How would you manage a patient with Lynch syndrome who is s/p surgery and pelvic RT 20+ years ago for endometrial carcinoma with a new T3N0M0 squamous cell carcinoma of the anus?

I would restrict my radiation fields to the anal tumor only + margin (plus PET+scan volume) and to the inguinal nodes

I have treated some patients like this with cancer of the vulva with previous EBRT.

I would limit the dose and volume based on overlap and focus on the area at highest risk like the primary and groins.