I'd already had radiation on 4 different occasions before my breast cancer came back the 2nd time and then I had my 5th round of radiation. I knew from past experience that I would be feeling pretty weak, sick, and tired for at least 4-6 weeks after radiation was completed and I was also recovering from chemo at the same time. I took a 6 week break between chemo ending and the start of radiation. After radiation I took a 6 week break before starting the Letrozole.

My reasoning was based on not being able to tell if I was having side effects from a new medication when I was still feeling crappy and dealing with side effects from radiation and chemo. My doctor agreed with me. I was also supposed to take Ibrance and my doctor wanted me to begin that med at the same time as the Letrozole. I have tons of drug allergies and hate taking new medications. I told my doctor there was no way I was starting 2 new meds at the same time because if I was allergic I wouldn't be able to tell if the allergy was from one or both new meds and there was the same issue with potential side effects. I asked the doctor which one was most important to take first and she said Letrozole so that's the one I started with. I waited a month to see if I had an allergic reaction and all was good. I started the Ibrance a month after the Letrozole. My blood work was good with just the Letrozole, but when the Ibrance was added my liver wasn't happy. If I started both meds at the same time we wouldn't have known which med caused the problems.

I guess what I'm trying to say is it can be helpful to recover a while between finishing one treatment and starting the next. That way you have a better idea of what's going on with any potential side effects, especially since all the different cancer drugs and treatments can have overlapping side effects and it's important to know what treatment(s) is causing each different side effect. In most cases waiting 4-6 weeks isn't going to harm you and your body will be in a better place physically and mentally if it has some down time to recover before adding something new to the mix.

It depends on your age and your stage and the condition that you are in after radiation.
This is a discussion with your doctor.

If you saw your medical oncologist before radiation and this was set up then, you need to contact their office if you are having a lot of symptoms after radiation such as exhaustion.

My oncologist just said to start after radiation is complete