Optimal timing for the first cystoscopic follow-up using time-to-treatment initiation analysis of oncologic outcomes in primary non-muscle invasive bladder cancer.

Various guidelines recommend the first follow-up cystoscopy at 3 months; however, no data exist on the optimal timing for initial follow-up cystoscopy. We tried to provide evidence on the timing of the first cystoscopy after the initial transurethral resection of bladder tumor (TUR-BT) for patients with non-muscle invasive bladder cancer (NMIBC) using big data. This was a retrospective National Health Insurance Service database analysis. The following outcomes were considered: recurrence, progression, cancer-specific mortality, and all-cause mortality. Exposure was the time-to-treatment initiation (TTI), a continuous variable representing the time to the first cystoscopy from the first TUR-BT within 1 year. Additionally, we categorized TTI (TTIc) into five levels: < 2, 2-4, 4-6, 6-8, and 8-12 months. A landmark time of 1 year after the initial TUR-BT was described to address immortal-time bias. We identified the optimal time for the first cystoscopy using Cox regression models with and without restricted cubic splines (RCS) for TTI and TTIc, respectively. Among 26,660 patients, 16,880 (63.3%) underwent cystoscopy within 2-4 months. A U-shaped trend of the lowest risks at TTI was observed in the 2-4 months group for progression, cancer-specific mortality, and all-cause mortality. TTI within 0-2 months had a higher risk of progression (aHR 1.36; 95% confidence intervals [CI] 1.15-1.60; p < 0.001) and cancer-specific mortality (aHR 1.29; 95% CI 1.05-1.58; p = 0.010). Similarly, TTI within 8-12 months had a higher risk of progression (aHR 2.09; 95% CI 1.67-2.63; p < 0.001) and cancer-specific mortality (aHR 1.96; 95% CI 1.48-2.60; p < 0.001). Based on the RCS models, the risks of progression, cancer-specific mortality, and all-cause mortality were lowest at TTI of 4 months. The timing of the first cystoscopy follow-up was associated with oncologic prognosis. In our model, undergoing cystoscopy at 4 months has shown the best outcomes in clinical course. Therefore, patients who do not receive cystoscopy at approximately 4 months for any reason need more careful follow-up to predict a poor clinical course.