In what ways can cancerous fibroids be diagnosed other than surgery?

I have 4 uterine fibriods the largest is 9 cm.They have not caused any discomfort in the last 4 years I've had them.I know they are non-cancerous ,but my gyn wants me to have a hysterectomy because of the size.I sought a second opinion and I got the same answer.The two doctors say that they may be cancerous because of the size.But I have sonograms every year and they are not getting bigger.Everywhere I read ,I see that uterine fibroids are benign tumors. I am 38 years and I do not want a hysterectomy at this age.Some doctors are quick to do unnecessary surgeries and I do not know If I should go ahead with the hysterectomy or wait.