2399 - One Fiducial for All Modalities: Optimized Trackability and Minimized Beam Perturbation for Fiducials Used across Multi-Modalities

J. Liu¹, C. Bamberger¹, N. Iqbal¹, K. Reynolds², S. Shah², X. Li¹, P. Wang¹, J. Fan¹, K. S. Choe¹, A. K. Chawla¹, D. Kim¹, and R. Rahimi¹; ¹Inova Schar Cancer Institute, Fairfax, VA, ²Medical device manufacturer, IZI Medical Products, Owings Mills, MD

Purpose/Objective(s): Requirements for the use of fiducials vary across different modalities. For the frameless robotic radiosurgery system treatments, a standard fiducial with a minimum diameter of 0.75 mm is essential, whereas for proton therapy, a maximum fiducial diameter of 0.50 mm is allowed. Larger fiducials pose challenges for proton treatments due to excessive material within the beam path, resulting in clinically unacceptable beam perturbations. This necessitates accurate knowledge of the treatment modality at the time of fiducial insertion, leading to operational complications since such treatment plans might not be available at simulation time. Our study aimed to investigate the feasibility of modulating fiducial parameters to preserve trackability while minimizing proton beam perturbation, thereby enabling the use of a single fiducial across all modalities. Materials/

Methods: Custom-made fiducials with a diameter of 0.75 mm and varying pitch values, resulting in lower amounts of material within the fiducial structure, were fabricated (Table 1). The trackability of these fiducials was evaluated using the frameless robotic radiosurgery system. Furthermore, a phantom containing both standard and customized fiducials was positioned under a proton beam, with radiographic films placed at various depths to capture and quantify beam perturbation.
Results: Trackability of fiducials seemed to be independent of the pitch value. The customized fiducials, all with diameter of 0.75 mm, demonstrated a notable reduction in proton beam perturbation. Particularly, at greater depths, such as 1.2 cm, they exhibited lower levels of perturbation compared to a standard fiducial with diameter of 0.5 mm, (Table 1).
Conclusion: Fiducials can be optimized to ensure compatibility across various radiation treatment modalities. An ideal fiducial would possess a larger diameter for enhanced trackability while incorporating less materials, achieved through higher pitch values, to minimize beam perturbation. Further clinical investigation is warranted to assess the mobility and performance of customized fiducials within patient anatomy. Table 1 - Abstract 2406: Fiducial diameter(mm) (pitch, gap(mm)); Proton beam perturbation at different depts.

	0 mm	1 mm	3 mm	5 mm	7 mm	9 mm	12 mm
0.50 (0.00, 0.10)	0.064	0.056	0.071	0.089	0.093	0.089	0.092
0.75 (0.00, 0.10)	0.073	0.077	0.084	0.119	0.126	0.134	0.130
0.75 (0.02, 0.25)	0.073	0.092	0.088	0.084	0.083	0.097	0.099
0.75 (0.03, 0.50)	0.071	0.083	0.087	0.092	0.077	0.088	0.093
0.75 (0.04, 0.77)	0.068	0.076	0.087	0.087	0.080	0.079	0.085
0.75 (0.05, 1.07)	0.074	0.078	0.083	0.083	0.071	0.076	0.084
0.75 (0.08, 1.56)	0.073	0.078	0.072	0.079	0.079	0.072	0.076