I used it during IVF -- a pill, perhaps you are talking about a cream? I thought it was fine, quite benign, but again the situations are different since you have the breast-cancer history to consider. Your oncologist should be able to advise you. I think there is said to be a difference between vaginal and oral as far as how much estrogen travels in your system.
I have been using Vaginal oestrogen for a few years now and it has helped me a lot. My vaginal skin is healthier and this has also helped my prolapses from progressing like they were. Before using the cream my vaginal skin was lax and dry but now things are a lot better.
Also, due to perimenopause then menopause the lack of natural oestrogen I had was giving me Vaginal Bacterial infections, but now I use the cream the BV infections are much much less