Objective To explore the relationship between dosimetric parameters and radiation pneumonitis in patients with early breast cancer after breast conserving surgery.

Methods  140 cases of early breast cancer treated with volume modulated radiation therapy after breast conserving surgery in our hospital from March 2020 to March 2023 were selected as the research objects. The patients were divided into radiation pneumonitis group (46 cases) and non radiation pneumonitis group (94 cases) according to whether radiation pneumonitis occurred after radiotherapy. There were 37 cases of grade 1 radiation pneumonitis, 32 cases of grade 2 radiation pneumonitis, and 14 cases of grade 3 radiation pneumonitis. The clinical data and dosimetric parameters of the two groups were compared. Use multiple linear stepwise regression to investigate the correlation between lung function indicators and dosimetric parameters. Multivariate logistic regression model was used to determine the independent risk predictors of radiation pneumonitis. Compare the basic information and dosimetric parameters of patients with different grades of radiation pneumonitis. The generalized mixed effect model was used to analyze the relationship between dosimetric parameters and the degree of radiation pneumonitis. The dose-response relationship between dosimetric parameters and grade 3 radiation pneumonitis was analyzed by restricted cubic spline model.

Results There were significant differences in age, diabetes mellitus, FEV1, MLD, V₅, V₁₀, V₂₀ and V₃₀ between the two groups (P<0.05). Age, diabetes, FEV1, V₅ and V₂₀ were independent risk factors for radiation pneumonitis. Age, proportion of patients with diabetes, FEV1< 2 L, V₅ and V₂₀ in patients with grade 3 radiation pneumonitis were significantly increased (P<0.05). The results of generalized mixed effect model showed that V₅ and V₂₀ were related to the degree of radiation pneumonitis. The results of restricted cubic spline model showed that V₅ and V₂₀ had a nonlinear dose-response relationship with grade 3 radiation pneumonitis (P_{non-linearity}=0.426,0.537).

Conclusion V₅ and V₂₀ were independent risk factors for radiation pneumonitis, and had a certain correlation with the degree of radiation pneumonitis. The dose volume parameters should be strictly controlled to reduce the incidence of radiation pneumonitis.