Nervous to start

The studies you are asking about don't exist.

A mastectomy is a localized treatment. Endocrine (anti-hormone) therapy is a systemic treatment. One does not replace the other. So there will be no studies that compare having a mastectomy to going on endocrine therapy. It's comparing apples to oranges. With the 5mm of invasive cancer, your mother would be prescribed endocrine therapy even if she'd had the mastectomy.

With invasive cancer, there is always a possibility that some cancer cells escaped the breast and moved into the body prior to the surgical removal of the cancer. The role of endocrine therapy is to track down any rogue cells that might be in the body and, hopefully, kill them off. In this way, endocrine therapy reduces the risk of a metastatic recurrence and increases survival rates.

Endocrine therapy does the same thing with any rogue cells that might be left in the breast after a lumpectomy, thereby reducing the risk of a localized (in the breast area) recurrence. Endocrine therapy also reduces the risk that the patient might develop a new primary breast cancer in either breast, i.e. a second breast cancer, unrelated to the first diagnosis. These are additional benefits of endocrine therapy for those who have a lumpectomy, however because the most significant benefit is the reduction in the risk of mets, endocrine therapy is virtually always recommended to patients who have ER+ invasive cancer, whether they've had a lumpectomy or a mastectomy.

That said, with a small invasive cancer, your mother's risk of mets is likely quite low. Endocrine therapy will reduce this risk by about one third. It is her decision as to whether this risk reduction is worth the risk of side effects.

Your mother's Oncologist should have provided her with an estimate of her risk of a metastatic recurrence. If he/she hasn't, your mother should ask for this information. Until that discussion, there are two on-line risk models available which you can use to estimate your mother's metastatic risk, and see the amount of risk reduction from either Tamoxifen or an Aromatase Inhibitor.

https://breast.predict.nhs.uk/tool

http://www.lifemath.net/cancer/breastcancer/therapy/

A couple of other comments. First, although your mother has a greater amount of DCIS than invasive cancer, the invasive cancer is the more serious condition, so other than surgically removing the DCIS, the DCIS can be ignored. Anything done to treat the invasive cancer will be more than sufficient to treat the DCIS.

Second, your mother has the choice between two very different types of endocrine therapy. Usually, an AI is given to post-menopausal women, but Tamoxifen, the treatment given to pre-menopausal women, is also available to those who are post-menopausal. Tamoxifen comes with it's own side effects (every drug has side effects) but it is actually good for the bones. Your mother's oncologist should take her through the pros and cons of each therapy.

Hope that helps.