TDF Factor (Time-Dose-Fractionation)

The TDF system was developed by Orton and Ellis in 1973 as a practical clinical tool to simplify the Nominal Standard Dose (NSD) equation. It was designed to allow clinicians to easily add together the biological effects of different treatment phases. While largely superseded by the BED (Linear-Quadratic model) in modern practice, it remains a valuable reference for historical data and some long-term clinical trial analysis.