Patient Positional Uncertainty and Margin Reduction in Lung Stereotactic Ablative Radiation Therapy Using Pneumatic Abdominal Compression.

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Motion management presents a significant challenge in thoracic stereotactic ablative radiation therapy (SABR). Currently, a 5.0-mm standard planning target volume (PTV) margin is widely used to ensure adequate dose to the tumor. Considering recent advancements in tumor localization and motion management, there is merit to reassessing the necessary PTV margins for modern techniques. This work presents a large-scale analysis of intrafraction repositioning for lung SABR under forced shallow breathing to determine the margin requirements for modern delivery techniques. Treatment data for 124 lung SABR patients treated in 607 fractions on a linear accelerator were retrospectively collected for analysis. All patients were treated using pneumatic abdominal compression and intrafraction 4-dimensional (4D) cone beam computed tomography (4D CBCT)-guided repositioning halfway through treatment. Executed repositioning shifts were collected and used to calculate margin requirements using the 2-SD method and an analytical model which accounts for systematic and random errors in treatment. A total of 85.7% of treated fractions had 3-dimensional repositioning shifts under 5.0 mm. Fifty-three fractions (8.7%) had shifts ≥ 5.0 mm in at least 1 direction. Margins in the right-left, inferior-superior, and posterior-anterior directions were 3.62 mm, 4.34 mm, and 3.50 mm, respectively, calculated using the 2-SD method. The analytical approach estimated that 4.01 mm, 4.37 mm, and 3.95 mm margins were appropriate for our workflow. Executing intrafraction repositioning reduced margin requirements by 0.73 ± 0.07 mm. Clinical data suggest that the uniform 5.0-mm margin is conservative for our workflow. Using modern techniques such as 4D CT, 4D CBCT, and effective motion management can significantly reduce required margins, and therefore necessary healthy tissue dose. However, the limitations of margin calculation models must be considered, and margin reduction must be approached with caution. Users should conduct a formal risk assessment prior to adopting new standard PTV margins.