I am so sorry you are dealing with this. I was diagnosed with Stage 2, ER/PR+, HER- breast cancer in the fall of 2022, and had a mastectomy, followed by 19 rads treatments. I have been diving since then with no ill effects, though I have noticed that my air consumption is not as good as it used to be. My radiation oncologist assured me that diving was fine as is travel to high altitudes (I also ski). My bigger complaint has been the anti-hormone drug, anastrazole in my case. Every old injury I have ever had has returned, I get trigger fingers (never had before), and recently had to have a meniscus repair. I’ve been taking the anastrazole since Feb 23. I plan to ask my oncologist to change at my next appointment. Best wishes to you as you go through a not-very-fun process.