Influence of tumor characteristics on correction differences between cone‐beam computed tomography‐guided patient setup strategies in stereotactic body radiation therapy for lung cancer

Abstract

Background: To evaluate the correction differences between vertebra and tumor matching as cone-beam computed tomography (CBCT)-guided setup strategies in lung stereotactic body radiation therapy (SBRT), and the correlations with tumor characteristics such as size, mobility, and location. Methods: The manual registrations for 33 lung tumors treated with SBRT were retrospectively performed by matching thoracic vertebrae for vertebra matching and then by matching CBCT-visualized tumors within the internal target volume obtained from a four-dimensional CT dataset for tumor matching. Results: The mean correction difference between the two matching methods during the SBRT fractions was larger in the anterior-posterior direction (2.7 mm) than in the superior-inferior (2.1 mm) and left-right (1.4 mm) directions, with differences of less than 5 mm in 90% of the total 134 CBCT fractions. The X-axis and direct distances from the central axis to the tumor had significant correlations with the correction differences in all three directions, while the mobility-related parameters were correlated only in the superior-inferior direction. The absolute differences in lung-dose parameters after applying the margins (3.4-6.5 mm) required for the setup errors from vertebra matching relative to tumor matching were mild, with values of 1.95 Gy for the mean lung dose and 3.9% for V20. Conclusion: The setup differences between vertebra and tumor matching in the CBCT-guided setup without rotation correction were increased in tumors located long distances from the central axis. The additional safety margins of 3.4-6.5 mm were required for the setup errors from vertebra matching. Key pointsSignificant findings of the study The correction difference between the vertebra and tumor matching as CBCT-guided setup strategies was the largest in the anterior-posterior direction and significantly correlated with the X-axis and direct distances from the central axis to the tumor. What this study adds Setup differences between vertebra and tumor matching in the CBCT-guided setup were increased in tumors located long distances from the central axis. The additional safety margins of 3.4-6.5 mm were required for the setup errors from vertebra matching.