Patients

This study was a retrospective, multicenter, cohort study conducted in four Italian University Hospitals.

Patients with metastatic solid tumors with MSI‐high status and receiving an anti‐PD‐1 monoclonal antibody with or without an anti‐CTLA‐4 agent were eligible for the study. Inclusion criteria were MSI‐high status and mismatch repair deficiency status independently from the primary tumor site of origin, ECOG PS 2 or 3 that had to be clearly related to progressive cancer and not to pre‐existing comorbidities as assessed by Charlson Comorbidity Index score ≤ 7 (reflecting presence of up to one mild among 19 considered comorbid diseases as previously reported 4), at least one previous treatment line for metastatic disease, and presence of measurable disease. During immunotherapy, tumor assessments with computed tomography (CT) or magnetic resonance imaging scans were performed at baseline and every 8–9 weeks (depending on the treatment schedule) until disease progression. Radiological evaluations could have been anticipated based on decisions of the treating physicians in case of clinical progression. MSI status was confirmed by both polymerase chain reaction and immunohistochemistry, each assessed locally. The primary outcome measure was overall response rate (ORR) according to RECIST 1.1 criteria. Secondary outcome measures were clinical benefit rate, time‐to‐response and duration of response, progression‐free survival (PFS), overall survival (OS), rate of ECOG performance status recovery, and safety. This study was approved by institutional review board of Fondazione IRCCS Istituto Nazionale dei Tumori (INT 117/15), and each patient signed a written informed consent.