considering preventative mastectomy

Dear Doctor,
I am 60 years old, have been dignosed recently with breast cancer, and carrier of BRCA1 mutation.
Underwent lumpectomy, tumor 1.5 cm. stage 1, grade 1, node neg ,hormone receptors 75% posititive, Her-2 neg, Ki-67 less than 5%.
I am undergoing radiation therapy, have had 15 sessions out of 30, but since I received the result of the genetic test, I am considering prophylactic bilateral mastectomy. What are the pros and contras of this option in my case ?  may I just stay under close surveillance (MRI, mammography, US) and take the Tamoxifen therapy for 5 years as the protocol proposes ?
If I decide on mastectomy may I and should I STOP RADIATION therapy now  because in such case it only does the damage without the benefit ?
If I decide on the mastectomy, will I still be treated with Tamoxifen ?
Also, I read on the internet that Tamoxifen does not have the same preventative effect on carriers of the mutation as it has on non-carriers;  is this true ?  And what happens after 5 years of Tamoxifen, even if I was lucky enough not to have a recurrence of the disease ?  
Please forgive my imperfect english, I write from overseas.
Thank you !

[Cleveland Clinic]

Dear overseas:  There are two reasons that tamoxifen may be used: One is for prevention in high risk women and the other is to reduce the risk of recurrence in women who have been diagnosed with breast cancer.  One reason tamoxifen may be less effective in preventing breast cancer in BRCA1 carriers is that cancers in those individuals are often hormone receptor negative and therefore are not dependent on estrogen. If your cancer is estrogen and/or progesterone receptor positive, there is no reason to think that tamoxifen or an aromatase inhibitor would be less effective even though you caarry a BRCA1 mutation.



In terms of considering mastectomy, this is a viable option for prevention of local recurrence and breast cancer in the other breast.  However, as you are midway through lumpectomy and radiation, you should discuss pros and cons with your surgeon, oncologist and radiation oncologist as it relates specifically to your situation.  Likely, tamoxifen (or one of the aromatase inhibitors if you are postmenopausal) would be recommended regardless as the purpose of this is to reduce the risk of systemic recurrence as well as local recurrence.  After 5 years, surveillance and/or treatment would continue as with any woman with a history of breast cancer.