Why do some doctors give chemo before surgery to shrink breast tumor?

Depends: It depends on several things, such as the cell type of the cancer, if it is hormone sensitive. Larger cancers, if there are multiple sites of breast cancer, or if the cancer has already spread may be conditions where chemotherapy is used before surgery to shrink the tumor first so as to possibly minimize the amount of surgery needed, or to avoid a mastectomy.

Neoadjuvant ChemoRx: It is not uncommon to have sufficient information about a breast cancer from biopsy and a mammogram to decide whether or not chemotherapy is needed. Accordingly, it may be advantageous to give the chemo before surgery. The main benefit is to shrink a large tumor and avoid a mastectomy. Other potential benefits are starting the most important rx first and being able to gauge cancer sensitivity.

Yes for shrinking : I agree with several of the other answers. It is used as a means to reduce a tumor that could not be treated with breast conservation, so that it can. We are learning that strongly hormone receptor sensitive tumors might do better with hormone blocking agents. Her-2 postitive tumors may have the best benefit. This treatment does not impact on cure in a major way.

To shrink big tumors: Neoadjuvant chemotherapy is for three main reasons: 1. To treat inflammatory breast cancer, when chemotherapy should always be given before surgery 2. To shrink large breast cancers that are very close to the chest wall muscle before they have a chance to invade 3. To shrink large tumors in order to avoid a mastectomy and have only the lump removed instead (lumpectomy). The risk of cancer returning is no different that having chemotherapy after surgery.

Less invasive: The smaller the tumor, the less invasive the surgery to attain clear borders, and the less disfiguring it will be. Also, this requires less reconstructive surgery as an added benefit.