Comparison of Different Head Tilt Angles in Tomotherapy and Volumetric Modulated Arc Therapy for Hippocampal-Avoidance Whole-Brain Radiotherapy

Purpose: Hippocampal-avoidance whole-brain radiotherapy (HA-WBRT) planning can present challenges. This study examines the influence of head tilt angles on the dosimetric characteristics of target and organs at risk (OARs), aiming to identify the optimal tilt angle that yields optimal dosimetric outcomes using tomotherapy (TOMO). Methods: Eight patients diagnosed with brain metastases underwent CT scans at five tilt angles: [0 degrees, 10 degrees), [10 degrees, 20 degrees), [20 degrees, 30 degrees), [30 degrees, 40 degrees), and [40 degrees, 45 degrees]. Treatment plans were generated using TOMO and volumetric modulated arc therapy (VMAT). Dosimetric parameters including conformity index (CI), homogeneity index (HI), D-2cc, D-98%, and D-mean of PTV, as well as D-max, and D-mean of OARs were analyzed. Furthermore, a comparison was made between the dosimetric parameters of TOMO and VMAT plans. Finally, delivery efficiency of TOMO plans were assessed. Results: For the PTV, [40 degrees, 45 degrees] tilt angle demonstrated significantly better conformity, homogeneity, lower D-2cc, and lower D-mean for the PTV. Regarding the OARs, the [40 degrees, 45 degrees] head tilt angle demonstrated significantly lower D-max and D-mean in hippocampus, eyes, optic chiasm, and optic nerves. The [40 degrees, 45 degrees] tilt angle also showed significantly lower D-max for brainstem and cochleas, as well as a lower D-mean for lens. In the [40 degrees,45 degrees] tilt angle for HA-WBRT, TOMO showed superior performance over VMAT for the PTV. TOMO achieved lower D-max for brainstem, cochleas, optic nerves, and optic chiasm, as well as a lower D-mean for hippocampus. Furthermore, a significant correlation was found between delivery time and the PTV projection length in the sagittal plane. Conclusion: The TOMO plan utilizing a tilt angle range of [40 degrees, 45 degrees] demonstrated superior PTV conformity and uniformity, along with enhanced OARs sparing. Furthermore, it exhibited a dosimetric advantage over VMAT for PTV and most OARs at the same angle range.