What are the current timing recommendations for initiating radiation after lumpectomy?

For me, this is dependent on the RT technique and plans for chemotherapy.

If planning partial breast (regardless of plan for chemotherapy), I will recommend the RT prior to chemotherapy. I will usually simulate 3-4 weeks post-op and then start RT a week later.

If planning whole breast, if no plan f...

I agree with Dr. @Dr. First Last. The only thing I would add to that is the healing component is sometimes a factor as well. For example, in a patient with a large seroma/hematoma (e.g. patient on blood thinners) postoperatively, I may give a bit more time to promote healing, and if really concerned...