Universal Fiducial Markers for Multi-Modal Radiotherapy.

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Biomedical Engineering

Background:

• Integrating photon and proton therapy requires universal fiducial markers for reliable performance across imaging and treatment systems.
• Standard fiducials face challenges: large sizes for computed tomography (CT) and image-guided radiotherapy (IGRT) cause proton beam perturbation, while smaller sizes compromise CyberKnife tracking accuracy.

Purpose of the Study:

• To design and implement customized fiducials that balance imaging visibility, tracking reliability, and minimal proton beam perturbation for multi-modality radiotherapy.
• To enable the use of universal fiducial markers across different radiotherapy systems, including photon and proton therapy.

Main Methods:

• Developed customized coil-shaped platinum fiducials (0.75 mm diameter) with variable coil gaps.
• Assessed visibility using cone-beam CT (CBCT) and kilovoltage (kV) imaging on photon and proton systems.
• Evaluated CyberKnife tracking reliability with an anthropomorphic phantom and respiratory motion platform.
• Measured proton beam perturbation using radiochromic films and a triple-channel film dosimetry method.

Main Results:

• Customized fiducials with increased coil gaps showed visibility comparable to standard fiducials in IGRT.
• These fiducials maintained reliable tracking performance in the CyberKnife system under static and dynamic conditions.
• Fiducials with coil gaps >0.5 mm/coil significantly reduced proton beam perturbation compared to standard fiducials.

Conclusions:

• Redesigned fiducial markers can maintain large diameters for visibility and trackability while minimizing proton beam perturbation by increasing coil gaps.
• A universal fiducial marker design is feasible, potentially streamlining clinical workflows and integrating various radiotherapy modalities.