Incidence of Malignant Oral Cavity Tumors in the Republic of Serbia: Trend Analysis (1999–2020) and Projections to 2040

Background and Objectives: Malignant tumors of the oral cavity are an important and growing component of the global cancer burden, with well-documented sex disparities in incidence. This study aimed to characterize temporal trends in the incidence of these tumors in the Republic of Serbia between 1999 and 2020, overall and by sex, and to forecast incidence through 2040. Materials and Methods: All histopathologically verified cases (ICD-10: C01–C06, excluding the lip) reported to the Cancer Registry of the Republic of Serbia (excluding the Autonomous Province of Kosovo and Metohija, for which data are unavailable) were analyzed retrospectively. Age-standardized incidence rates (ASRs; world standard population) were calculated, and joinpoint regression estimated the annual percentage change (APC) with 95% confidence intervals (CI). Autoregressive Integrated Moving Average (ARIMA) models projected incidence to 2040, with sensitivity analyses for the 2016 extension of Registry coverage. Results: A total of 5695 cases were registered (3872 men, 1823 women). Restricted to the population under consistent Registry coverage (Central Serbia, 1999–2015), incidence did not change significantly in either sex (men: APC = −0.30%, 95% CI: −1.63, 1.06; women: APC = −3.31%, 95% CI: −7.41, 0.96). Over the full 1999–2020 series, which includes the 2016 extension of coverage to Vojvodina, incidence appeared to rise among men (APC = 1.71%; 95% CI: 0.47, 2.96; p = 0.009), with an apparent acceleration after 2013 (APC = 8.39%; p = 0.002); a level-shift model and a formal comparison of competing trend descriptions both attributed this apparent rise to the 2016 coverage change rather than to a genuine trend (+50.7%; p < 0.001). Male incidence was about twice that in women (mean ASR 3.63 versus 1.64). Under an unchanged-coverage scenario, ARIMA models projected a flat mean incidence through 2040 (2.6 per 100,000 overall; 4.7 in men, with a 95% prediction interval widening from 3.6–5.8 in 2021 to 1.2–8.2 in 2040), with a considerably less precise and less reliable forecast for women. Conclusions: Oral cavity cancer incidence in Central Serbia was stable from 1999 to 2015, with a persistent male excess. The apparent post-2013 rise is attributable to the 2016 extension of registry coverage rather than to a genuine trend, and a counterfactual estimate suggests considerably higher incidence in Vojvodina. Region-stratified reporting is needed before national trends can be interpreted reliably.