Forecasting Trends in Androgen Deprivation Therapy Intensification for Metastatic Hormone-Sensitive Prostate Cancer: A Retrospective Population-Based Cohort and Time-Series Analysis.

Treatment intensification with androgen receptor pathway inhibitors (ARPIs) and/or docetaxel in addition to androgen deprivation therapy (ADT) improves survival for men with metastatic hormone-sensitive prostate cancer (mHSPC), yet real-world uptake has historically been low. We conducted a population-based retrospective cohort study of Ontario men aged ≥66 years diagnosed with de novo mHSPC between 2014 and 2022 using linked administrative health data, defining treatment intensification as initiation of an ARPI and/or docetaxel with ADT within six months of diagnosis. Quarterly intensification rates were modeled using autoregressive integrated moving average (ARIMA) time-series methods with nonlinear trend specifications, and competing models were compared using information criteria, out-of-sample hold-out forecast accuracy, and long-horizon extrapolation behaviour to project uptake through 2030. Among 6099 men, 24% received treatment intensification, with quarterly intensification rates increasing from 3% in 2014 to 56% in 2022. A restricted cubic spline ARIMA model (ARIMA(1,0,1) + RCS3) was selected as the primary base-case forecast because it showed superior out-of-sample hold-out accuracy and more tempered long-horizon extrapolation. The cubic specification was retained as an upper-bound scenario, reflecting the possibility of continued aggressive momentum in treatment adoption. Both specifications captured a marked inflection after 2020 that temporally coincided with guideline updates and funding expansions. Near-term base-case projections (through 2026) suggest continued growth in intensification toward 80-85%, with the upper-bound scenario approaching saturation more quickly. Projections beyond 2026 are exploratory and presented for methodological completeness, given the eight-year horizon relative to a nine-year observation window and the widening prediction intervals at extended horizons. Despite substantial growth over time, treatment intensification remains incomplete in routine practice. These findings are temporally consistent with the impact of policy and funding changes on the adoption of evidence-based therapy and underscore the need for ongoing implementation efforts to address persistent clinical and system-level barriers to equitable access.