# Advanced External Beam Stereotactic Radiotherapy for Skull Base Reirradiation

## TL;DR

This study compares advanced radiation therapy techniques for re-treating skull base tumors, focusing on their ability to deliver precise doses while protecting nearby organs.

## Contribution

The study introduces two novel gradient indices to evaluate dose fall-off and spread in skull base reirradiation treatment plans.

## Key findings

- Gamma Knife showed the highest border gradient, indicating superior dose fall-off near targets.
- Proton therapy plans had the least dose spread in low-dose regions.
- All modalities achieved good target coverage, but IMPT and VMAT offered better dose uniformity.

## Abstract

Skull base stereotactic radiotherapy (RT) is particularly challenging due to prior radiation and the proximity of several critical organs. This study reviewed four advanced external beam RT modalities and their corresponding or available modern treatment planning systems (TPSs). The plan quality and potentials were evaluated in terms of target coverage and dose gradient. The steepest border gradient was used to assess the fall-off speed achievable near the target to spare adjacent critical structures, while the volume gradient was used to evaluate dose spread at a distance. Gamma Knife demonstrated the highest border gradient, followed by small-spot-size proton beams and CyberKnife. The proton beam exhibited the least dose spread in the low-dose region.

Background/Objectives: Stereotactic body radiation therapy (SBRT) for skull base reirradiation is particularly challenging, as patients have already received substantial radiation doses to the region, and nearby normal organs may have approached their tolerance limit from prior treatments. In this study, we reviewed the characteristics and capabilities of four advanced external beam radiation delivery systems and four modern treatment planning systems and evaluated the treatment plan quality of each technique using skull base reirradiation patient cases. Methods: SBRT plans were generated for sixteen skull base reirradiation patients using four modalities: the GK plan for the Elekta Leksell Gamma Knife Perfexion/ICON, the CyberKnife (CK) plan for the Accuray CyberKnife, the intensity-modulated proton therapy (IMPT) plan for the Hitachi ProBeat-FR proton therapy machine, and the volumetric-modulated arc therapy (VMAT) plan for the Varian TrueBeam STx. These plans were evaluated and compared using two novel gradient indices in addition to traditional dosimetry metrics for targets and organs at risk (OARs). The steepest border gradient quantified the percent prescription dose fall-off per millimeter at the boundary between the target and adjacent critical structures. This gradient index highlighted the system’s ability to spare nearby critical OARs. The volume gradient assessed the extent of dose spread outside the target toward the patient’s body. Results: All plans achieved comparable target coverage and conformity, while IMPT and VMAT demonstrated significantly better uniformity. The GK plans exhibited the highest border gradient, up to 20.9%/mm, followed by small-spot-size IMPT plans and CK plans. Additionally, IMPT plans showed the benefit of reduced dose spread in low-dose regions and the lowest maximum and mean doses to the brainstem and carotid artery. Conclusions: The advanced external beam radiotherapy modalities evaluated in this study are well-suited for SBRT in skull base reirradiation, which demands precise targeting of tumors with highly conformal doses and steep dose gradients to protect nearby normal structures.