Impact of Internal Mammary Node Irradiation and Thoracic Lateral Diameter on Organ-at-Risk Doses and Normal Tissue Complication Probability in Free-Breathing Right Breast Volumetric Modulated Arc Therapy

OBJECTIVE
To evaluate the independent impact of internal mammary node irradiation (IMNI) and thoracic morphology on organ-at-risk (OAR) doses and normal tissue complication probability (NTCP) in free-breathing right breast volumetric modulated arc therapy (VMAT), and to identify computed tomography (CT)-based markers of higher OAR dose.
METHODS
Fifty consecutive right-sided breast cancer patients treated with free-breathing VMAT were retrospectively analyzed. Doses were converted to the equivalent dose in 2-Gy fractions (EQD2; α/β=3 Gy). Pulmonary NTCP (Lyman-Kutcher-Burman model) and major adverse cardiac event risk (Darby model) were calculated. Multivariate analysis identified independent OAR-dose predictors.
RESULTS
IMNI (13 patients, 26%) significantly increased lung Dmean, pulmonary NTCP, and cardiac Dmean on univariate analysis (all p<0.001). After adjustment, IMNI independently predicted lung Dmean (β=+0.97 Gy; p=0.029), lung V5 (β=+8.28%; p=0.024), cardiac Dmean (β=+0.43 Gy; p=0.027), and NTCP (β=+0.60%; p=0.020). Thoracic lateral diameter independently correlated with lung Dmean (ρ=0.368; p=0.009) and cardiac Dmean (ρ=0.311; p=0.028). The Haller Index showed no significant correlation with OAR doses.
CONCLUSION
IMNI independently increases lung and cardiac doses and thoracic lateral diameter independently predicts OAR dose in free-breathing right-breast VMAT. These markers may identify higher-risk patients who could be prioritized for deep inspiration breath-hold (DIBH) evaluation, pending confirmation in paired planning studies.

Keywords: Breast cancer radiotherapy; internal mammary node irradiation; normal tissue complication probability; organ-at-risk dose; volumetric modulated arc therapy.