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Proton PBS Plan Design and Robustness Evaluation for Head and Neck Cancers

X Liang

X Liang*, S Tang , H Zhai , M Kirk , A Kalbasi , A Lin , P Ahn , Z Tochner , J McDonough , S Both , University of Pennsylvania, Philadelphia, PA


SU-E-T-266 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To describe a newly designed proton pencil beam scanning (PBS) planning technique for radiotherapy of patients with bilateral oropharyngeal cancer, and to assess plan robustness.

Methods: We treated 10 patients with proton PBS plans using 2 posterior oblique field (2F PBS) comprised of 80% single-field uniform dose (SFUD) and 20% intensity-modulated proton therapy (IMPT). All patients underwent weekly CT scans for verification. Using dosimetric indicators for both targets and organs at risk (OARs), we quantitatively compared initial plans and verification plans using student t-tests. We created a second proton PBS plan for each patient using 2 posterior oblique plus 1 anterior field comprised of 100% SFUD (3F PBS). We assessed plan robustness for both proton plan groups, as well as a photon volumetric modulated arc therapy (VMAT) plan group by comparing initial and verification plans.

Results: The 2F PBS plans were not robust in target coverage. D98% for clinical target volume (CTV) degraded from 100% to 96% on average, with maximum change ΔD98% of -24%. Two patients were moved to photon VMAT treatment due to insufficient CTV coverage on verification plans. Plan robustness was especially weak in the low-anterior neck. The 3F PBS plans, however, demonstrated robust target coverage, which was comparable to the VMAT photon plan group. Doses to oral cavity were lower in the Proton PBS plans compared to photon VMAT plans due to no lower exit dose to the oral cavity.

Conclusion: Proton PBS plans using 2 posterior oblique fields were not robust for CTV coverage, due to variable positioning of redundant soft tissue in the posterior neck. We designed 3-field proton PBS plans using an anterior field to avoid long heterogeneous paths in the low neck. These 3-field proton PBS plans had significantly improved plan robustness, and the robustness is comparable to VMAT photon plans.

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