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Total Variation Regularization of Fluence Fields for 3D Printed Compensators in Small Animal IMRT

X Liu1*, E Pearson1, G Redler2, H Halpern3, R Wiersma4, (1) University of Chicago, Chicago,IL, (2) Moffitt Cancer Center, Tampa, FL, (3) University of Chicago, Chicago, IL, (4) University of Pennsylvania, Philadelphia, PA


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

Room: AAPM ePoster Library

Purpose: Recently, 3D printed compensators have enabled preclinical small animal IMRT. Traditional IMRT optimization leads to highly-modulated fluence patterns that can be difficult for 3D printers. In this work, total variation regularization (TVR) IMRT optimization is proposed to reduce modulation complexity and provide practical advantages, without reducing overall dose conformity.

Methods: During IMRT optimization, an L1 norm gradient operator is defined on nearby beamlet intensities in both column-wise and row-wise directions, and a mixed L1/L2 norm optimization with non-differentiable and convex cost functions is approximated and solved efficiently by an in-house software. The attenuation coefficient of Copper/Polylactic-acid (80/20%) filament used for compensator fabrication was empirically determined for XRad225Cx (Precision X-ray Inc.) small animal irradiation system. This enables conversion from optimized beamlet intensity to local compensator thickness. Five-field IMRT plans were generated with and without TVR to conformally treat a murine tumor with a simultaneous integrated boost to hypoxic tumor. Dose delivered to a phantom using 3D-printed compensator IMRT was measured with calibrated gafchromic film. Gamma analysis was used to compare measured to planned dose per-field.

Results: Implementing TVR reduced beamlet intensity total variation (TV) by 48-58%, compensator thickness TV (a measure of smoothness of a surface) by 45-54%, and sum of compensator thickness by 20-28%, and radiation beam-on time by 22-28%, while DVH curves remained acceptable/comparable and gamma 3%/0.7mm passing rate was comparable at 95.7% with TVR and 94.0% without. Reducing beam-on time reduces overall treatment time and non-focal dose to normal tissue from leakage and scatter. Reduced compensator thickness decreases printing time/material.

Conclusion: This work uses TVR to reduce fluence map complexity for 3D printed compensator based IMRT. Experimental results show TVR can produce more easily printable compensators, which reduce printing time, save costs in material usage, and reduce treatment times while maintaining the planning advantages of IMRT.

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