The aim is to investigate the dosimetric characteristics of planning target volume (PTV) and organs at risk (OAR) during the radiotherapy of left-sided breast cancer treated with modified radical mastectomy by comparing IMRT plans based on CT images and kilo-voltage cone beam computed tomography (KV-CBCT) images. 18 patients with left-sided breast cancer treated with radical mastectomy were selected. Treatment plans (PCT) based on CT images were designed and performed. KV-CBCT images were also acquired on the 1st day, 6th day, 11th day, 16th day, and 21st day of radiotherapy. All structures on planning CT images including PTV and OAR were applied by bulk density assignment (BDA) and plan parameters of PCT were replanted to a copy of planning CT images, which generated a new plan (PBDA). The prescription dose of PBDA was 50 Gy with conventional fractionation: 2 Gy per fraction, 5 fractions per week, total 25 fractions. The prescription dose was 50 Gy/25 F/5 week, 90% of the target area were covered by the prescription dose. The 5 series of KV-CBCT images acquired were imported into the planning system and matched with the planning CT for the same treatment position. The same structures of planning CT images were copied and applied by BDA on 5 series of KV-CBCT images. 5 replanted plans (PREP) and 5 new KV-CBCT plans (PCBCT) were designed based on these KV-CBCT images. Radiation parameters of PREP were same to those of PCT, but all parameters of PCBCT were new. 5 Gy given to each PREP and PCBCT covered 90% PTV. Two composite plans (P∑REP and P∑CBCT) were compounded of the five PREPs and five PCBCTs, respectively. Total prescription dose of P∑REP and P∑CBCT were 50 Gy/25 F/5 week. According to dose volume histograms (DVHs), dose distribution of PCT, PBDA, P∑REP and P∑CBCT were compared and analyzed by t-test. The results showed that PBDA has similar conform index (CI) and inhomogeneity coefficient (HI) to PCT(p>0.05). HI and CI of P∑REP were significantly worse than those of PCT(t=−2.96, −3.57, p<0.05). HI and CI of P∑CBCT were significantly better than those of P∑REP (t=−4.01, −6.24, p<0.05), but no significant difference with PCT(p>0.05). Comparing to PCT, PBDA gave similar dose indexes of OAR(p>0.05). Several parameters with P∑REP were significantly higher than with PCT: breast_R Dmax, lung_L V5, lung_L V30, lung_L Vmean and heart V30(t=−5.32, −3.57, −2.81, −2.99, −4.06, p<0.05). Compared with P∑REP, all OARs dose indexes of P∑CBCT were significantly decreased(t=4.65, 3.88, 5.32, 3.16, 4.95, 2.79, 4.55, p<0.05). In a conclusion, due to many factors such as the impact of cardiopulmonary exercise and significant dose changes of PTV and OARs, it is necessary that a real-time or adaptive treatment plan based on KV-CBCT images guided and BDA method should be designed to achieve the higher therapeutic gain ratio for IMRT radiotherapy of modified radical mastectomy for left-sided breast cancer.Cited