Follow up mammo

In April 2015 I found a small lump, felt but not seen on mamma or ultrasound. There's periodic pain on that breast too. I saw 2 surgeons and neither recommended to remove it. Had an MRI that showed a 4mm cyst. (Dec 2015)

I'm wondering how they know for sure it's a benign cyst from the MRI? Also they are recommending a follow up mammo with ultrasound on the side with the lump (this month). I just turned 40 and have dense tissue, but it seems strange that they would recommend imaging that didnt show anything the first time.

My biggest concern is the radiation from the mammo. My GP said there is concerns with accumulative radiation, but it's the only option right now. Can the benefits (when they couldn't see anything the first time) outweigh the risk?