In this retrospective study, all patients undergoing breast-conserving therapy between 2005 and 2009 were screened, and qualifying consecutive patients were included in 1 of 2 cohorts: large-breasted patients (chest wall separation>25 cm or planning target volume [PTV]>1500 cm3) (n=97) and small-breasted patients (chest wall separation<25 cm and PTV<1500 cm3) (n=32). All patients were treated prone or supine with hypofractionated IMRT to the whole breast (42.4 Gy in 16 fractions) followed by a boost dose (9