For a patient with a solitary kidney, what dose constraints would you use for SBRT to a nearby site?  

I use the UK Consensus on Normal Tissue Dose Constraints for SRS & SBRT.For a solitary kidney, the SBRT 5 fxn OAR constraint is Mean <10 Gy V10 Gy <10% (optimal), but can go up to <45% (mandatory) This is based on the ABC-07 & SPARC trial protocols.

Assuming you're talking dose constraints on the solitary kidney, here is the process I would go through (in my own mind) when I try to make dose constraint decisions in the SBRT sphere. First, oftentimes we may have opinions only and not much hard data to go on when making dose constraint decisions ...