2178 - Intensity Modulated Proton Therapy with Robust Optimization to Mitigate Inter-Fractional Variations

02:30pm - 04:00pm PT

Hall F

Screen: 7

POSTER

Presenter(s)

Romy Megahed, MD - UAMS Radiation Oncology, Little Rock, AR

R. J. Megahed¹, K. Schneider², C. L. Chien³, S. Samanta³, and W. C. Hsi⁴; ¹Department of Radiation Oncology, University of Arkansas for Medical Sciences, Little Rock, AR, ²University of Arkansas for Medical Sciences, Little Rock, AR, ³Department of Radiation Oncology, UAMS Winthrop P. Rockefeller Cancer Institute, University of Arkansas for Medical Sciences, Little Rock, AR, ⁴Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL

Purpose/Objective(s): Intensity modulated proton therapy (IMPT) is thought to be more sensitive to interplay effects, inter-fractional anatomic variation and positioning inaccuracies than traditional photon IMRT due to the sharp dose gradient. This is especially true for head and neck cancers small changes in set up can impact target coverage and dose to OARS. This study aims to identify the effect of inter-fractional variation on dose distribution to target covergae and OAR dose for head and neck proton plans.

Materials/Methods:

Eight patients treated for oropharyngeal cancers with robustly optimized proton therapy plans were selected. The clinical workflow involves recalculating the dose on repeated quality assurance CTs (QACTs) throughout treatment to assess continued target coverage under the possibility of anatomical changes.

For this study, the dose calculated on the QACTs was then transferred to the planning CT (pCT) using deformable registration. Doses to relevant head and neck OARs including brainstem, bilateral cochleas, bilateral parotids, larynx, and esophagus were recalculated on each QACT and compared to the pCT as well as with historical QUANTEC constraints. Target coverages were evaluated using the conformality index (CI) for the target on both the pCT and QACTs and compared using a one way ANOVA.

Results: All dose constraints met on the pCT were again met on the dose recalculated on the QACTs. CI values for 6 of 8 patients were above 1 for both the planned dose and subsequent QACT calculated doses. Values less than 1 resulted from tight contouring due to proximity of multiple OARs. No statistically significant differences were observed in CI values between planned and QACTs doses. OAR dose for planned and QACT doses adhered to the dose constraint for the brainstem, cochlea, larynx, esophagus and contralateral parotid. The constraint for the ipsilateral parotid was violated by both planned and QACT doses. There were no cases where the constraint was met with the planned dose and violated by the QACT dose

Conclusion: IMPT with robust optimization offers excellent and consistent achievement of OAR doses. This indicates appropriate target coverage and OAR sparing for IMPT regardless of inter-fractional variation.