How do you approach SBRT liver constraints when the total liver volume is <700 cc?

We consider 3 different methods of thinking about liver constraints when evaluating a plan:

1) Sparing 700 cc's to < 15 Gy (for 3 fractions), and <17 Gy (for 5 fractions).

2) Liver mean dose constraints (as in RTOG 1112 for CP A cirrhosis, <14 Gy), and <15-16 Gy for non-cirrhotic patients depending ...

I'm not aware of data that addresses this question. My understanding is that our constraint of sparing 700 cc of liver is extrapolated from surgical data, in which this is considered the minimal volume of residual liver required for adequate hepatic function following a partial hepatectomy. My pract...