In locally advanced rectal cancer treated with total neoadjuvant therapy, do you adjust boost volumes to only include post-chemo gross disease/nodes plus a margin?
The answer to this question is that there is usually no risk reduction advantage to making the boost volume smaller for the last 5.4Gy. it may be better to make the boost volumes larger than a uniform expansion on the GTV.
There is no definitive guide to boost volumes in rectal cancer. We published our pattern of pelvic failure after 45Gy* which showed 10% pelvic recurrence rate with 50% in the presacral area and 25% in the rest in the low pelvis. We have used generous margins (3 cm above and below pre chemo GTV and always include the lateral non-TME nodes because that is where they tend to recur.
We only reduce the volume or even omit the boost altogether if there is a lot of small bowel in the volume or in an effort to spare the anal canal if we can.