Using the Cancer Aging and Research Group- Breast Cancer (CARG-BC) predictive model in older adults (OA) with early breast cancer: an external validation study

Purpose: Decision-making for chemotherapy in early breast cancer (EBC) in OA (older adults: age ≥65 years) is complex due to frailty, multimorbidity, and competing risks for mortality. Magnuson (2021) developed a chemotherapy toxicity prediction score, CARG-BC; its external validation can improve generalizability. Objectives: CARG-BC’s ability to predict grade 3+ chemotoxicity in OA with EBC (primary), unplanned healthcare use, and changes to chemotherapy protocol (secondary). Methods: A single centre retrospective cohort study comprising OA with EBC who received (neo)adjuvant chemotherapy from 2013-2023. Clinical, demographic, CARG-BC, and healthcare usage variables were extracted from patient records. Risk groups based on CARG-BC score were compared using T-test (continuous variables) & χ2 test (categorical variables). Toxicity risk based on CARG-BC score was assessed using logistic regression. The predictive ability of the CARG-BC score was evaluated by calculating AUC. Results: Of 243 patients, the median age was 70 years (range 65-86), 99.6% female, 80.2% with comorbidities, 33.7% with polypharmacy, 28.8% living alone, and 8.2% seen in the geriatric oncology clinic. Over half (53.9%) had grade 3+ toxicities. Healthcare utilization included 19.8% of patients with at least one unplanned clinic visit, 29.6% an emergency care visit, and 14.4% a hospitalization. The median CARG-BC score was 7 (IQR 3, 8) and the CARG-BC AUC was 0.76 (95% Confidence interval [CI] 0.70, 0.82). The odds of grade 3+ toxicity is increased by 1.33 times per CARG-BC point increase. Conclusion: The CARG-BC model retained good discrimination for grade ≥3 chemotoxicity and should be used in shared-decision making with OA.