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Plan Quality of Proton Vs. Photon IMRT for Whole Ventricle Irradiation

K Kainz*, W Wang, S Firat, J Wilson, C Schultz, X Li, Medical College of Wisconsin, Milwaukee, WI

SU-E-CAMPUS-T-4 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To assess whether photon IMRT is an alternative to protons for whole ventricle irradiation (WVI), we compare dosimetric plans of helical tomotherapy with passively-scattered protons for WVI based upon target dose uniformity and conformity and organ-at-risk (OAR) sparing.

Methods: Six representative pediatric patients treated for CNS disease were evaluated retrospectively. A 3 mm isotropic expansion of the ventricles' contour was designated as the PTV and prescribed to 24 Gy. Among the OARs considered were the temporal lobes, optic chiasm, optic nerves, inner ears, and lenses. Tomotherapy plans were generated using the TomoTherapy planning system v. 4.2 (Accuray, Inc.) with the longitudinal jaw size of 2.5 cm and the pitch of 0.287. Three-field proton plans were created using CMS XiO v. 4.70 (Elekta, Inc.), using a passively-scattered proton beam model with maximum range of 32 g/cm2 (225 MeV), range modulation in 0.5 g/cm2 increments, and Lucite range compensators. Plan quality was compared using a series of parameters including conformity index (CI), uniformity index (UI), and a EUD-based plan quality index (fEUD), a composite value calculated using the generalized EUD of all targets and OARs.

Results: Among the 6 patients, proton plans exhibited slightly better CI for 4 cases and slightly better UI for 5 cases. If all OARs are considered, the proton plans had slightly better fEUD (0.20 vs. 0.17). This was attributed to the total avoidance of the eyes and lenses among the proton plans. If the eyes and lenses are excluded, the average fEUD is equal to 0.20 for both planning techniques.

Conclusion: For WVI, if the sparing of distal ROIs such as the eyes and lenses is a concern, classical proton planning is a more favorable choice. Otherwise, the overall dose to the target and proximal OARs are comparable for both planning techniques.

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