Immune checkpoint inhibitors (ICIs) have become essential in cancer treatment, offering significant survival benefits across various malignancies. By enhancing antitumor immune responses via blockade of co-inhibitory immune checkpoints, ICIs have transformed the therapeutic landscape and become a cornerstone of modern oncology​ [1]. However, patients with autoimmune diseases have been frequently excluded from clinical trials evaluating ICIs due to hypothetical concerns about increased frequency of immune-related adverse events (irAEs) and flares of pre-existing autoimmune conditions. The potential mechanisms of flare include the disruption of co-inhibitory immune checkpoints such as PD-1 and CTLA-4, which are essential for maintaining immune tolerance, leading to the activation of autoreactive T cells and disease flares [2,3]. Additionally, ICIs may increase the production of Th1 cytokines such as TNF-α, and IFN-γ, as well as chemokines responsible for T cell migration including CXCL9/10/11, which may also contribute to the incidence of disease flares [[4], [5], [6]].

Most published studies evaluating the use of ICIs in patients with cancer and pre-existing autoimmune diseases are limited to case reports, small case series, or retrospective cohorts. Only a few analyses have examined specific autoimmune diseases individually, as most prior reports grouped these conditions as a single entity, or “autoimmune disease” [[7], [8], [9]]. In particular, the safety of ICIs in patients with pre-existing rheumatologic diseases remains poorly understood, but most flares and irAEs are mild to moderate and are managed with systemic corticosteroids or other immunosuppressive therapies, and permanent discontinuation of ICI therapy is only required in a minority of cases [[10], [11], [12]]. These conditions warrant detailed analysis of irAEs and their flares because of their systemic manifestations and high potential for flares under immune modulation [13]. In this study, we aimed to systematically analyze the incidence and characteristics of rheumatologic disease flares in patients with solid tumors and pre-existing rheumatologic conditions undergoing ICI therapy.