Variation of V105% between pre- and postmerged subfields in field-in-field hypofractionated breast radiotherapy plans.

Hypofractionated whole-breast irradiation has emerged as a viable alternative to conventional fractionation. In the field-in-field forward planning technique, a merged plan with 2 to 4 segmental fields is the final plan delivered to the machine. As per the ASTRO guidelines for the hypofractionation regimen, the volume of breast tissue receiving V105% of the prescription dose should be less than 200 cc. However, we have noticed substantial changes to this volume (change in V105% between -55 cc and + 47.1 cc) after merging the subfields. This study compares the V105% of 29 breast plans before and after merging the subfields.