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Fraction-Variant Beam Angle Optimization in Intensity Modulated Proton Therapy

W Gu

W Gu*, D O'Connor , D Nguyen , V Yu , D Ruan , K Sheng , UCLA School of Medicine, Los Angeles, CA


SU-H2-GePD-T-4 (Sunday, July 30, 2017) 3:30 PM - 4:00 PM Room: Therapy ePoster Lounge

Purpose: To develop a novel algorithm that selects variant beams in different treatment fractions for intensity modulated proton therapy (IMPT).

Methods: In the optimization, 1162 non-coplanar beams uniformly distributed across 4π steradians were included as the candidates. For each beam, the doses of all scanning-spots covering the PTV and a margin were calculated. The fraction-variant beam angle optimization (FVBAO) problem was formulated to include four terms: two L2-norm dose fidelity terms to penalize the deviation of PTV fractional dose and OAR total doses from prescription, respectively; an L1-norm sparsity term on spots to reduce the number of active spots and improve delivery efficiency; an L2,½-norm group sparsity term to control the number of active beams per fraction to between 2 and 4. Fast Iterative Shrinkage-Thresholding Algorithm (FISTA) was applied to solve this problem. The FVBAO-IMPT algorithm was tested on a head-and-neck patient of 5 treatment fractions. The result was compared with a 2-field IMPT plan that uses the same beams every fraction.

Results: For the FVBAO-IMPT plan, a total number of 10 beams were selected and optimized, among which 9 beams were unique. Compared with plan using the same beam sets for all fractions, improved PTV coverage was observed with a maximum dose reduction of 2.17%, a 4.19% improvement of conformity index and a 2.47% increase of homogeneity index. The OAR doses were comparable in the two plans.

Conclusion: This work tested an IMPT optimization approach to vary beam angles during different treatment sessions. It provides an approach to use and optimize a larger total number of IMPT beams in the entire treatment, and improve the plan quality while still maintaining a low number of beams in each fraction. This method will be further validated in more complex cases where OAR sparing is difficult with only 2 to 4 beams.

Funding Support, Disclosures, and Conflict of Interest: DOE DE-SC0017057 NIH R44CA183390 NIH R01CA188300 NIH R43CA183390 NIH U19AI067769

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