Why would patient take arimidex after bilateral mastectomy?

My mother's physician has recommended bilateral mastectomy and sentinel node removal.  The mastectomy could not be scheduled until mid-May, and the physician stated that she was going to prescribe 'arimidex' to halt the progression of any hormone-sensitive cancer cells in the breasts.  She said that my mother will be taking Arimidex for five years.

If a patient has a bilateral mastectomy, why would they take a medicine to halt progression of breast cancer cells.  Since they are removing both breasts, I don't understand why Mom would take medicine that is used to 'prevent' or halt the spread of cancer in the breasts.  There would be no breast remaining for the cancer cells to exist?  

Is it because there are cancer cells in other areas of the body?  

Thank you for your replies.  We gratefully appreciate your thorough answers to question posted yesterday.  dp

[CCF-RN,MSN-rf]

Dear millerpuryear:  It is unusual to prescribe arimidex prior to surgery, especially if the hormone status of the tumor is unknown.  Once the tumor is removed, it should be tested for estrogen and progesterone receptors.  If these are positive, it means that estrogen may feed the growth of the cancer or cancer cells.  This is the reason to take arimidex or tamoxifen or another anti-estrogen drug to stop the growth ability of any possible stray cancer cells.  If the tumor is estrogen/progesterone receptor negative, there would be little or no reason to take these types of medications.

[goldenlover]

I also had bilateral mastectomies and took Tamoxifen afterwards to prevent a recurrence.  Both oncologists I consulted told me that no mastectomy removes 100% of the breast tissue in the body, some always remains.  Theoretically it is possible to develop breast cancer in this remaining tissue.

[VinceRost]

A related discussion, Why Arimidex (Anastrozole) after bilateral mastectomy was started.