Introduction: Despite the emergence of atezolizumab and bevacizumab (A + B) as standard first-line systemic therapy for unresectable hepatocellular carcinoma (HCC), a comprehensive understanding of the clinical significance of immune-related adverse events (irAEs) remains limited. We aimed to assess the impact of irAEs on patients with HCC undergoing A + B treatment. Methods: This multicentre retrospective study included consecutive patients with HCC who were treated with the A + B regimen from September 2020 to December 2022. Patients were categorized into three groups based on the severity of irAEs, ranging from those without any experience of irAEs to those with severe irAEs. Results: This study included 150 patients with HCC, with a mean age of 63.3 years. Among them, 93.3% of patients were classified as Barcelona Clinic Liver Cancer stage C, 52.0% had portal vein tumour thrombosis (PVTT), and 60.7% extrahepatic spread. Patients were classified as follows: group 1 (n = 84) had no irAEs, group 2 (n = 37) had mild irAEs (grade 1-2), and group 3 (n = 29) had severe irAEs (grade ≥3). The median overall survival (OS), progression-free survival (PFS), and time-to-treatment discontinuation (TTD) were 13.6, 5.7, and 3.6 months, respectively. Group 2 demonstrated significantly superior OS compared to group 1 (9.5 months) and group 3 (5.6 months), with a median OS of 23.0 months (p < 0.001). Furthermore, group 2 demonstrated significantly better outcomes in terms of PFS and TTD compared to both group 1 and group 3 (p < 0.001 for both). Multivariate analysis identified mild irAEs (hazard ratio [HR], 0.353; p = 0.010), ALBI grade 1 (HR, 0.389; p = 0.006), Child-Pugh class A (HR, 0.338; p = 0.002), and the absence of PVTT (HR, 0.556; p = 0.043) as independent predictors of better OS. Conclusion: Our study highlights the significant impact of irAE severity on the outcomes of patients with HCC receiving A + B. Notably, the occurrence of mild irAEs was independently associated with favourable survival, suggesting their potential role as surrogate indicators of HCC prognosis.
