Cardiac Substructure Dose Quantification and Optimization in Left-Sided Lung Cancer Radiation Therapy

Because curative-intent radiation therapy provides excellent local control for lung tumors, patients are surviving longer to experience treatment-associated adverse cardiac events. Patients treated for left-sided lung tumors are particularly vulnerable because of the heart's anatomic proximity. Although the whole heart is conventionally contoured as an organ at risk, emerging research highlights cardiac substructure doses better predict cardiotoxicity after radiation therapy than mean heart dose.

An institutional data set of 50 contrast-enhanced computed tomography (CT) scans was compiled from patients treated between 2015 and 2025 for left-sided locally advanced primary lung cancers with definitive photon radiation therapy to 60 Gy in 30 fractions. Twenty-one cardiac substructures were manually contoured on each CT scan per gold-standard atlases and novel protocols. Mean and maximum substructure doses were extracted from treatment dose-volume histograms, and associations between substructure dose, target volume, tumor location, and demographics were analyzed. Strategic treatment replanning of the 15 cases exceeding a mean whole heart dose of 9 Gy evaluated feasibility of substructure dose reduction.

Mean substructure radiation doses were compared against the optimal institutional whole-heart mean dose constraint of 10 Gy. The 4 highest average mean dose results with 95% confidence intervals (CIs) were: aortic arch 38.6 (95% CI, 32.7-43.80) Gy; pulmonary artery 33.3 (95% CI, 29.4-36.80 Gy; pulmonary veins 28.8 (95% CI, 24.9-32.4) Gy, and left circumflex coronary artery 27.3 (95% CI, 21.8-32.7) Gy. In the full cohort, average left anterior descending artery V15 Gy was 52.7%; in the elevated heart dose cohort, it reached 83.8%. After replanning, left anterior descending artery V15 Gy was reduced to 13.7%.

This is one of few quantifications of substructure dosimetry in left-sided lung radiation therapy. Eleven substructures initially received mean doses exceeding the institutional whole-heart threshold, underscoring the need to consider substructures in treatment planning. Strategic replanning demonstrates clinical feasibility in reducing substructure doses, particularly the left-sided coronary arteries. The resulting contour and dose data sets provide a foundation for predictive modeling and substructure-sparing treatment strategies.