Hi! I Have Been Taking Letrazole For The Past 15

Hi! I have been taking Letrazole for the past 15 months w/ good results (and minimal SE's) until now. My recent CT scan showed progression - increased size & # of mediastinal lymyph nodes/involvement (DX: St IV breast cancer; s/p mastectomy; no chemo or RT TX). My ONCO prescribed Arimidex today. What is the difference between the 2 drugs. I am 61 YO and concerned about the potential cardiac SE's. Are there any other options?