Reliability of CA15-3 Tumor Marker in Monitoring Therapeutic Response in Different Molecular Subtypes of Metastatic Breast Cancer

The prognostic impact of CA15-3 level in different molecular subtypes of metastatic breast cancer is not well elucidated yet; therefore, we conducted the present study to determine the reliability of CA15-3 tumor marker in terms of monitoring therapeutic response in different molecular subtypes of breast cancer. In this prospective study, we assessed the levels of CA15-3 in 83 patients with metastatic breast cancer assessable by RECIST, who were treated and followed up in Mosul Oncology hospital during 2017 and 2018. We evaluated the mode of changes of CA15-3 level after two cycles of systemic therapy (chemotherapy, endocrine therapy, or target therapy) and analyzed the relation between CA15-3 level changes and response to therapy in different molecular subtypes of breast cancer. Herein, CA15-3 level was more frequently elevated in Luminal subtypes of metastatic breast cancer compared with that in other subtypes. Additionally, the reduction in CA15-3 level after two cycles of systemic therapy was significantly correlated with the good response and longer progression-free survival. The mode of change of CA15-3 level was closely correlated with the clinical therapeutic response and survival advantage rather than the pretreatment level of CA15-3 in metastatic breast cancer. This finding revealed equivalent quality of CA15-3 with medical imaging at lower cost. Therefore, measurement of CA15-3 level at regular intervals before and after starting systemic therapy could predicate the clinical response and replace imaging examination used routinely for monitoring the responses in patients with Luminal subtypes of metastatic breast cancer.