Effect of Breathing Motion on Dose Accumulation in the Lung Bronchial Tree

Abstract

To improve the accuracy of the calculated dose to the bronchial tree during stereotactic body radiation therapy (SBRT) for non–small-cell lung cancer (NSCLC), to better understand the relationship between radiation dose and bronchial complications and to aid in SBRT eligibility decisions. Ten NSCLC patients treated under a 60 Gy in 3 fractions or 48 Gy in 4 fractions SBRT protocol were retrospectively evaluated. Inhale and exhale reconstructions from 4D-CT images at treatment planning were obtained. The external contour, lungs, and tumor were contoured in both images for each patient. MORFEUS, a finite element model-based deformable registration algorithm with the capability to model the sliding interface between the lungs and the rib cage was used to model the breathing motion. The model was expanded to include the bronchial tree, which was segmented from the 4D-CT using a threshold technique. Triangular elements with a thickness of 0.1 cm were used to model the bronchial trees, while tetrahedral elements were used to model the lungs. Radiation dose was calculated in a commercial treatment planning system on 4D-CT inhale and exhale images and accumulated over the breathing cycle using MORFEUS. Dose results for the breathing model were compared to standard treatment planning (exhale images). Six patients showed a maximum dose deviation larger than 30 Gy to an individual surface element within the bronchial tree. The average maximum change to a surface element over all patients was 32 Gy (range, 9–61 Gy), with an average maximum dose reduction of 30 Gy (range, 6–61 Gy) and an average maximum dose increase of 21 Gy (range, 5–40 Gy). Four patients showed changes in dose larger than 5 Gy to more than 10% of the bronchial tree (of treated lung) when breathing motion and deformation are considered. These same 4 patients showed changes in dose larger than 10 Gy to more than 3.5% of total treated lung bronchial tree. One patient showed a change in dose larger than 20 Gy to 4% of the bronchial tree within the treated lung. The change in overall maximum dose to a 2 cc volume in the bronchial tree exceeded 2 Gy. The change in mean lung dose (range, 0–0.05 Gy) and lung V20 (range, 0.05–1.4%) was insignificant. Results indicate that large changes in dose to the bronchial tree can occur due to breathing during SBRT treatment compared to standard (static) treatment planning. Furthermore, large changes in local maxima within the bronchial tree can occur without similarly large changes in mean lung dose or V20, indicating that the structures should be separately considered since maximum dose to the bronchial tree is predictive of late complications. Biomechanical modeling may help to develop a more quantitative tool for determining eligibility for SBRT treatment and improve understanding of late bronchial complications.