Comparison of Intrafractional Motions in Six Degrees of Freedom between Two Immobilization Methods of Robotic Stereotactic Ablative Radiation Therapy for Head and Neck Tumors

Abstract

Maintaining immobilization to minimize spine motion or rotation is very important during robotic stereotactic ablative radiation therapy (SABR) for head and neck tumors. This study aimed to compare the intrafractional motions in six degrees of freedom between two immobilization methods. With spine tracking system for image guiding, 9,094 records from 41 patients receiving robotic SABR for head and neck tumors were obtained. Twenty-one patients were immobilized with thermoplastic masks and head rests (Group A), and twenty patients used not only thermoplastic masks and head rests but also vacuum bags to support head and neck area (Group B). The intrafractional motions in X (superior-inferior), Y (right-left), Z (anterior-posterior) axes, 3D (three-dimensional) vector, Roll, Pitch and Yaw between two groups were compared and the margins of planning target volume (PTV) to cover 95% intrafractional motions were estimated. The translational motions in the X axis, Y axis and 3D vector in Group A were significantly smaller than those in Group B. The rotational errors in the Roll and Yaw in Group A were also significantly smaller than those in Group B; conversely, those in the Pitch in Group A were larger. To cover 95% motions, margins of 0.96, 1.55 and 1.51 mm in the X, Y and Z axes, respectively, were needed in Group A, and 1.06, 2.86, and 1.34 mm were needed in Group B. Thermoplastic masks and head rests with vacuum bags could not provide better immobilization than those without vacuum bags in most axes. The clinical use of 2mm as margins of PTV to cover 95% intrafractional motions was adequate in Group A but not in Group B, especially in the Y axis.