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Sensitivity of a Surface Imaging System in Assessing Robotic Couch Isocentricity

O El-Sherif

O El-Sherif*, N Remmes , J Kruse , Mayo Clinic, Rochester, MN


SU-I-GPD-J-49 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: A proton therapy treatment room consists of a 190⁰ gantry and a robotic couch. Isocentric couch rotations are used during treatment to change the laterality of the proton beam. Isocentricity of each couch rotation is currently validated with an x-ray IGRT system. This study assesses the sensitivity of a commercial surface imaging system, AlignRT (Vision RT, London UK) for validating couch rotations.

Methods: A reference surface image of the patient is acquired after x-ray based localization. After couch rotation AlignRT reports a 3D displacement vector. Non-zero 3D vectors may be attributed to patient motion, changes in patient surface, non-coincidence between AlignRT and couch isocenters, and finally, to couch run-out. To assess AlignRT sensitivity in detecting couch run-out, volunteers were positioned orthogonal to the proton gantry, without x-ray localization, and reference surface images were captured. Subjects were repeatedly rotated ±90⁰ to typical treatment angles and 3D vectors were recorded. Additional measurements were performed in which intentional translations of 2, 4, 6, and 8 mm were combined with the intended isocentric rotations. Data sets were collected for subjects with a thoracic isocenter and no immobilization as well as subjects with a cranial isocenter and thermoplastic immobilization. A total of 300 rotations (with and without intentional translations) were measured. AlignRT and couch coincidence was also measured using a phantom.

Results: For true isocentric rotations the mean AlignRT displacement vectors for the immobilized and non-immobilized volunteers were 0.1 ± 0.6 mm and 0.1 ± 1.1 mm respectively. 95% of the AlignRT measurements for the immobilized and non-immobilized subjects were within1mm and 2 mm of the actual displacement respectively.

Conclusion: A clinical action level of 2mm 3D displacement will reliably detect couch runouts exceeding 1 mm with sensitivity and specificity of 89% and 100% respectively.

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