BACKGROUND: EAU guidelines recommend a single instillation (SI) of intravesical chemotherapy (e.g. Mitomycin C) within 24 hours after transurethral resection of a bladder tumour (TURBT) in patients with low- to intermediate risk non-muscle invasive bladder cancer without (suspected) bladder perforation or bleeding requiring bladder irrigation. However, remarkable variation exists in the use of SI. The risk of severe complications is likely to contribute to this variation, but evidence is

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... . OBJECTIVE: To investigate the absolute severe complication and mortality risk after SI in low- and intermediate risk bladder cancer. METHODS: In this observational, historic cohort study, data on 25,567 patients diagnosed with TaG1G2 urothelial bladder carcinoma (UBC) between 2009 and 2018 who underwent TURBT were collected from the Netherlands Cancer Registry. Data were supplemented with information on cause of death and severe complications after cancer treatment by re-examining the electronic health records and the 14-day complication risk and the 30-day mortality risk were evaluated. RESULTS: On average, 55% of patients had a SI after TURBT, varying from 0–>80% between hospitals. The 30-day mortality risk was 0.02% and the 14-day risk of severe complications was 1.6%. CONCLUSIONS: As the absolute risk of mortality and severe complications is very low, SI after TURBT can be considered a safe treatment in patients with low- to intermediate UBC without contraindications for SI. These results imply that a part of eligible patients is denied effective treatment.