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

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 POINTS: Significant 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.