Beam angle optimization using angular dependency of range variation assessed via water equivalent path length (WEPL) calculation for head and neck proton therapy |
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Institution: | 1. Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, Arizona;2. Department of Radiation Physics, University of Texas MD Anderson Cancer Center, Houston, Texas;3. Department of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, Texas;4. School of Business Administration, Southwestern University of Finance and Economics, Chengdu, Sichuan, China;5. Varian Medical Systems, Inc, Palo Alto, California;6. Department of Radiation Oncology, Emory University, Atlanta, Georgia;7. Scripps Proton Center, San Diego, California;2. Department of Radiation Oncology, Oklahoma Proton Center, Oklahoma City, OK 73142, USA;3. Department of Medical Physics, Apollo Proton Cancer Centre, Chennai, Tamil Nadu, India;4. Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, USA |
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Abstract: | PurposeTo investigate angular sensitivity of proton range variation due to anatomic change in patients and patient setup error via water equivalent path length (WEPL) calculations.MethodsProton range was estimated by calculating WEPL to the distal edge of target volume using planning CT (pCT) and weekly scatter-corrected cone-beam CT (CBCT) images of 11 head and neck patients. Range variation was estimated as the difference between the distal WEPLs calculated on pCT and scatter-corrected CBCT (cCBCT). This WEPL analysis was performed every five degrees ipsilaterally to the target. Statistics of the distal WEPL difference were calculated over the distal area to compare between different beam angles. Physician-defined contours were used for the WEPL calculation on both pCT and cCBCT, not considering local deformation of target volume. It was also tested if a couch kick (10°) can mitigate the range variation due to anatomic change and patient setup error.ResultsFor most of the patients considered, median, 75% quantile, and 95% quantile of the distal WEPL difference were largest for posterior oblique angles, indicating a higher chance of overdosing normal tissues at distal edge with these angles. Using a couch kick resulted in decrease in the WEPL difference for some posterior oblique angles.ConclusionsIt was demonstrated that the WEPL change has angular dependency for the cohort of head and neck cancer patients. Selecting beam configuration robust to anatomic change in patient and patient setup error may improve the treatment outcome of head and neck proton therapy. |
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Keywords: | Water equivalent path length Angular dependency Proton therapy Anatomic change Patient setup error Cone-beam CT Scatter correction |
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