The most important point to be understood is that, fibroids are not cancerous mass of tissues. So their origination from cancer cells is just a myth. These cells develop due to the abnormal growth of uterus cells, forming a mass or lump of varied sizes. However the exact cause of fibroid formation is not known.
It is observed from various cases that, hormones seem to impact the growth of fibroids to a certain extent, since, the existence of fibroids is there until the menstrual cycles exist. And, after attaining menopause, they seem to shrink. However again, this is not valid in all the cases and might differ for every individual.