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 s...