fibroid tumors

I have gone to the doctor for my yearly and have been having trouble with my periods (irregular) now for about three months now. She sent me for an ultrasound and took a biospy and the tests came back...I have 4 fibroid tumors on my uterus and the largest one is the size of a grape. They are benign, but my question that I forgot to ask my doctor is what are the chances of them becoming cancerous? I am going to have an Endometrial Ablation and then she is going to insert a birth control object called Mirena. I asked her can the Mirena cause cancer and she said no. I am having the procedure done in August and need any information that will help ease my mind. Please help me.