End‐to‐end validation of fiducial tracking accuracy in robotic radiosurgery using MRI‐only simulation imaging
Authors: Singhrao Kamal , Zubair Muhammad , Nano Tomi , Scholey Jessica E. , Descovich Martina 
Published: December 2023

Background

Image‐guided radiation‐therapy (IGRT)‐based robotic radiosurgery using magnetic resonance imaging (MRI)‐only simulation could allow for improved target definition with highly conformal radiotherapy treatments. Fiducial marker (FM)‐based alignment is used with robotic radiosurgery treatments of sites such as the prostate because it aids in accurate target localization. Synthetic CT (sCT) images are generated in the MRI‐only workflow but FMs used for IGRT appear as signal voids in MRIs and do not appear in MR‐generated sCTs, hindering the ability to use sCTs for fiducial‐based IGRT.

Purpose

In this study we evaluate the fiducial tracking accuracy for a novel artificial fiducial insertion method in sCT images that allows for fiducial marker tracking in robotic radiosurgery, using MRI‐only simulation imaging (MRI‐only workflow).

Methods

Artificial fiducial markers were inserted into sCT images at the site of the real marker implantation as visible in MRI. Two phantoms were used in this study. A custom anthropomorphic pelvis phantom was designed to validate the tracking accuracy for a variety of artificial fiducials in an MRI‐only workflow. A head phantom containing a hidden target and orthogonal film pair inserts was used to perform end‐to‐end tests of artificial fiducial configurations inserted in sCT images. The setup and end‐to‐end targeting accuracy of the MRI‐only workflow were compared to the computed tomography (CT)‐based standard. Each phantom had six FMs implanted with a minimum spacing of 2 cm. For each phantom a bulk‐density sCT was generated, and artificial FMs were inserted at the implantation location. Several methods of FM insertion were tested including: (1) replacing HU with a fixed value (10000HU) (voxel‐burned); (2) using a representative fiducial image derived from a linear combination of fiducial templates (composite‐fiducial); (3) computationally simulating FM signal voids using a digital phantom containing FMs and inserting the corresponding signal void into sCT images (simulated‐fiducial). All tests were performed on a CyberKnife system (Accuray, Sunnyvale, CA). Treatment plans and digital‐reconstructed‐radiographs were generated from the original CT and sCTs with embedded fiducials and used to align the phantom on the treatment couch. Differences in the initial phantom alignment (3D translations/rotations) and tracking parameters between CT‐based plans and sCT‐based plans were analyzed. End‐to‐end plans for both scenarios were generated and analyzed following our clinical protocol.

Results

For all plans, the fiducial tracking algorithm was able to identify the fiducial locations. The mean FM‐extraction uncertainty for the composite and simulated FMs was below 48% for fiducials in both the anthropomorphic pelvis and end‐to‐end phantoms, which is below the 70% treatment uncertainty threshold. The total targeting error was within tolerance (<0.95 mm) for end‐to‐end tests of sCT images with the composite and head‐on simulated FMs (0.26, 0.44, and 0.35 mm for the composite fiducial in sCT, head‐on simulated fiducial in sCT, and fiducials in original CT, respectively.

Conclusions

MRI‐only simulation for robotic radiosurgery could potentially improve treatment accuracy and reduce planning margins. Our study has shown that using a composite‐derived or simulated FM in conjunction with sCT images, MRI‐only workflow can provide clinically acceptable setup accuracy in line with CT‐based standards for FM‐based robotic radiosurgery.