ObjectiveTo analyze the cardiac dosimetry of lymph node irradiation in the internal breast region after left-sided breast cancer surgery and to assess its impact on patients' quality of life.MethodsThe clinical data of 108 patients who underwent inverse intensity modulated radiotherapy （IMRT） after left-sided breast cancer surgery in Cancer Hospital of Nantong University from May 2019 to May 2021 were collected and retrospectively analyzed， and divided into a study group （with internal breast， 55 cases） and a control group （without internal breast， 53 cases） according to whether the postoperative radiotherapy included lymph node irradiation in the internal breast region. The dosimetric indexes of planned target area （PTV）， cardiac tolerance， serum myocardial injury markers and quality of life before and after radiotherapy were compared between the two groups.ResultsIn terms of PTV dosimetry， the conformality index （CI） of the study group and the control group were 0.73±0.07 and 0.75±0.08， the homogeneity index （HI） were 0.17±0.03 and 0.17±0.02， the D_maxwere （55.69±1.02） Gy and （55.46±1.13） Gy， the D_meanwere （50.54±0.23） Gy and （50.48±0.21） Gy respectively， there were no statistically significant differences （t=1.38，P=0.169；t<0.01，P>0.999；t=1.11，P=0.269；t=1.41，P=0.160）. In terms of cardiac receptivity， the D_meanof the two groups were （5.93 ± 0.32） Gy， （5.64 ± 0.30） Gy， V₄₀were （0.47 ± 0.10） %， （0.41 ± 0.11） %， and V₃₀were （2.48 ± 0.51） %， （2.06 ± 0.49） % respectively， and there were statistically significant differences （t=4.86，P<0.001；t=2.97，P=0.004；t=4.36，P<0.001）. The levels of serum troponin Ⅰ （cTnⅠ） before radiotherapy in the study group and the control group were （0.09±0.02） ng/ml and （0.09±0.01） ng/ml， creatine kinase isoenzyme MB （CK-MB） were （0.27±0.08） U/L and （0.25±0.08） U/L， myoglobin （MYo） were （3.84±1.02） μg/L and （3.69±0.97） μg/L， and brain natriuretic peptide （BNP） were （172.35±16.24） pg/ml and （169.81±15.93） pg/ml respectively， there were no statistically significant differences （t<0.01，P>0.999；t=1.30，P=0.197；t=0.78，P=0.436；t=0.82，P=0.414）. One month after radiotherapy， the levels of serum cTnⅠ in the two groups were （0.09±0.03） ng/ml and （0.09±0.02） ng/ml， CK-MB were （0.29±0.09） U/L and （0.28±0.08） U/L， MYo were （4.06±1.08） μg/L and （4.01±1.03） μg/L， and BNP were （175.13±17.09） pg/ml， （172.47±16.28） pg/ml respectively， there were no statistically significant differences （t<0.01，P>0.999；t=0.61，P=0.544；t=0.25，P=0.806；t=0.83，P=0.410）. The European Organization for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 （EORTC QLQ-C30） scores before radiotherapy in the study and the control groups were （60.24±5.13） points and （61.19±5.46） points， （74.12±7.20） points and （75.35±7.88） points at 1 month after radiotherapy， （77.53±7.14） points and （78.95±7.08） points at 6 months after radiotherapy， and （75.02±6.93） points and （76.68±6.74） points at 1 year after radiotherapy respectively， there were no statistically significant differences （t=0.93，P=0.353；t=0.85，P=0.399；t=1.04，P=0.302；t=1.26，P=0.210）. The EORTC QLQ-C30 scores at 1 month， 6 months， and 1 year after radiotherapy were higher than those before radiotherapy in the two groups， and there were statistically significant differences （allP<0.001）.ConclusionIMRT containing lymph node irradiation in the internal breast region after left breast cancer surgery brings a certain degree of increased cardiac dose， but it is feasible to control it within a certain range and does not affect the patients' cardiac function or quality of life in the short term.