Influence of Modulation Factor on Treatment Plan Quality and Irradiation Time in Hippocampus-Sparing Whole-Brain Radiotherapy Using Tomotherapy

Abstract

Objectives: Hippocampus-sparing whole-brain radiotherapy (HS-WBRT) using tomotherapy is known to provide a better dose distribution than volumetric-modulated arc therapy but requires an extended irradiation time. The present study aimed to investigate whether irradiation time can be shortened by reducing the modulation factor (MF) without losing the target dose distribution. Methods: Using six tilted computed tomography images in the head area, the planning target volume (PTV) and hippocampal doses, and the irradiation time was investigated with a jaw width of 1 cm, a pitch of 0.200, and the MF changed from 3.0 to 2.6, 2.2, 1.8, and 1.4. Results: No significant changes in the PTV or hippocampus were found with MF in the range from 3.0 to 1.8, but marked deterioration was found with that of 1.4. The irradiation time showed a linear relationship with the MF within the range from 3.0 to 1.8, with 1334, 1158, 986, and 817 s at modulation factors of 3.0, 2.6, 2.2, and 1.8, respectively. However, when the MF was 1.4, the irradiation time was 808 s. Conclusions: When HS-WBRT is performed with a tilted body position and a jaw width of 1 cm, with a MF of 1.8, a favorable balance between dose parameters and irradiation time is achieved, whereas with a MF of 1.4, the quality of the radiotherapy plan deteriorates, and the irradiation time is approximately the same as that with a MF of 1.8.

Plan Quality Comparison Between Hippocampus-Sparing Whole-Brain Radiotherapy Treated With Halcyon and Tomotherapy Intensity-Modulated Radiotherapy

“…Miura et al 15 reported that the irradiation time was reduced by approximately 10% after applying tilt in tomotherapy. Ishibashi et al 16 reported a reduction in the irradiation time by 30% after reducing the modulation factor. Moreover, Shimizu et al reported that irradiation time can be shortened to 6 min using a 2.5 cm jaw.…”

“…As in this study, 66–75 rotations, with each rotation lasting for 21–23 s, were used for HS-WBRT; at least 11.8 s per revolution is required for tomotherapy. 16 , 18 …”

“…As in this study, 66-75 rotations, with each rotation lasting for 21-23 s, were used for HS-WBRT; at least 11.8 s per revolution is required for tomotherapy. 16,18 Determining the superiority of a device superior among all the IMRT-dedicated radiotherapy devices using an O-ring type LINAC is difficult. As evident from the data of the present study, tomotherapy produces a better dose distribution; however, the irradiation time is considerably longer.…”

Tomotherapy: Comparison of Hi-ART, Tomo-HD, and Radixact

“…In contrast, when the pitch was large (≥0.25), lowering the MF until the gantry speed was maximized reduced both treatment time and plan quality [8]. Furthermore, Ishibashi et al reported that MF reduction from 3.0 to 1.8 in hippocampus-sparing whole-brain irradiation resulted in almost no deterioration in plan quality and a shorter irradiation time [9]. The irradiation time at an MF of 1.8 was approximately 60% of that at an MF of 3.0, and the irradiation time decreased linearly during this period.…”