Author: Anthony J. Doemer, Ian Gallagher, Yimei Huang, Jeff A. Riess, Kundan S Thind, Bo Zhao 👨🔬
Affiliation: Henry Ford Health 🌍
Purpose: Stereotactic body radiation therapy (SBRT) is commonly used for early-stage lung cancer. While pre-treatment CT captures some tumor motion, it may not fully represent intra-fraction motion during treatment. This study evaluates the need for increased intra-fraction imaging frequency in lung SBRT by identifying patients with greater than average motion.
Methods: A retrospective study was performed on 100 SBRT lung patients treated with 45~60Gy in 5 fractions from 09/2022 to 08/2024. Internal target volume (ITV) was constructed from free-breathing 4DCT, and PTV included the ITV plus a patient-specific margin of 0.3~0.8cm.
Cone-beam CT (CBCT) was used for image guidance for all treatments. The treatment workflow included an initial CBCT (CBCTi) followed by a verification CBCT (CBCTv). Additionally, at 1st fraction and the discretion of treatment team at subsequent fractions, 280 out of the 500 fractions had mid-treat CBCT (CBCTm). Any deviation observed by the CBCTs at the target was corrected. The additional deviation at CBCTv or CBCTm, and time between successive CBCTs was recorded.
Results: The correction at CBCTv was 0.13±0.21cm (N=500). While majority patients did not have intra-fraction correction ≥0.5cm at CBCTv, deviation ≥0.5cm was observed in 1 fraction on 9 patients, 2 fractions on 4 patients, 3 fractions on 3 patients, and 4 fractions on 1 patient. Of the 280 fractions that had both CBCTv and CBCTm, intra-fraction corrections decreased from 0.17±0.21cm at CBCTv to 0.10±0.16cm at CBCTm, with a 90th percentile of 0.46 and 0.28cm, correspondingly. The time between CBCTi and CBCTv, and between CBCTv and CBCTm, was 5.8±2.1 and 6.5±1.9min, respectively.
Conclusion: A small percentage of SBRT lung patients had more intra-fraction motion than normal. Multiple CBCTs can identify such patients and potentially mitigate the impact of any intra-fraction motion.