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Session: Advances in Treatment Planning I [Return to Session]

Direct Thickness Optimization (DTO) for Compensator Based IMRT

X Liu*, R Wiersma, University of Pennsylvania, Philadelphia, PA


SU-E-206-3 (Sunday, 7/10/2022) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Room 206

Purpose: 3D printed compensator approaches to implement intensity-modulated radiation therapy (IMRT) have drawn attention in recent years for implementing cost effective IMRT for developing countries and implementing IMRT in preclinical small animal models. Current approaches are two-step in that they first use inverse optimization to calculate the ideal IMRT fluence map, and then converted the beamlet intensities to corresponding compensator thickness. However, such two-step approaches lead to complex modulation printing patterns, excess filament usage, and long beam-on times. To address these issues, we have developed a novel direct thickness optimization (DTO) method by directly imposing the radiation filament transmission curve in the objective function.

Methods: The polychromatic transmission through copper-PLA filaments were measured with an ionization chamber and were fitted as a function of the filament thickness by a two-term exponential model. The total variation of compensator thickness inside the beam's-eye-view, sum of thickness and total beam-on times were considered as soft constraints and were included in the objective function. This is a non-convex problem and was solved by a modified LBGFS algorithm, and the compensator thickness and beam-on time of each gantry angle were directly optimized. Both traditional IMRT and DTO plans were generated to treat a mouse heart while sparing lung tissue.

Results: 6 mice heart plans were generated by both traditional IMRT and DTO. Across all cases, it was found that DTO reduced the total variation of compensator beamlet thicknesses by 82+/-6%. On average, the total mass of compensator material consumed, and radiation beam-on time were reduced by 75+/-7% and 22+/-5%, respectively, while dose metrics remained comparable.

Conclusion: DTO approach can generate 3D-printed compensator patterns with significantly less complexity while still maintaining similar dose conformity to traditional IMRT. This can simplify the 3D printing process, reduce the amount of filament used, and reduce radiation delivery time.


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