Objective To explore the dosimetric differences between abdominal deep inspiration breath hold(ADIBH)mode and free breath(FB)mode in intensity modulated radiation therapy(IMRT)for left breast cancer.Methods From July 2022 to May 2023,a total of 22 patients who needed adjuvant radiation therapy after left breast cancer surgery in the hospital were selected as the research objects.The simulated computed tomography(CT)positioning images of ADIBH and FB modes were collected,the planned target volume(PTV)and endangered organs were outlined,the IMRT plan was designed,and the dosimetric param-eters of the two modes were compared.Results There was no significant difference in the mean dose(Dmean),homogeneity index(HI)and conformity index(CI)of PTV between the ADIBH and the FB modes(P＞0.05).Compared with the FB mode,the heart Dmean,V5,V10,V20,V30 and V40 in the ADIBH mode decreased by 2.95 Gy,12.21％,8.26％,6.56％,5.41％and 3.48％,respectively,and the left anterior descending(LAD)coronary artery Dmean,maximum dose(Dmax),minimum dose(Dmin)and V40 decreased by 15.99 Gy,16.10 Gy,0.82 Gy and 13.73％,respectively,with statistical significance(P＜0.05).Compared with the FB mode,the dose and volume of heart irradiation in the ADIBH mode at the same level were significantly reduced.Pearson correlation analysis showed that there was a positive correlation between heart Dmean and LAD Dmean in the ADIBH mode(r=0.72),and between heart Dmean and LAD Dmean in the FB mode(r=0.69).Compared with the FB mode,the left lung Dmean of the ADIBH mode decreased by 0.99 Gy,and the difference was statistically significant(P＜0.05).However,there was no significant difference in left lung V5,right lung Dmean and right breast Dmean between the two breathing modes(P＞0.05).Conclusion ADIBH mode can effectively reduce the dose to the heart and LAD,and play a good protective role.