What SBRT dose and tissue constraints would you choose for a presacral rectal recurrence status post preoperative chemoradiation to 50.4 Gy followed by surgical resection in a patient who is not a candidate for surgical re-resection?

By first principes, irradiation should be given with lower doses per fraction, unless the recurrence is not near a critical structure such as bowel, ureter, periferal nerve or lumbosacrail plexus. Theses recurences are also multifical with nests of microscopic cells typically found in the surgical specimen. Keep in mind reducing the dose to a tolerable level such as 25Gy in 5 fx makes the treatment non-ablative (BEDs that are palliative).

We have published multiple papers on this. 39Gy in 26 fx BID avoiding small and large bowel <50Gy point dose with low expectations. We typically give I-125 seed implants under CT guidance to 80Gy instead and have published 25% 5yr OS.