The trend in radiation for breast cancers caught that early has evolved a lot. I had a similar situation, with a very small lesion caught early, and negative sentinel lymph node.
The radiation recommended, as an option to consider, was five sessions, each two days apart at Cleveland Clinic. I know that other cancer centers are looking at very short-term radiation as well. [While I was figuring out my decision, I read of a woman who had a similar situation and went to Mayo Clinic and had five sessions of radiation also over a 10-day period of time so I think that that's going to be a general trend.]

One limiting factor will be if the radiation facility has the state of the art technology and people trained to use it. One thing I've learned from a cancer journey is that some recommendations that are made may not be the best state of the art protocol but are the best that a facility can offer at that time and dependent upon the age or quality of its technology. That is one serious argument for going to a top-ranked well-endowed facility that can provide the best current options. I'm always inclined to get a second opinion but any medical decision that is essentially irreversible because that tilts the risk reward balance a lot in my opinion.

For what it's worth, I did have the OncotypeDX test done at the recommendation of my oncologist. It was very reassuring in my case and I'm glad that I did it because it provides more data than just that upon which a chemo decision is made.