This study aimed to compare estimates of secondary cancer risks using five radiotherapy modalities to treat breast cancer:intensity-modulated radiotherapy(IMRT),3-dimensional conformal radiation therapy(3DCRT),field-in-field forward-planned intensity-modulated radiation(FinF),volumetric modulated arc therapy(VMAT),and TomoHDA(TOMO).Each of 10 selected breast patients was re-planned with the five different modalities.The planning target volume(PTV)was defined as the contoured breast subtracted 5 mm from the skin,and lungs,heart,and contralateral breast were contoured as OARs.The prescription(Rx)was 50.4 Gy in 28 fractions,which covered 95%of the PTV.Dose constraints for the contralateral lung of volumes receiving 20 Gy and 10 Gy(V20Gy and V10Gy)were less than 20%and 40%,respectively,a maximum cord dose less than 45 Gy and for the rest.Qrgan equivalent doses(OEDs),dose characteristics,and lifetime attributable risks(LARs)were derived from dose-volume histograms for in-field regions and radio-photoluminescence glass dosimeter(RPLGD)measurements for out-of-field regions.TOMO had a better target dose distribution than 3D-CRT and FinF.OEDs for TOMO were similar to those of 3D-CRT in in-field region but increased as distance from the field boundary increased.LARs of the lung and contralateral breast were highest for VMAT,followed by IMRT,FinF,3D-CRT,and TOMO.IMRT and VMAT had a 40-50%lower in V20Gy than 3D-CRT.Our results indicate that TOMO provides comparable plan quality for breast cancer radiotherapy compared to IMRT/VMAT,and,at the same time,the lowest cancer risk in in-field regions among all studied treatment modalities.Therefore TOMO could be a better alternative treatment modality than 3D-CRT for breast cancer patients.