Treatment with steroids is lengthy and usually requires about 6 months. While some source report very good success with steroids  most report a considerable risk of recurrence after a treatment with steroids alone. Steroids are known to cause elevation of prolactin levels and increase risk of several conditions such as diabetes, and other endocrinopathies which in turn increase the risk of IGM. Treatment with topical steroids to limit side effects was also reported in one case.  For surgical treatment recurrence rates of 5-50% have been reported. 
I realize that I am posting several years later, but my story is similar. One night in the shower, I felt numerous, very painful lumps in both breasts. It really scared me, needless to say. I saw my PCP a couple of days later, who referred me to have a mammogram. After the mammogram, I was taken to have an ultrasound performed and then to a patient room and waited while the radiologist reviewed the films. After what seemed like an eternity, she came in and said that, initially, she was concerned about cancer. But because the lumps were in both breasts, she believed I have "Chronic granulomotous mastitis". She then performed a biopsy. I was referred to an oncologist who confirmed that diagnosis. She also told me that it is very rare and that I was only her 2nd patient in a year. She prescribed methotrexate, which was quite effective. After about 8-10 months all seemed well and resolved.