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Mixed Orthovoltage and Electron Treatments Offer An Inexpensive Alternative to Protons in Treating Peri-Ocular Lesions

Y Poirier

Y Poirier1*, K Martell2,3 , R Khan4 , T Zhang4 , A Hudson2,3 , N Wolfe3 , D Spencer2,3 , J Voroney2,3 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) University of Calgary, Calgary, AB, (5) Tom Baker Cancer Centre, Calgary, AB, (4) Washington University School of Medicine, St Louis, MO


WE-RAM1-GePD-T-5 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Lounge

Purpose: Peri-orbital lesions are typically difficult to treat as they are surrounded by sensitive optical structures, which limit the dose that can be delivered to the target. Electrons are typically used, but suffer from a low surface dose and wide penumbra. Orthovoltage treatments offer higher surface dose and sharp penumbras, but suffer from rapid falloff and poor coverage of deeper lesions. While protons are ideal, they are an expensive modality and their access depends highly on geographical location. It is possible to combine the strengths of orthovoltage (high surface dose, sharp penumbra) with those of electrons (constant dose at depth, finite range) to create a more ideal dose distribution. In this study, we compare the dose distributions of a mix consisting of 30% 200 kVp orthovoltage + 70% 9 MeV electrons (renormalized to 110% to bring the sum Dmax to 100%) to those of orthovoltage, electrons, and protons alone.

Methods: Dose distributions were retrospectively calculated for four previous peri-orbital lesion cases. The orthovoltage dose distributions were calculated using our previously validated kV dose calculation engine, kVDoseCalc. The electron dose distributions were calculated using the Varian Eclipse Monte Carlo model, and the proton dose distributions were calculated with Eclipse for Mevion S250 using a double scattering technique.

Results: Mixed orthovoltage and electron therapy produced dose distributions superior to either modality alone. The surface dose increased from ~60-80% with electrons alone to ~90-100% in mixed electron/orthovoltage plans. Combining electrons and orthovoltage treatments also increased the area of tissue receiving > 95% of the prescribed dose, due to the high scattering of electrons in small fields.

Conclusion: Combining electrons and orthovoltage treatments can produce a superior dose distribution for treating shallow peri-orbital tumors compared to either modality alone. While protons give superior dose distributions, the proposed technique is a viable inexpensive alternative.

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