Individuals aged over 80 years (the “oldest-old”) represent the fastest-growing demographic in clinical oncology. While the Age-Adjusted Charlson Comorbidity Index (ACCI) is a traditional benchmark for multimorbidity, its predictive accuracy in the ultra-elderly may be limited by competing risks of death and the physiological burden of active malignancy.

To evaluate the prognostic utility of the ACCI and to develop and internally validate a streamlined stratification tool, the ABC Score, for octogenarian cancer patients.

We conducted a retrospective cohort study of 203 patients aged ≥ 80 years, first evaluated between 2022 and 2024 at a Brazilian public institution. Overall survival (OS) was analysed using Cox proportional-hazards models. Variables significant at p < 0.20 in univariate screening were entered into a multivariable model with backward elimination to derive a points-based score from the β coefficients. Discrimination was assessed by Harrell’s C-index and internally validated by bootstrap resampling with optimism correction; calibration was assessed at 6, 12, and 24 months.

Over a median follow-up of 30.5 months, 80 deaths (39.4%) occurred among 203 patients. The ACCI was significant in univariate analysis (HR = 1.28, p < 0.001) but lost independent prognostic value after adjustment (p = 0.30). Independent predictors were ECOG performance status, clinical stage, and chronic pulmonary disease. The ABC Score had an apparent C-index of 0.778 and an optimism-corrected C-index of 0.771 (95% CI 0.721–0.825) after bootstrap validation, with excellent calibration at 6, 12, and 24 months. Three risk tiers stratified survival (log-rank p < 0.001).

The ABC Score is a promising, pragmatic exploratory tool that prioritises functional and respiratory reserve over aggregate comorbidity counts; it requires external, multicentre validation before clinical use.