# Dosimetric Study of Biaxially Rotational Dynamic Radiation Therapy for Hippocampal-Sparing Whole Brain Irradiation

## TL;DR

This study shows that BROAD-RT improves hippocampal protection during whole brain irradiation without compromising treatment quality or increasing treatment time.

## Contribution

The study introduces BROAD-RT as a novel radiation therapy method for hippocampal-sparing whole brain irradiation.

## Key findings

- BROAD-RT significantly reduced hippocampal dose compared to conventional VMAT.
- BROAD-RT achieved acceptable PTV coverage and homogeneity similar to conventional VMAT.
- BROAD-RT treatment time was clinically acceptable at an average of 313.6 seconds.

## Abstract

In patients with brain metastasis and adequately long-term survival, whole brain irradiation (WBI) leads to progressive memory loss and dementia. In this study, hippocampus-sparing WBI (HS-WBI), which has potential benefits in neurocognitive preservation, was planned using a novel radiation treatment device, biaxially rotational dynamic radiation therapy (BROAD-RT) using a novel O-ring-type linear accelerator. BROAD-RT improved hippocampal sparing with acceptable planning target volume coverage with a clinically acceptable treatment time. These data suggest that BROAD-RT is a treatment device that is clinically feasible in terms of both dose distribution and treatment time for HS-WBI and may be a possible therapeutic option.

Objectives: Although hippocampal-sparing whole-brain irradiation (HS-WBI) offers potential neurocognitive benefits, it poses challenges in treatment planning. This study aimed to compare the dose distributions of biaxially rotational dynamic radiation therapy (BROAD-RT) with a novel O-ring-type linear accelerator (OXRAY) and conventional non-coplanar volumetric modulated arc therapy (VMAT) planning (Conv-VMAT) in HS-WBI treatment plans. Methods: This study included 10 patients with brain metastases from lung cancer at our institution. The hippocampus was contoured using gadolinium-based contrast-enhanced magnetic resonance imaging, and hippocampal-sparing regions were created using a 5 mm margin around the hippocampus. Two virtual plans (BROAD-RT and Conv-VMAT) with 30 Gy in 10 fractions were created to compare the dose distributions in the planning target volume (PTV), hippocampus, eyes, and lens. All plans were analyzed using a paired t-test. Results: The mean (standard deviation [SD]) hippocampus-Dmax, -Dmean, -D100%, and -V10 were 11.10 (0.61), 7.95 (0.20), 7.01 (0.19), and 0.42 (0.34) for BROAD-RT and 16.10 (0.57), 9.89 (0.75), 8.24 (0.34), and 39.05 (25.89) for Conv-VMAT, respectively. All hippocampal parameters were significantly better with BROAD-RT than with Conv-VMAT (p < 0.01). The PTV-D98, -D50, -D2, -V35, and -homogeneity index did not exhibit significant differences between BROAD-RT and Conv-VMAT. Although lens-Dmax was significantly better in BROAD-RT than in Conv-VMAT (p < 0.01), no significant differences were observed in the eye-Dmax and chiasm-Dmax between BROAD-RT and Conv-VMAT. The mean (SD) BROAD-RT beam delivery time was 313.60 (34.91) s. Conclusions: BROAD-RT improved hippocampal sparing with acceptable PTV coverage and PTV homogeneity in HS-WBI planning. In addition, BROAD-RT has a clinically acceptable treatment duration.