Reading so much I'm confused.

HER2, an acronym for human epidermal reproductive protein factor 2, is a naturally-occurring growth hormone. If your tumor cells are HER2-, that means they don’t overexpress that growth hormone. HER2+ means they do. In IDC, the latter is considered a hallmark of aggressiveness and a higher risk of spread outside the breast.

But HER2 is pretty much irrelevant in DCIS, and often not even measured—most DCIS does overexpress HER2, but since DCIS can’t escape the ducts, no matter how large it grows (unless a rogue cell does mutate into IDC and remains dormant for years, maybe decades) there is no reason to give targeted therapy (e.g., Herceptin or Perjeta) because targeted therapy works better on aggressive cancers (like triple-positive or ER/PR- HER2+IDC) treated concurrently with chemo—and pure DCIS doesn’t warrant chemo. In fact, the reason DCIS is often large and multifocal is that it is HER2+, but that growth still is confined to the ducts. Your tumors, measured in the aggregate, were quite large (>6cm, or at least 2-1/4”) so mastectomy was a good choice for you. (Because of multifocal and/or large tumors, that's why so many patients with DCIS choose mastectomy rather than multiple lumpectomies).

Your tumors were 100%-estrogen-receptor positive, and very highly progesterone-receptor positive. That, despite their being Grade 3, means that once removed (especially by mastectomy) whatever tumor cells might theoretically have escaped beforehand depend on estrogen for their survival and are highly vulnerable to estrogen-deprivation. How long you will stay on Tamoxifen alone—rather than some form of ovarian suppression followed by an AI—would depend on your age, whether you are planning and how close you are to being peri-menopausal, rather than still strictly premenopausal. If you are in your 40s or early 50s you’re probably looking at living at least another 20-30+ yrs. if you starve those tumor cells of estrogen. Good question to ask your oncologist.

The reason you would need no further treatment after a double mastectomy even though your tumor was ER/PR+, is that almost all the breast tissue would have been removed.

In people with invasive cancer, the anti-hormonals are given for two reasons. To protect the breast against recurrences and new primaries, and more importantly to protect against any cells that might have already left the breast prior to surgery (distant metastasis). If you have pure DCIS, then by definition, no cells could have escaped so that reason becomes irrelevant and the only reason to take it would be to protect the remaining breast tissue.

With a double MX, there is so little breast tissue remaining that even though the risk of recurrence/new primary is not zero, it is low enough that the risks of the medication begin to outweigh the benefits. Edited to add: Even with a grade 3 tumor, the chances of any cells escaping the breast without first colonizing outside the ducts (and therefore being noticed even as a micro invasion during the pathology) are infinitesimal (even if not zero) and the risks of the drugs would again outweigh any benefit.

For those of us who have had either lumpectomies or single MX, there is more breast tissue left to worry about, and therefore the benefits of the medication often outweigh the risks.

Hope that helps clarify things for you.