Calendar-year seasonality in colorectal and breast cancer screening and first-time uptake: A 6-year single-center time-series to optimize capacity level-loading (2020–2025).

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Background: Within-year peaks and troughs in screening utilization can create early-year capacity constraints and late-year underutilization. We quantified month-of-year seasonality in colorectal cancer (CRC) and breast screening and translated it into an operational planning metric for capacity level-loading. Methods: We analyzed monthly screening volumes from January 2020 to December 2025 (72 months) at one hospital. Outcomes were total monthly screens and first screens for CRC and breast screening. Month-of-year effects were estimated using Poisson regression with month indicators and year fixed effects, using March as the reference month. We summarized operational impact using an annual level-loading fraction, defined as the sum of above-mean monthly excess volume divided by annual total volume, representing the proportion of annual volume that would need redistribution from high- to low-volume months to achieve a flat monthly schedule. Results: We observed 36,158 screenings (23,266 CRC; 12,892 breast), mean 502/month. Combined volume peaked in March (mean 810/month; 1.61x mean) and was lowest in December (mean 364/month; 0.73x) (peak-to-trough 2.22). Q4 volume was 41% lower than Q1 (391 vs 662/month). After year adjustment, December remained lower than March (CRC + breast IRR 0.45, 95% CI 0.33–0.61; P < 0.001), with a larger decline for CRC (Dec vs Mar IRR 0.35, 95% CI 0.25–0.49; P < 0.001) than breast (Dec vs Mar IRR 0.70, 95% CI 0.53–0.93; P = 0.013); breast volume was lowest in February (Feb vs Mar IRR 0.67, 95% CI 0.49–0.92; P = 0.012). First screens totaled 5,062 for CRC (21.8%) and 3,105 for breast (24.1%). CRC first screens peaked in February (mean 118.8/month) vs January (34.8/month) (ratio 3.41), whereas breast first screens peaked in December (64.8/month) vs January (29.0/month) (ratio 2.24). The median annual level-loading fraction was 13.4% (range 6.9–24.8) across years. Conclusions: CRC and breast screening utilization showed substantial, reproducible month-of-year seasonality with program-specific first-screen peaks. From a health services perspective, these findings support calendar-aware capacity allocation—protecting early-year CRC first-screen capacity, shifting repeat CRC volume into late-year months, and reserving Q4 breast first-screen slots—to reduce peak-month access constraints and better utilize late-year latent capacity.