When doing a tumor bed boost following whole breast irradiation, what do you typically use for CTV and PTV margin for photon and electron plans?  

This is one of the few times that I do not do a traditional CTV/PTV expansion. "Old school" was a block margin of 2 cm around the scar - and then boost to a selected depth + choose IDL for coverage ("90% at 2 cm depth). We do know now that we risk missing the cavity if we rely on the scar, as we are...