What small bowel dose constraints do you use when treating with short course radiation for rectal cancer?

In my experience, I have set a small bowel max of 25 Gy; sometimes allowing up to 27 Gy. This was out of an abundance of caution; our initial experience (1) was with 20 Gy/5 fractions and there was concern about increasing the radiation dose to 25 Gy. Also, we wanted to make sure that patients with a lot of small bowel in the pelvic field (most commonly, women with prior pelvic surgery) would be evaluated for more complicated planning if necessary.

When treating external iliac lymph nodes, have used IMRT to meet the same constraint.

However, there is no data driving this practice. In the RAPIDO study, where short-course radiation followed by chemotherapy is showing improved disease-related treatment failure compared to conventional chemorad...

None.