Validating robotic couch isocentricity with 3D surface imaging

Background: A proton therapy system with 190° gantries uses robotic couch rotations to change the treatment beam laterality. Couch rotations are typically validated clinically with post-rotation radiographic imaging.

Aims: This study assesses the specificity and sensitivity of a commercial 3D surface imaging system, AlignRT (Vision RT, London UK) for validating couch rotations.

Materials & methods: In clinical operation, a reference surface image of the patient is acquired after radiographic setup with couch at 270°, perpendicular to the gantry axis of rotation. The couch is then rotated ±90° to a typical treatment angle, and AlignRT reports a 3D displacement vector. Patient motion, changes in patient surface, non-coincidence between AlignRT and couch isocenter, and mechanical couch run-out all contribute to the 3D vector magnitude. To assess AlignRT sensitivity in detecting couch run-out, volunteers were positioned orthogonal to the proton gantry and reference surface images were captured without x-ray localization. Subjects were repeatedly rotated ±90⁰ to typical treatment angles and displacement vectors were recorded. Additionally, 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 using a phantom; subjects with a thoracic isocenter and no immobilization; and subjects with a cranial isocenter and thermoplastic immobilization. A total of 300 rotations were measured.

Results: During isocentric rotations, the mean AlignRT displacement vectors for the phantom, immobilized, and non-immobilized volunteers were 0.1 ± 0.1 mm, 0.8 ± 0.1 mm, and 1.1 ± 0.2 mm respectively. 95% of the AlignRT measurements for the immobilized and non-immobilized subjects were within 1 mm and 2 mm of the actual displacement respectively.

Discussion: After characterizing the accuracy using phantoms and volunteers, we have shown that a three-pod surface imaging system can be used to identify gross non-isocentric patient rotations. Significant positional deviations, either due to improper couch rotation or patient motion, should be followed by radiographic imaging and repositioning.

Conculsion: AlignRT can be used to verify patient positioning following couch rotations that are applied after the initial x-ray guided patient setup. Using a three-pod AlignRt system, positional deviations exceeding 4 mm were flagged with sensitivity and specificity of 90% and 100% respectively.