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

Abstract

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°, 10°), [10°, 20°), [20°, 30°), [30°, 40°), and [40°, 45°]. 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°, 45°] tilt angle demonstrated significantly better conformity, homogeneity, lower D_2cc, and lower D_mean for the PTV. Regarding the OARs, the [40°, 45°] head tilt angle demonstrated significantly lower D_max and D_mean in hippocampus, eyes, optic chiasm, and optic nerves. The [40°, 45°] tilt angle also showed significantly lower D_max for brainstem and cochleas, as well as a lower D_mean for lens. In the [40°,45°] 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°, 45°] 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.

Keywords: dosimetry; efficacy; hippocampus; radiotherapy; tomotherapy.

Figure 1.

The representative CT simulated images depicted in panels (a) through (e) correspond to the following head tilt angle groups, respectively: (a) 5°, (b) 15°, (c) 25°, (d) 35°, and (e) 45°.

Figure 2.

The comparison of (a) PTV conformity index, (b) PTV homogeneity index, (c) PTV D_2cc, (d) PTV D_98%, and (e) PTV mean dose among five groups for TOMO. Note: Bold values indicate significance at P < 0.05.

Figure 3.

Comparison of the D_max and D_mean of hippocampus for TOMO plans in various angles ((a) the D_max of hippocampus, (b) the D_mean of hippocampus). Note: Bold values indicate significance at P < 0.05.

Figure 4.

An illustration showcasing the dose distribution in the sagittal view for different tilt angles. The tilt angles of (a-e) were 5°, 15°, 25°, 35°, and 45°, respectively. The red arrows indicate regions with high dose (hot spots), while the blue arrows indicate regions with low dose (cold spots).

Figure 5.

The comparison of (a) PTV conformity index, (b) PTV homogeneity index, (c) PTV D_2cc, (d) PTV D_98%, and (e) PTV D_mean for TOMO and VMAT plans. Note: Bold values indicate significance at P < 0.05.

Figure 6.

Comparison of (a) delivery time, (b) PTV projection length in the sagittal plane, (c) hippocampus projection length in the sagittal plane, and (d) actual modulation factor among five groups. Note: Bold values indicate significance at P < 0.05.