I'm assuming your sister has the typical invasive ductal type rather than DCIS(non-invasive type)?
1) There are three main types of treatment for invasive breast cancer ): Surgical excision, radiation, and chemotherapy/hormonal therapy.
Re: Surgical Therapy. Most people have a choice between 1) Mastectomy or 2) Lumpectomy with Radiation therapy. If you opt for mastectomy, you generally do not need radiation, unless there is a possible chest wall, skin or adjacent structure or matted axillary lymph node involvement. If you opt for lumpectomy (surgical excision of lump only with good margins), you definitely need radiation therapy for rest of the breast. This is an absolute must because there is always a possibility that there may be a few cells around the original cancer site or elsewhere that can grow later to spread. So you definitely need radiation to kill the few cells that generally can linger, even with great margins. Studies have proven this. I personally know two people who opted out of radiation and had a recurrence 2 and 3 years later. The decision whether you are a candidate for lumpectomy is mainly cosmetic and whether you can handle the many trips to radiation therapy. If the tumor is too big and you can't get a decent cosmetic result with just a lumpectomy, the surgeon will not offer it. There is no definite size criteria. It may also depend on the size of the breast. Also, if the tumor is big, sometimes the oncologist will offer neoadjuvant chemotherapy to shrink it down before lumpectomy like MK said. Also, after trying lumpectomy, if the margins are not clear, and the surgeon tries for reexcision and the margins are still not clear, the surgeon will generally recommend mastectomy. Also, radiation therapy requires multiple sessions monday through friday for 5 to 8 weeks (There is newer techniques to shorten this (brachytherapy)).
The second part to surgical therapy is the Sentinel Lymph Node surgery. Either a radionuclide (Tc sulfur colloid) or blue dye (or both) is injected into the skin of the breast. The idea is that the tracer or dye will go the first lymph node in the axilla. The surgeon will take out only this lymph node (one or a few) and do a frozen section to see if the sentinel lymph node is positive. If this is positive, the surgeon will go ahead and do a full axillary dissection (take out a bunch more, 10-12) mainly for staging purposes. If it is negative, we're assuming that the rest are negative as well.
Re: Radiation Therapy. Required for lumpectomy. (Sometimes not if pathology is non-invasive lower grade DCIS) Generally not required after mastectomy except as above. Sometimes if you have lots of positive axillary lymph nodes, and especially if they're matted (stuck together), the radiation oncologist may irradate the axilla regardless of mastectomy or lumpectomy.
Re: Chemotherapy/hormonal therapy. Most people are recommend hormonal therapy. Tamoxifen for pre-menopausal women and Arimidex for post-menopausal women, especially if the cancer is ER/PR positive (estrogen, progesterone receptor positive). The side effects are generally ok and most people should get this. As for Chemotherapy, this is an option people should decide. Chemo does prolong life, but you really have to weigh the pros and cons since it is pretty hard to take. The marker used is her 2 neu. Positive generally means the cancer is more receptive to chemo. This should all be discussed the medial oncologist.
2) Breast cancer aggressiveness like most cancers is measured in two ways. Grade and Stage.
Grade: There are Grades I, II, III. This tells you how aggressive the type of tumor you have, III being the most aggresive. Younger women generally have more aggressive cancers, and older women can have any aggressiveness. However, this is not as important as Stage.
Stage: There are 4 stages (true for most cancers). Staging takes into account the T - tumor size, N - number of positive axillary lymph nodes, and M - presence of Mets. To put it simply, Stage I: small (<2cm) tumor with negative lymph nodes. Stage II: Either bigger tumor and/or positive axillary lymph nodes. Stage III: Either very large/chest wall involvement and/or multiple positive lymph nodes, matted together. Stage IV: spread to distant areas. This is a very simplified version of staging. The most important issue is whether you have positive lymph nodes.
I hope this information is useful.
Good luck Les,