Lung and Heart Dose Variability During Radiation Therapy of Non-Small Cell Lung Cancer

Purpose: To investigate the hypothesis that positional and anatomic variations during radiation therapy induce changes in lung and heart volumes and associated radiation doses. Methods and Materials: In this longitudinal investigation, variations in lung and heart volumes and standard dose parameters of mean lung dose, lung V_20Gy, mean heart dose, and heart V_40Gy were analyzed on weekly 4-dimensional CT scans of 15 lung cancer patients during conventionally fractionated radiochemotherapy. Tumor, individual lung lobes, and heart were delineated on the mid-ventilation phase of weekly 4-dimensional CT scans. Lung lobes and heart were also contoured on individual breathing phases of pre-, mid-, and end-of-treatment scans. Planning dose was transferred to consecutive scans via rigid registration. Volume and dose variations were assessed relative to the initial planning scan. Results: Interfraction lung volume variability relative to week 0 was twice as large as tidal volume variability (8.0% ± 5.3% vs 4.0% ± 3.3%, P=.003). Interfraction lung volume variation ranged between 0.8% and 17.1% for individual patient means. Lower lung lobes had larger volume variability compared with upper lobes (13.5% ± 8.1% vs 7.0% ± 5.0%, P<.00001). Average mean lung dose variation was 0.5 Gy (range, 0.2-1.0 Gy for individual patient means) and average lung V_20Gy variation 0.9% (range, 0.2%-1.6%). Average heart volume variation was 7.2% (range, 3.4%-12.6%). Average mean heart dose variation was 1.2 Gy (range, 0.1-3.0 Gy) and average heart V_40Gy variation 1.4% (range, 0%-4.2%). Conclusions: Anatomic and positional variations during radiation therapy induce changes in radiation doses to lung and heart. Repeated lung and heart dose assessment will provide a better estimate of the actual delivered dose and will improve prediction models for normal tissue toxicity, if assessed in larger cohorts.