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Investigating the Dosimetric Effect of the Qfix Encompass SRS Immobilization System On Cone-Based SRS Treatment Planning

Y Lei1*, S Wang1, A Granatowicz2, Q Fan3, W Nie1, S Wisnoskie1, S Zhou1, (1) University of Nebraska Medical Center, Omaha, NE, (2) Nebraska Medicine, Omaha, NE, (3) Memorial Sloan Kettering Cancer Center, Commack, NY

Presentations

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

Room: AAPM ePoster Library

Purpose:
To investigate the dosimetric effect of the Qfix Encompass SRS immobilization system on cone-based SRS treatment planning.

Methods:
We first created and compared two SRS circular plans using Varian’s Cone Planning treatment planning system (TPS). These two plans share the same beam settings except the inclusion of the Qfix Encompass immobilization system as supporting structure, which is provided in Varian Eclipse v15.6. We then similarly created two SRS circular plans in BrainLab’s iPlan TPS based on the same CT image set. Both structure sets of these two plans include head contours with HU overridden to be water to simulate the situation in Varian’s Cone planning TPS, but one’s outer contour includes Encompass immobilization structure and the other does not. Both plans share the same beam settings and all arcs were deliberately configured to pass through the Encompass structure to evaluate its dosimetric effect. The heterogeneity correction was enabled for both plans to account for the immobilization structure in dose calculation. The dosimetric parameters were compared.

Results:
For Varian’s Cone planning TPS, we found no dosimetric difference between the two plans (with and without Encompass device included). For BrainLab’s iPlan TPS, compared to the plan with Encompass device (plan1), the plan does not include immobilization system inside the outer contour (plan2) is 3.2% hotter in isocenter dose, 2.7% hotter in minimum target dose, 3.2% hotter in maximum target dose, and 3.2% hotter in mean target dose. The normal brain’s V(12Gy) and V(18Gy) are 2.11cc vs. 1.86cc and 0.19cc vs. 0.13cc for plan2 and plan1, respectively.

Conclusion:
The Qfix Encompass SRS immobilization system affects the dose distribution in SRS cone treatment planning. Planners should be meticulous about the beam geometry to minimize the uncertainties if the dose calculation algorithm does not account for the Encompass support device.

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