What GTV to CTV margin do you typically use for the primary lesion in anal cancer?

With the availability of modern on-board imaging, I use a smaller margin, typically a 1.5 to 1.8 cm expansion from what I contour as the anal canal, which includes the GTVp. I've found that this margin is adequate and can help reduce skin toxicity and dose to the genitals.

I have concerns about even attempting to delineate a GTV for the cephalad portions of many anal canal cancers. An advanced tumor that extends cephalad above the pelvic floor can infest the mesorectal compartment with highly irregular margins. For axial slices at the upper extent of macroscopic disea...

Not exactly. But when you include the entire canal at that level and the MRE, it is often 2.5 cm. CT is not great for delineation. PET-CT resolution is low, and FDG avidity will likely overestimate the extent of disease. MRI is occasionally helpful. The other thing is that the anal canal is about 3-...