BCG in the fight against cancer: exploring its applications in diverse tumour types and future directions

Bacillus Calmette-Guérin (BCG), originally a tuberculosis vaccine, is a standard immunotherapy for non-muscle-invasive bladder cancer (NMIBC). This review summarises BCG biology, including trained immunity, and evaluates clinical and translational evidence for BCG across solid tumours. We aim to distinguish guideline-supported indications from investigational uses and identify contexts where BCG may remain clinically relevant or trial-ready. Evidence is strongest in NMIBC, where outcomes depend on induction plus maintenance schedules and appropriate patient selection. Outside the bladder, intralesional and vaccine-adjuvant approaches show signals in selected melanoma and vaccine settings, while historical lung and colorectal trials largely failed to translate into durable benefit. Emerging preclinical work in hepatocellular carcinoma and breast cancer suggests potential synergy with modern immunotherapy strategies. BCG should be framed as a platform immunomodulator whose value beyond NMIBC requires mechanism-guided trials, clear safety boundaries and clinically anchored endpoints.