# Universal Fiducial Markers for Multi-Modal Radiotherapy

## TL;DR

This study develops universal fiducial markers for radiotherapy that work well with both photon and proton systems while minimizing beam interference.

## Contribution

The paper introduces customized coil-shaped platinum fiducials that balance imaging visibility and proton beam compatibility.

## Key findings

- Customized fiducials with larger coil gaps showed similar visibility to standard fiducials in imaging systems.
- These fiducials maintained reliable tracking in CyberKnife systems under simulated motion.
- Increased coil gaps significantly reduced proton beam perturbation compared to standard designs.

## Abstract

Integrating multiple radiotherapy modalities, including photon and proton therapy, within a single clinical facility requires universal fiducial markers that perform reliably across all imaging and treatment systems. For computed tomography (CT) and image-guided radiotherapy (IGRT), fiducials should be large enough to ensure visibility without introducing significant image artifacts. In CyberKnife treatments, a fiducial diameter of at least 0.75 mm is necessary to maintain accurate tracking, particularly in the abdominal and pelvic regions. However, fiducials of this size are unsuitable for proton therapy due to the considerable beam perturbation they cause. This study investigates the design and implementation of customized fiducials that balance imaging visibility, tracking reliability, and proton beam perturbation, enabling their use across multiple radiotherapy modalities.

Customized coil-shaped platinum fiducials with a 0.75 mm diameter and variable coil gaps were developed to reduce the amount of metal in the proton beam path. For image visibility and trackability assessments, both standard fiducials (0.5 and 0.75 mm diameters with nominally zero coil gaps) and customized versions were inserted in an anthropomorphic phantom. Visibility was evaluated using cone-beam CT (CBCT) and kilovoltage (kV) imaging on both photon and proton treatment systems. Trackability was assessed in the CyberKnife system using an anthropomorphic phantom and a respiratory motion platform. Proton beam perturbation was measured using radiochromic films that were placed downstream of the fiducials and irradiated with a broad spread-out Bragg peak (SOBP) field. A triple-channel film dosimetry method was employed for analysis.

Customized fiducials with increased coil gaps demonstrated visibility comparable to standard fiducials in IGRT for both photon and proton systems. In the CyberKnife system, these customized fiducials also provided reliable tracking performance under both static conditions and simulated respiratory motion. The film measurements revealed that the customized fiducials with coil gaps larger than 0.5 mm/coil could significantly reduce proton beam perturbation compared to both the standard 0.75 and 0.5 mm diameter fiducials.

This study demonstrates the feasibility of redesigning fiducial markers to maintain a large diameter for reliable imaging visibility and trackability while minimizing proton beam perturbation by increasing the coil gap. Adopting a universal fiducial marker design has the potential to streamline clinical workflows and support seamless integration across various radiotherapy modalities.