What cumulative dose constraints would you use for the normal liver when repeating SBRT for liver metastases with a longer time interval (i.e. 2-3 years)?

I will assess cumulative dose within the normal liver with retreatment but if the interval is 2-3 years, I think there has been enough repair in a non cirrhotic liver that one could consider it nearly de novo treatment. There has been very little literature directly examining the impact of re-irradi...