Quantification of incidental dose to potential clinical target volume (CTV) under different stereotactic body radiation therapy (SBRT) techniques for non-small cell lung cancer - Tumor motion and using internal target volume (ITV) could improve dose distribution in CTV

Purpose: Clinical target volume (CTV), although present, is usually not considered during stereotactic body radiation therapy (SBRT) for non-small cell lung cancer. This study aimed to quantify the incidental dose to the potential CTV under different SBRT techniques. Materials and methods: Ten patients with various tumor motions were included in the study. Gated-4DCT was performed for all patients. Three treatment plans were generated. Plan A was based on free breathing gross tumor volume (GTV) from a regular CT. Plan B was based on internal target volume (ITV) from gated 4DCT. Plan C was a perfect gated treatment at the exhale phase. The hypothetical CTV was represented by three CTV shells (5, 10, and 15 mm). Time-averaged dose for different respiratory phases was calculated for 18 representative points in each shell. Results: The minimum doses for plans A, B, and C were 84 ± 20%, 94 ± 3%, and 80 ± 17% of the isocenter dose to the 5 mm shell, 72 ± 27%, 64 ± 7%, and 20 ± 11% to the 10 mm shell, and 38 ± 27%, 27 ± 17%, and 6 ± 7% to the 15 mm shell, respectively. The caudal and cranial ends of each shell usually had lower dose compared to the other points on the shell. Plan B had the most uniform and reasonable doses to the CTV shells, and patients with large respiratory motion had significantly higher minimum dose than patients with less motion. Conclusion: The potential CTV may incidentally receive adequate and relatively homogeneous doses when ITV is used and the patients have large respiratory motion. However, it could be underdosed for gated treatment or for patients with little motion.