Hi @sue417,
I exanded the title of your discussion to reflect your question. Here's some info from Mayo Clinic "Radiation is typically used after lumpectomy. But it might not be necessary if you have only a small area of DCIS that is considered low grade and was completely removed during surgery." Read more https://www.mayoclinic.org/diseases-conditions/dcis/diagnosis-treatment/drc-20371895

There are many discussions about breast cancer and radiation in the group, some of which may be related to the experience and information you're looking for. See the list of discussions here:
https://connect.mayoclinic.org/group/breast-cancer/?search=Radiation&index=discussions
Radiation to the entire breast or to part of the breast will depend on the results of your pathology. I can understand your concerns. Make sure your cancer team knows about your heart condition. You may wish to ask about a cardio-oncology specialist or department where you are getting care.

Mayo Clinic offers a program to address heart problems associated with cancer treatment called cardio-oncology or for people who have a pre-existing heart condition when diagnosed with cancer. The Cardio-Oncology Clinic (https://www.mayoclinic.org/departments-centers/cardio-oncology-clinic/overview/ovc-20442193) evaluates people prior to cancer treatment and patients who have experienced side effects due their treatment, and can help evaluate treatment when you have both cancer and heart conditions.

Do you have access to a similar cardio-oncology program?