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The most commonly used breast cancer mucin assay is designated CA 15.3. CA 15.3 is not specific for breast cancer because levels are also increased in patients with gastric, colorectal, lung, pancreas, ovarian, uterine, and cervical cancers. Levels are also elevated in a few healthy women, 10% of patients with benign breast diseases, and 40% of patients with inflammatory liver diseases. CA 15.3 levels are not elevated during pregnancy or lactation.
CA 15.3 is not very sensitive in detecting early breast cancer; only 5 to 30% of patients with stage I and II cancer have elevated levels. CA 15.3 is not very useful in the differential diagnosis of breast masses, since it is elevated in some benign breast diseases and often not elevated in early breast cancer. CA 15.3 levels do not correlate with known prognostic factors, such as histologic grade, receptor status, or age. It does not have prognostic value for predicting which patients with metastatic disease will respond to therapy.
CA 15.3 is not sensitive enough to accurately monitor the disease course in patients with no evidence of disease after their primary therapy. CA 15.3’s positive predictive value for relapse was only 41% for the first antigen elevation and 58% for two consecutive antigen elevations.
The American Society of Clinical Oncology published a practice guideline on the use of tumor markers in 1996. The expert panel concluded that CA15-3 is of limited value for detecting disease recurrence in postoperative patients for three reasons: (1) it is not a sensitive indicator of micrometastases, (2) no curative therapy exists for recurrent breast cancer, and (3) the test generates many false positive and false negative results. A more promising application for CA 15.3 is monitoring the response to metastatic cancer therapy. All of the breast cancer mucin assays appear to be more sensitive for monitoring patients with metastatic disease than CEA. In metastatic breast cancer, 70 to 90% of patients have elevated levels of mucin antigens, compared to 50 to 60% for CEA. When mucin antigens are elevated, there is better correlation between mucin antigen serial levels and disease activity than between CEA serial levels and disease activity. Antigen levels must change at least 25% above the pre-change value before they are considered clinically significant.
For sensitivity, the correlation of changes in antigen levels with clinical course is defined as an increase of 25% or more in patients with progressive disease, a decrease of 25% or more in patients with regressive disease, and as a variation of less than 25% in patients with stable disease. Care must be taken not to over interpret rising antigen levels, since transient spikes lasting 30 to 100 days may occur shortly after therapy, due to tumor cell lysis. Monitoring the disease course of patients with metastatic disease is the most successful application of breast cancer mucin assays to date.
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