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4Pi Optimization to Compare Single Versus Multiple Gaze Angle Selection in Ocular Proton Therapy

E Fleury1 2*, C Thomas3 4, B Little3, JW Beenakker5, JP Pignol3 4, P Trnkova1 6, M Hoogeman1 2, (1) Erasmus Medical Center, Rotterdam, The Netherlands (2) Holland Proton Therapy Center, Delft, The Netherlands (3) Dalhousie University, Halifax, Canada (4) Queen Elizabeth II Health Sciences Centre, Halifax, Canada (5) Leiden University Medical Center, Leiden, The Netherlands (6) Medizinische Universitat Wien, Vienna, Austria


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

Room: AAPM ePoster Library

Purpose: To investigate the feasibility and the clinical benefit of using a 2-beams approach in ocular clipless proton therapy with a fixed horizontal eye line using a 4Pi optimization based on beam-eye-view overlap of tumor and organs-at-risk (OARs).

Methods: An in-house developed ocular melanoma proton treatment planning system (TPS) was used to calculate dose distributions on 3D MRI-based reconstructed eyes. Five uveal melanoma patients were included in the study. High-resolution 3D MRI images were acquired on a 7T Philips Achieva MRI (Best, The Netherlands). Target volume and OARs were semi-automatically delineated, including optic nerve, ciliary body, lens and sclera ring. The macula was geometrically defined as it is not visible on MRI. A previously published 4Pi beam angle optimization algorithm was modified for the purposes of ocular proton therapy to determine optimal gazing angles. This optimizer, based on beams-eye-view overlap between tumor and OARs, calculated two optimal gazing angles within a clinically feasible space (±30° upward/downward, ±30° left/right). Dose calculations were performed for the gazing angles optimally selected by 4Pi, resulting in a two equally-weighted beams plan vs. two individual beam plans. Dosimetric outcomes were extracted from each plan to evaluate which approach was more favorable for OARs sparing.

Results: Among the patients of our cohort, the mean dose to OARs was reduced by 8.3% on average by using a 2-beams approach. An overall mean dose to OARs decrease of 6.3% and 10.3% was achieved on average with the 2-beams configuration vs. the single beam 1 and single beam 2 configuration, respectively.

Conclusion: This work confirms that using more than one beam in ocular proton therapy yields a better trade-off in OARs sparing. It also shows the usefulness of an automated and objective method applied to gaze angle selection using a 4Pi beam angle optimization approach.

Funding Support, Disclosures, and Conflict of Interest: This research was co-founded by the research program PROTONS4Vision (Grant NWO 14654), which is financed by the Netherlands Organization for Scientific Research (NWO), Technology Foundation STW, the Top consortium for Knowledge & Innovation (TKI-HTSM) and Varian Medical Systems, Inc., Palo Alto.


Protons, MRI, Optimization


TH- External Beam- Particle/high LET therapy: Proton therapy – dose optimization

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