Discovery of a Sinusoidal Pattern in the Cancer Antigen 15-3 Time Series of a Patient with Metastatic Breast Cancer

Cancer Antigen 15-3 (CA 15-3) is a glycoprotein commonly linked to breast cancer. Monitoring CA 15-3 levels is crucial for tracking disease progression and evaluating treatment response in patients with metastatic breast cancer. Although fluctuations in CA 15-3 levels are common in patients with metastatic breast cancer, no periodic pattern has been identified to date. In this study, a sinusoidal pattern was observed for the first time in CA 15-3 time-series data of a patient with metastatic breast cancer. This pattern recurred every 175 days, from February 2024 to January 2025. Statistical analysis strongly indicated that the observed sinusoidal variation was highly unlikely to have occurred by chance, thus providing an excellent fit to the data. This finding is both novel and captivating, as changes in CA 15-3 levels are usually linked to disease progression or treatment responses rather than following a predictable sinusoidal rhythm.