Room: Karl Dean Ballroom A1
Purpose: To evaluate the geometric accuracy of implanted fiducials as a surrogate of tumour position and to inform the subsequent impacts of motion management strategies.
Methods: Thirteen patients have had beacons implanted endo-bronchially in surrounding airways to the tumour under fluoroscopic guidance. The centroids of two or three implanted electromagnetic beacons have been used for real-time MLC adaptation. The geometric error of inferring tumour motion based on beacon centroid motion was determined from a simulation 4DCT. The tumour and beacon positions across all phases of the respiratory cycle were identified. The surrogacy error was defined as the maximum difference between the tumour and beacon centroid relative to the reference phase (maximum exhale).
Results: All thirteen patients had three beacons implanted for this study. The beacon centroid to tumour distance varied from 8.0 to 46.0mm. A surrogacy error up to 3.6 mm (n=12) was measured using the simulation 4DCT; one patient excluded due to imaging artefacts. Surrogacy analysis informed the decision to track the tumour using the centroid of three beacons in 11 out of 13 patients. The beacon to tumour distance didnâ€™t trend with surrogacy error. Furthermore, the surrogacy error remained similar across motion amplitudes of 2.0 to 14.0mm. No migration of beacons or toxicities was observed between simulation and treatment. The maximum surrogacy error was typically observed at the inhale phase of breathing.
Conclusion: Accuracy of localising lung tumours using fiducial markers is dependent on the surrogacy error of the marker. The surrogacy error should be calculated per patient to determine the optimal use for treatment. The magnitude of tumour motion may be used to identify cases that will benefit from fiducial-based motion management. Implantation is limited by patient anatomy however surrogacy error analysis can inform the use of customised margins and should be considered for planning and treatment.
Funding Support, Disclosures, and Conflict of Interest: Keall and Booth are investigators on MLC tracking clinical trials partially supported by Varian Medical Systems.