With vaginal cuff brachytherapy, do you treat to active or treatment length?

When you prescribe your dose at 0.5 cm depth of the vagina? For instance, if you measure a vaginal length to be 8 cm, would you prescribe to treat 4 cm active length or 4 cm treatment length? Or you would do something differently? There seems to be a variability in clinical practice in regards to the prescription length. If local control is equivalent between two different approaches (active vs treatment length), why not use a definition with a shorter prescription length which may potentially yield better toxicity outcomes?