Which tumor marker is used to determine the usefulness of trastuzumab (Herceptin) therapy for breast cancer?

The appropriate tumor marker for assessing the utility of trastuzumab (Herceptin) therapy in breast cancer is HER-2/neu. This is a protein that can be overexpressed in some breast cancer cells, indicating a more aggressive disease and often correlating with poorer prognosis. Trastuzumab is specifically designed to target and inhibit the HER-2/neu receptor, making it particularly effective for patients whose tumors demonstrate enhanced levels of this marker.

When the HER-2/neu status is positive, it signals that trastuzumab may be a viable treatment option, as it can significantly improve outcomes in these patients. Monitoring HER-2/neu levels before starting therapy allows healthcare providers to personalize treatment plans, selecting therapies that are most likely to be effective based on the cancer's characteristics.

The other options listed do not serve the same purpose in relation to trastuzumab therapy. For instance, progesterone receptor (PR) status can inform about hormone therapy options but is not related to HER-2 status. Carcinoembryonic antigen (CEA) is primarily used in monitoring treatment response and disease recurrence, especially in gastrointestinal cancers, not specifically for breast cancer and Herceptin use. Myc is an oncogene but does not directly