His doctor says Brown's prognosis is excellent. The 74-year-old governor is continuing a full work schedule during the treatment.
"Fortunately, this is early stage localized prostate cancer, which is being treated with a short course of conventional radiotherapy," Dr. Eric Small said in a statement.
"Localized" means the cancer is not spreading. This is Brown's second bout with cancer. He received treatment for skin cancer on his nose in April 2011. He had the growth removed and was seen walking around with a bandage after the surgery.
Brown is being treated at UC San Francisco and treatment will continue through the week of Jan. 7.