Room: AAPM ePoster Library
Purpose: 4DCT and 4DCBCT are commonly used for determination and verification of ITVs in lung cancer treatment. However, there are limited studies to understand if 4DCT based ITV is sufficient in locally advanced lung cancer patients. We aim to quantify the additional ITV margin needed by comparing 4DCT and 4DCBCT based motion.
Methods: This study included forty-six patients with locally advanced non-small-cell lung tumors treated with IMRT from 2018 to 2019. At simulation, 4DCT was performed for tumor motion evaluation and ITV contouring. Weekly 4DCBCT was performed for tumor motion verification. The two extreme phases of inhalation and exhalation from two types of scans were analyzed in Velocity with rigid registration focusing on PTV region. The shifts in three directions were considered as tumor motion.
Results: The tumor motions in Superior-Inferior(SI), Anterior-Posterior(AP), and Left-Right(LR) directions are (mean ± SD) 2.71±2.75 mm, 1.20±1.08 mm, and 0.93±0.90 mm for 4DCT, and 2.71±2.57 mm, 1.18±1.01 mm, and 1.00±0.91 mm for 4DCBCT, respectively. Motion from 4DCT and 4DCBCT were highly correlated (SI: R=0.95, P<0.0001, AP: R=0.85, P<0.001, R=0.86, LR: P<0.001). For 97.8% of the patients, 2 mm uniform expansion from 4DCT ITV will cover the motion observed in 4DCBCT; for 80.4% of the patients, 1 mm margin is enough. For only one of forty-six patients, 4DCT/4DCBCT motion difference is larger than 3 mm, re-simulation confirmed 4DCBCT’s prediction. This indicates 4DCBCT is more robust for irregular breath. In addition, tumor motion is weakly correlated to PTV volume (R=-0.26, P=0.08) but strongly depending on location (upper lobe vs. lower lobe, P<0.00004).
Conclusion: The overall discrepancies of motion observed in 4DCT and 4DCBCT are small, while 4DCBCT is less susceptible to irregular motion. It is recommended to include an additional 2 mm uniform expansion in 4DCT based ITV to cover the tumor motion at the treatment.