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Accuracy Evaluation for 6-Dimensional Patient Position Detection Using An Orthogonal OBI KV Image Pair of a Truebeam Linear Accelerator


Michael Cheung

Michael L M Cheung1*, K Y Lee1, Y K Ngar1, K H Yu1, Anthony T C Chan1,2, (1) Department of Clinical Oncology, Prince of Wales Hospital, Hong Kong SAR, China (2) State Key Laboratory in Onology in South China, Chinese University of Hong Kong, Hong Kong SAR, China

PO-BPC-Exhibit Hall-9 Saturday  Room: Exhibit Hall

Purpose:
To evaluate the accuracy of 6-dimensional patient position detection using an in-house developed software based on a pair of orthogonal kV images taken using the on-board-imager (OBI) of a Varian Truebeam linear accelerator.

Methods:
An in-house software was developed to detect the 6D position (lateral, longitudinal, vertical, pitch, row, yaw) of a patient on the treatment couch by performing 2D-3D image registration between a pair of orthogonal OBI kV images and the planning CT of the patient. A 6D simulation phantom designed by Ngar et.al., 2012, was used for the evaluation of the accuracy of 6D position detection of the software developed. The phantom was scanned using a CT simulator with its 6D shift set to zero. The phantom was then placed on the couch of a Truebeam linac. 20 sets of randomly generated known non-zero 6D shift with magnitudes within 20 mm and 4 degrees were applied to the phantom. A pair of orthogonal OBI kV images was taken at OBI angles 45 degree and 315 degree respectively for each set of 6D shift. The CT and the OBI images were imported into the in-house developed software for 6D position detection. The detection error was obtained by subtracting the actual shift from the software detected shift.

Results:
The detection error of the 6D positions of the 20 sets of non-zero 6D shifts of the phantom detected were found to be all within 1 mm and 1 degree. The mean translation detection error and mean rotation detection error were 0.68 mm and 0.56 degree respectively.

Conclusion:
With the help of a proper image registration software, a Truebeam linear accelerator was able to perform 6D patient position detection with an accuracy of <1 mm and <1 degree using an orthogonal OBI kV image pair.


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