Newly Diagnosed - Letrozole

Hi, this is my 2nd time on this forum, so I hope I’m in the right area to ask my question. I have just finished my radiation yesterday, after having 15 rounds. I have been prescribed Letrozole by my Oncologist, and I was told to wait 2 weeks after my radiation finished before I start taking it. But I don’t understand why I need to wait this long? I know I should have asked my doctor at the time but it just didn’t occur to me, and since then I have heard other people say they started taking Letrozole straight after their rads, so has anyone else had to wait this long, and is this a common practice? Thanks for any advice you can give me!