A. Groll, J. Schmall, H. Liang, J. Burns, M. Narayanan, and G. Kuduvalli Ph.D; RefleXion Medical, Inc., Hayward, CA
Purpose/Objective(s): BgRT can treat one or more positron emission tomography (PET) avid tumors. To enable multi-target BgRT testing, a motion platform was developed capable of supporting up to 3 structures moving asynchronously within a large torso phantom. We characterized the performance of the platform and created a BgRT plan intended to represent multiple metastatic masses with respiratory motion. Materials/
Methods: The anthropomorphic phantom was engineered to support up to three 3D printed 18F-fluorodeoxyglucose (FDG) fillable PET avid targets or organs at risk (OARs). In the studied configuration, two structures were selected to be 26-mm tumors, each with 3 degrees of freedom for motion. The third, the OAR using a c-shape insert, was allowed the freedom to move in only IEC-X and IEC-Y. The movement was supported by a set of translational stages equal in number to the degrees of freedom; arms overhanging the phantom connected the stages to the targets and OAR. The phantom was placed in a prototype medical technology company system. Using a no-motion simulation CT, 26-mm GTVs and 36-mm PTVs were contoured to both targets, and the c-shape was contoured as an OAR. Target 2 was selected as the therapeutic target and assigned a biology-tracking zone as a 10-mm, 25-mm, and 10-mm expansion in IEC-X, IEC-Y, and IEC-Z of its PTV. The modeling PET session used for planning was prepared with a target and OAR activity concentration of 55.57 kBq/ml and a background of 6.77 kBq/ml; the target-to-background ratio was 8.21:1. A programming environment script was written to generate 2D and 3D pseudorandomized sinusoidal respiration trajectory patterns as inputs into the motion platform during the modeling PET scan. For IEC-X, IEC-Y, and IEC-Z, target 1 had amplitude limits of 8 mm, 15 mm, and 8 mm. Target 2 was amplitude limited to 8 mm, 25 mm, and 8 mm. Target 1 was further constrained to a max velocity of 9.5 mm/s, and target 2 was constrained to 12.5 mm/s. The OAR was assigned a Cos4 motion pattern with an amplitude of 5 mm and a fixed frequency of 9.5 breaths per minute (bpm). The PET scan was used to prepare a 12.5 Gy/fraction plan. Analysis was performed by quantifying the accuracy between the theoretical and achieved trajectory positions as well as the achieved respiratory rate of each target. Plan quality was evaluated using the conformity index (CI), normalized target signal (NTS), and activity concentration (AC) for target 2. Results: The commanded positions for the desired trajectories resulted in less than 100-micron error when compared against the theoretical trajectories. Target 1 achieved a respiratory rate of ~16 bpm, and target 2 achieved a rate of 14 bpm. The plan using the modeling PET scan achieved a CI of 1.17, an NTS of 7.07, and an AC of 7.96 kBq/ml. Conclusion: The phantom achieved precise and asynchronous motion patterns within the range of normal respiration (12-20 bpm). The NTS and AC were above the minimum limit for a deliverable plan (NTS>2.7 and AC>5).
