The aim of this work is using multi-segmented conformal radiotherapy, field in field (FIF) for large breast cancer treatment, the dose coverage of the planning target volume (PTV) and evaluation the radiation load on the organs at risk (OARs). Ten patients with large breast of mean PTV was 1489 ± 335 Cm3 were included. Many parameters used in the dose evaluations in the planning target volume (PTV) and the organs at risk (OARs) volume including ipsi lateral lung, heart, and the contralateral breast, Dose homogeneity index (DHI) and conformity index (PITV). The FIF technique for all patients improved the homogeneity and conformity of the PTV, where the mean values of maximum dose of the PTV was 107%, the volumes over the prescription dose in irradiated volumes received very low doses. For each dosimetry of the organs at risk, the FIF technique reduced the dose received to theses organs. In conclude the FIF technique provided better dose distribution in the PTV and reduced the doses in the OARs and improving homogeneity and conformity of the planning volume.