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Image Guidance for Eye Tumors Treated with Proton Therapy: Noninvasive Eye Tracking Versus X-Ray Imaging

R Via1*, G Fattori1, A Pica1, G Baroni2, A Lomax1, D Weber1, J Hrbacek1, (1) Paul Scherrer Institut, Villigen Psi, AG, CH, (2) Politecnico di Milano, MI,IT

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
Ocular proton therapy (OPT) relies on an invasive surgical procedure to implant clips on the sclera that are used to verify the patient position through radiographic imaging. We have assessed the performance of a non-invasive eye tracking system (ETS) and compared it to X-ray imaging of clips.
Methods:
The ETS consists of four optical cameras that reconstruct the three-dimensional position of the pupil through stereoscopic imaging. The planned pupil position is obtained by applying the geometrical transformation from simulation to treatment eye geometry, retrieved using clips, to ETS measurement performing during simulation. As a result, the following positioning protocol could be applied for 12 uveal melanoma patients treated at out institution: first, real-time measurements of the pupil position from which the initial translational chair corrections could be calculated; second, X-ray imaging of clips was applied to fine tune these corrections. Thus, the accuracy of the pupil-based approach was directly measured by the first X-ray images, enabling dosimetric evaluation of target coverage and geometrical comparison to the actual treatment position (determined from the final X-ray images before delivery).
Results:
Lens misalignment exhibited similar distributions for pupil-based and clips-based approach (Kruskal-Wallis p-value=0.23) whereas the opposite is true for errors on the target center-of-mass (Kruskal-Wallis p-value<0.05) with a median misalignment of 0.49mm(IQR:0.53mm) and 0.32(IQR:0.19), respectively. However, this is enough to ensure V95 to the target volume greater than 95% for all pupil-guided patient positions considered here.
Conclusion:
This study shows how a pupil-based approach does not guarantee the same level of accuracy of the clinical standard due to its inability, contrarily to X-ray imaging of clips, to account for eye rotations. Including an estimate of rotational misalignment (gaze direction correction) is necessary to improve the performance. However, these results are encouraging, as sufficient target coverage was achieved even with this simplified approach

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Funding Support, Disclosures, and Conflict of Interest: The research leading to these results have been founded by Krebsliga Schweiz (Swiss Cancer League) under grant agreement KFS-4447-02-2018 and by Personalized Health and Related Technologies under grant agreement PHRT-524

Keywords

Ocular Proton Therapy, Eye Tracking, Image Guidance

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – Development (new technology and techniques)

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