Abstract P1-05-12: Evaluation of the benefit of the tumor marker cancer antigen 15–3 (CA 15-3) in different subtypes of metastatic breast cancer
Authors: Lindman Henrik , Molin Ebba Cederqvist , Jernling Martin , Schiza Aglia 
Published: March 2023

Abstract
Background: Cancer antigen 15-3 (CA 15-3) is the most commonly used tumor marker in patients with breast cancer (BC). However, the usefulness of CA 15-3 is still controversial and there is lack of evidence for its role in metastatic BC. We performed a retrospective analysis of the value of CA 15-3 for monitoring and diagnosis in subgroups of metastatic BC. Method: This retrospective study included three cohorts of a total of 196 patients with advanced or metastatic BC treated during the years 2015-2020. Patients’ data, such as subtype, hormone receptor status, and CA 15-3 values, were retrieved from a treatment database to find out how well the CA 15-3 levels followed the disease progression or remission over time. An increase or decrease of CA 15-3 less than 10% indicated stable disease and changes larger than 10% indicated progress or response of therapy. The patients were then categorized into one of six groups based on the proportion of correct CA 15-3 values in each patient during the course of the disease. A proportion of correct values of 95% or higher placed the patient in category 5, between 90% and 95% category 4, 75% to 90% were category 3, 50% to 75% category 2 and below 50% category 1. Patients without any elevated CA 15-3 values and with no evidence of association with a trend of CA 15-3 and disease progression were categorized as 0. Patients in categories 3, 4, and 5 were classified with approved CA 15-3 conformity and categories 4 and 5 with excellent conformity. Results: The median number of samples per patient was 27 (range 5-102) and at diagnosis of metastatic disease, 79%, 66% and 60% of patients had elevated CA 15-3 in the luminal, HER2-positive, and TNBC groups, respectively. Overall, 72.4%, 95% confidence intervals (CI) (66.2-78.7) of the patients had an approved conformity of CA 15-3 to monitor the course of the disease. The benefit of CA 15-3 differed significantly between the subtypes, luminal BC had an approved conformity for 94%, 95% CI (88.6-99.1) of the patients, HER2 BC 63%, 95% CI (52.1-73.5) and for TNBC 46% (95% CI 29.9-62.0) of patients. In patients with Luminal A, the conformity was excellent in 84% of patients compared to 60% of patients with Luminal B. Estrogen receptor status (ER) was the single most important factor for the value of CA 15-3, with an approved conformity of 91%, 95% CI (85.6-95.7) of ER-positive patients and 37%, 95% CI (25.3-48.2) of all ER-negative patients. Elevated baseline levels of CA 15-3 in the diagnosis of metastatic BC increased the benefit of the marker to approved levels in 62.5% of the ER-negative patients, but ER-positive patients mostly had approved conformity, despite negative CA 15-3 at baseline. Visceral or bone metastases further improved the utility of CA 15-3 and soft tissue or solitary CNS metastases decreased it. Conclusion: The benefit of serum marker CA 15-3 in monitoring patients with metastatic BC depends mainly on whether the tumor expresses ER or not. ER-positive patients were excellently monitored in most cases and elevated CA 15-3 values at baseline indicated potential utility in ER-negative patients.
Citation Format: Henrik Lindman, Ebba Cederqvist Molin, Martin Jernling, Aglia Schiza. Evaluation of the benefit of the tumor marker cancer antigen 15–3 (CA 15-3) in different subtypes of metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-05-12.