CA 15.3, Metastatic Burden, and Overall Survival in Hormone Receptor-Positive/HER2-Negative Advanced Breast Cancer: Real-World Evidence from Peru.

Background/Objectives: Serum cancer antigen 15.3 (CA 15.3) is frequently measured in advanced breast cancer, but its prognostic significance after adjusting for established clinical factors remains incompletely characterized in Latin American populations. We evaluated the association of baseline CA 15.3 with metastatic burden and overall survival (OS) in hormone receptor-positive/HER2-negative advanced breast cancer and descriptively explored baseline/follow-up patterns. Methods: We retrospectively evaluated 127 women treated at a Peruvian public oncology institution between 2018 and 2024, with CA 15.3 levels ≥32.4 U/mL being considered elevated. Baseline associations were assessed using multivariable logistic regression, and OS was evaluated using Kaplan-Meier estimates and Cox proportional hazards regression adjusted for age, ECOG performance status, de novo versus recurrent disease, luminal subtype, first-line treatment, number of metastatic sites, and metastatic site. Results: Baseline CA 15.3 was elevated in 41/127 patients (32.3%) and was independently associated with postmenopausal status (OR: 3.27; 95% CI, 1.33-9.03; p = 0.014) and ≥2 metastatic sites (OR: 2.72; 95% CI, 1.24-6.10; p = 0.013). Median OS was 55 months (95% CI, 43-78), while elevated baseline CA 15.3 was associated with shorter OS (41 vs. 65 months; log-rank p = 0.024) and remained independently associated with shorter OS after multivariable adjustment (adjusted HR: 3.42; 95% CI, 1.63-7.19; p = 0.001). Conclusions: Elevated baseline CA 15.3 was associated with greater metastatic burden and shorter OS, and baseline/follow-up patterns were exploratory because the sampling was not standardized. Thus, CA 15.3 should be interpreted as an adjunct to clinical and radiological assessment.