SU-E-T-568: Hippocampus and Neural Stemcell Sparing Using Proton Therapy in Whole Brain Irradiation.
PURPOSE
To investigate feasibility of using proton therapy (PT) in neural stemcell sparing and to compare its dosimetric advantage to IMRT. To investigate the robustness of the proton plan in hippocampus sparing by introducing translation and rotation errors in treatment plan.


METHODS
Hippocampal neurogenesis directly correlates to memory and other cognitive functions. Compared to the traditional whole brain (WB) uniform irradiation, Photon Intensity-Modulated-Radiation-Therapy (IMRT) can reduce hippocampal dose somewhat but limited by its beam exit dose. Pinnacle and Eclipse were used, respectively, for IMRT and PT planning (30Gy in 10 fractions) for hippocampal neural stemcell sparing WB irradiation. Both hippocampus and neural stemcell regions were contoured as avoidance region. Similar to RTOG0933, the target was defined as the WB subtracted by 5 mm expansion of the avoidance region. Other organs-at- risk (OARs) includes lenses, eyes, optical chiasm, optical nerves, brainstem and parotids. Twelve coplanar beams were used in IMRT plans with 2.5mm MLC width. Objectives were specified to maximize target coverage and reduce dose to avoidance region and to other OARs. There were 9 non- coplanar proton beams that include 2 pairs of patch fields, 1 pair of match fields and 4 open fields. Distal blocking was used in 5 beams.


RESULTS
Both plans' target coverage was scaled at 70% target covered by 30Gy. Plan comparison between proton and IMRT are, respectively, as follows: Target D2 (36.2, 37.3Gy); Mean dose to avoidance region (0.96, 5Gy); mean doses to left-lens (0.21, 3.35Gy); mean doses to right-lens (0.25, 3.57Gy); mean dose to left-parotid (0.82, 9.57Gy) and right-parotid (0.11, 11.6Gy). Similar dose distribution found in brainstem, eyes and optical nerves and chiasm for both proton and IMRT plans.


CONCLUSIONS
Proton plan demonstrated dosimetric advantage in neural stemcell and hippocampus sparing than IMRT plan. The proton plan is robust from translation and rotation setup errors.