What image guidance do you use when treating rectal and anal cancers?  

For anal cancer, daily CBCT for the first three fractions to evaluate bladder filling mostly (and to make sure there's not surprise excessive gas in the anus/rectum). After that, KV daily and CBCT weekly. If treating with a vaginal dilator in, I may CBCT more frequently to evaluate its position.

I use daily kv imaging, including for prone IMRT, which is how I most often treat anal cancer. I have not found that cone beam CT scans lead to changes in set up, but this is a controversial area and I am very open to hearing from others who find weekly or even daily CBCT scans helpful.

Agree that the above approaches are reasonable and @Harvey J. Mamon's comment that this is a controversial area. Nevertheless, IGRT parameters should be individualized. There are pitfalls in using across-the-board departmental IGRT policies based on organ site. A rectal cancer patient getting 3-fie...