Dear Tracie,
The question you pose is a good one. Unfortunately, it is the subject of some controversy. For example, Susan Love, MD and Dennis Slamon, M.D., Ph.D. (credited with the discovery of herceptin) have two somewhat different perspectives.
At Susan Love's cite, one of her "frequently asked question" responses is as follows:
"HER2/neu is not useful in guiding DCIS treatment because it is more common for DCIS to be HER2/neu positive than it is for invasive cancer to be HER2/neu positive (we don't yet know why this is). Also, we have no data on treating women with DCIS with Herceptin, which is the treatment used for HER2/neu-positive tumors. This is, though, an area of active research."
Dennis Slamon is quoted on the Abbott Laboratories website within the following context:
Most breast cancers begin in the milk ducts, narrow passageways that radiate throughout the breast. A few cells, for reasons that are not completely understood, start accumulating genetic mistakes that cause them to grow abnormally. Eventually the cells develop into DCIS (ductal carcinoma in situ). The good thing about dcis cells is that they haven't spread beyond the milk duct. The bad thing is that they are malignant. "Some people call DCIS precancer, but it's not precancer," says Dr. Dennis Slamon, director of breast-cancer research at the UCLA School of Medicine. "It's preinvasive. It's cancer that hasn't invaded outside the breast ducts."
It appears that Slamon leans toward your thinking that DCIS is malignant preinvasive cancer. It also appears that we do not know why a large percentage of DCIS is HER-2 positive, yet a lesser percentage of invasive cancer is HER-2 positive.
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