in the second-line immunotherapy group was higher than that in the chemotherapy group, 25.0% vs.0, respectively, and the DCR were both 75.0% in two groups.The median progression free survival(mPFS)was 9.2 months and median overall survival(mOS)was 19.0 months for patients receiving first-line treatment, and the mPFS was 3.3 months and mOS was 11.0 months for second-line therapy.The first-line immunotherapy provided more shorter mPFS(1.6 months vs.9.2 months, P=0.081)and longer mOS(not reached vs.18.1 months, P=0.147)than the chemotherapy group.The younger group had prolonged mPFS(9.7 months vs.7.2 months, P=0.305)while shorter mOS(18.1 months vs.23.9 months, P=0.289)compared with the older group, and none of them reached statistical differences.Both mPFS and mOS were prolonged in the epithelioid subtype compared with the non-epithelioid subtypes, 10.4 months vs.1.6 months( P<0.001)and 20.3 months vs.4.6 months( P=0.803), respectively.Both mPFS(7.1 months vs.4.7 months, P=0.583)and mOS(18.3 months vs.6.3 months, P=0.134)were prolonged in the second-line chemotherapy group compared with the immunotherapy group.The Cox regression analysis showed that gender, Eastern Cooperative Oncology Group, Performance Status(ECOG PS)and positive CK5/6 were both the independent predictors for the first-line PFS.Histological type was an independent prognostic factor for the first-line OS. Conclusions:MM in the Chinese elderly population exhibits unique clinicopathologic characteristics.The immunotherapy improves ORR, DCR and prolongs mOS in first-line use, and improves ORR in second-line.First-line treatment improves mPFS in the younger group compared with the older group.Multivariate Cox regression demonstrates that gender, ECOG PS and CK5/6 expression are both predictors of efficacy, and histological type is an independent prognostic factor for survival of the Chinese elderly population with MM.