I am sorry for your loss. It may very well may have been due to the radiotherapy. Left Breast cancer patients were at greatest risk. The Left Anterior Descending (LAD) has been identified as the "Organ At Risk."
We used to treat the heart as a parallel organ, like the lungs; damage a piece with no significant harm to the overall function. We've discovered that it may be a serial organ, like the spine; damage a piece and you impair the entire organ. Or in this case the LAD is a critical element.
The mechanism of action is believed to be accelerated atherosclerosis; hardening of the artery leading to blockage:
We now use breath hold techniques and completely spare the heart, but mistakes were made -- that only a long retrospective review of the data was able to reveal:
We used to treat the heart as a parallel organ, like the lungs; damage a piece with no significant harm to the overall function. We've discovered that it may be a serial organ, like the spine; damage a piece and you impair the entire organ. Or in this case the LAD is a critical element.
The mechanism of action is believed to be accelerated atherosclerosis; hardening of the artery leading to blockage:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4188157/
We now use breath hold techniques and completely spare the heart, but mistakes were made -- that only a long retrospective review of the data was able to reveal:
https://www.nejm.org/doi/full/10.1056/NEJMoa1209825