From others I know who've gone through breast cancer treatment, I think you may ending up changing your workouts or taking a break from them during the treatment -- it will be important not to overdo things if you're feeling tired or have to let things heal. But exercise will be an important part of your recovery, especially if you end up having the mastectomy.
Re: Sharon's comments, one of my neighbors has been involved with a support group and sports activities for breast cancer survivors through the hospital where she had her surgery. There are probably similar support services in your area. And, he may have cheated at cycling, but Lance Armstrong started his foundation to help cancer survivors deal with the type of problem she mentions, as well as others (such as financial or insurance issues). They also have some materials to help keep track of treatments. Many people have found their services to be helpful, so it might be worth looking at their website if you haven't already.
Best wishes to you and your family as you go through this.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles