What is the rationale for 6 hours between EBRT and HDR fractions for BID treatment?  

Based on half-life of repair of about 1.5 hours for most normal tissue (spinal cord is one of the exceptions). So 6 hours will be 90% repair with 4 half-life intervals.

In the early nineties, there was experience by @William M. Mendenhall's group, I believe, with hyper fractionation for head and neck. The 4-hour interval showed toxicity that was not apparent with a 6-hour interval. This was thought to speak to fundamental empirically derived normal tissue biology a...