When using EBRT to deliver APBI, what dose do you prescribe and to what volume?  

NSABP B-39/RTOG 0413 prescribed a dose of 38.5 Gy using two fractions of 3.85 each daily, but prescribed to the ICRU 50 reference point dose (usually isocenter). Many studies trying to replicate this treatment report worse cosmetic toxicity, but seem to prescribe the dose to the entire PTV volume (Leonardi 2013, Hepel 2009, Jagsi 2010).  However, the University of Florence phase 3 trial used 30 Gy in 5 fractions to the partial breast volume and found improved cosmetic outcomes compared to whole breast RT (Livi et al. 2015).