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Isodose-Driven Optimization for Automatic/Interactive Electron Bolus Design

M Chen

M Chen*, X Gu , T Chiu , T Long , J Tan , W Lu , University of Texas Southwestern Medical Center, Dallas, TX


SU-I-GPD-T-66 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Electron therapy has great potential to treat superficial tumors due to its finite beam range, which spares distal organs-at-risk (OAR). However, the electron beam range needs to be modulated to achieve the desired dose distribution in case of complex patient surfaces and tumor shapes. The electron range could be modulated via 3D bolus, which is typically manually generated. In this research, we developed an isodose-driven optimization approach for automatic/interactive electron 3D bolus design.

Methods: First, a 3D bolus is represented by a 2-dimensional thickness map T(x,y) on the fluence plane. To achieve reasonable accuracy and speed for dose update during optimization, the dose D_T(x,y,r) with bolus T is modeled using linear interpolation from two anchor doses with the thickness bounds L and U. Specifically, D_T=a⋅D_L+(1-a)⋅D_U, where the two anchor doses D_L and D_U are calculated using the electron Monte Carlo (MC) method and the weighting factor a is defined according to the proximity ratio ((U-T))⁄((U-L)). The anchor doses provide a reasonable estimate for dose with any bolus in between. The radiological distance between the prescription iso-dose without any bolus and the tumor boundary is used to define the initial bolus map T_0, which is down-scaled and up-scaled to obtain the bolus bounds L and U, respectively. The remaining steps iterate between updating the bolus, similarly to bolus initialization, and calculating dose with the updated bolus through linearly interpolating the anchor doses.

Results: The interpolation dose approximated the MC dose reasonably accurately with high efficiency. The optimized dose conformed well to the target boundary and met all prescriptions as demonstrated in clinical head-and-neck cases.

Conclusion: We have developed an optimization approach for electron bolus design. The ultra-fast dose updating scheme and isodose-driven approach facilitate automatic/interactive bolus design. It improves both efficiency and quality in electron modulated radiotherapy treatment planning.

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