Author: Yukio Fujita, Syoma Ide, Fumiki Ito, Kei Ito, Satoshi Kito, Keiko Murofushi, Yujiro Nakajima, Yuhi Suda, Kentaro Taguchi, Naoki Tohyama, Fumiya Tsurumaki, Riku Watanabe 👨🔬
Affiliation: Komazawa University Graduate School, Komazawa University, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital 🌍
Purpose: Spine stereotactic body radiotherapy (SBRT) demands high precision due to the target's proximity to the spinal cord and steep dose gradients, which complicate treatment planning. Consequently, setup errors can have a notable impact on dose distribution. This study aimed to evaluate the correlation between planning factors, including geometry and complexity, and robustness of SBRT against setup errors.
Methods: This retrospective study analyzed dual-arc VMAT plans prescribing 24 Gy in 2 fractions for 50 spinal segments with metastases (7 cervical, 26 thoracic, and 17 lumbar). The setup margin of planning target volume and planning organ at risk volume of spinal cord were set to 2 mm. The prescribed dose was 24 Gy in 2 fractions, and a 2-mm margin was added to the clinical target volume and spinal cord. Translational setup errors were simulated with magnitudes ranging from 0.5 mm to 3.0 mm, in 0.5-mm increments, in each of the six axial directions. The relationship between the rates of change in CTV D95% and spinal cord D0.03cc due to setup errors and 48 plan complexity metrics was analyzed using Pearson’s correlation coefficient.
Results: The direction with the largest dose variation due to a 2 mm setup error was the anterior direction for CTV D95%, with a reduction of -7.2%, and the posterior direction for spinal cord D0.03cc, with an increase of 28.7%. The strongest correlation for CTV D95% was observed between a 2-mm cranial direction error and the MU value (r = 0.40). For spinal cord D0.03cc, the highest correlation was noted between a 2-mm caudal direction error and the beam's-eye-view jaw area ratio (r = -0.37).
Conclusion: Setup errors in spine SBRT significantly affected the dose to the CTV and spinal cord. A weak correlation was identified between changes in dose metrics caused by setup errors and plan complexity.