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An Interdimensional Correlation Framework for Direct Real-Time Estimation of Six Degree of Freedom Target Motion Using a Single X-Ray Imager During Radiotherapy

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D Nguyen

D T Nguyen1*, J Bertholet2 , J Kim1 , R O'Brien1 , J Booth3 , P Poulsen2 , P Keall1 , (1) The University of Sydney, Sydney, (2) Aarhus University Hospital, Aarhus, (3) Royal North Shore Hospital, St Leonards, Sydney


TH-AB-205-3 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: 205

Purpose: Increasing evidence suggests that intrafraction tumor motion monitoring needs to include both 3D translations and 3D rotations. Presently, methods to estimate the rotation motion require the 3D translation of the target to be known first. However, ideally, translation and rotation should be estimated concurrently. We present the first method to directly estimate for six-degree-of-freedom (6DoF) motion from the target’s projection on a single rotating x-ray imager in real-time.

Methods: This novel method is based on the linear correlations between the superior-inferior translations and the motion in the other five degrees-of-freedom. The accuracy of the method was evaluated in silico with 81 liver tumor motion traces from 19 patients with three implanted markers. The ground-truth motion was estimated using the current gold standard method where each marker’s 3D position was first estimated using a Gaussian probability method, and the 6DoF motion was then estimated from the 3D positions using an iterative method. The 3D position of each marker was projected onto an orthogonally-gantry-mounted imager with an imaging rate of 11Hz. After an initial 110° gantry rotation (200 images), a correlation model between superior-inferior translations and the five other DoFs was built using a least square method. The correlation model was then updated after each subsequent frame to estimate 6DoF motion in real-time.

Results: The proposed algorithm had an accuracy (±precision) of -0.03±0.32mm, -0.01±0.13mm and 0.03±0.52mm for translations in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) directions respectively; and, 0.07±1.18°, 0.07±1.00° and 0.06±1.32° for rotations around the LR, SI and AP axes respectively on the benchmarking dataset.

Conclusion: The first method to directly estimate real-time 6DoF target motion from segmented marker positions on a 2D imager was devised. The algorithm was evaluated using 81 motion traces from 19 liver patients and found to have sub-mm and sub-degree accuracy

Funding Support, Disclosures, and Conflict of Interest: This work is supported by two Cancer Australia grants. Author P Keall is supported by an Australian NHMRC Senior Professorial Fellowship.

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