2047 - Treated and Replanned Hippocampal Doses in Patients Receiving Photon vs. Proton RT for Favorable Grade 2-3 IDHmt Gliomas

Purpose/Objective(s):

Radiation (RT) dose to the hippocampus is associated with neurocognitive toxicity after whole brain radiation (WBRT). Patients with favorable-risk gliomas have a better prognosis than patients receiving WBRT and may benefit from hippocampal-sparing, but the hippocampus is not standardly delineated for gliomas. We herein compare hippocampal doses in pts treated with photons vs protons, and degree of potential improvement with hippocampal contouring and optimization for both modalities.

Materials/Methods:

This was a retrospective study of pts with IDHmt grade 2-3 gliomas treated from 2013-2025 at 1 academic center. We recorded whether hippocampi were contoured in treated RT plans. All missing hippocampi were contoured, and treatment plans reoptimized (photons: IMRT; protons: pencil beam) to minimize hippocampal dose without losing target coverage. For dosimetric analysis, pts were only included if neither hippocampus was infiltrated by tumor (n=86). We recorded treated and replanned bilateral hippocampus mean, V10Gy, V30Gy, amongst other covariates. Independent t-test compared photon vs proton doses. Absolute improvement in hippocampal doses after replanning was determined for both photon and proton cohorts.

Results:

103 pts were included (42 photon receiving mean 57 Gy; 61 proton receiving mean 56 Gy). At least one hippocampus was contoured in treated plans in 3/52 (6%) vs 25/51 (49%) from 2013-2019 vs 2020-2025, and 8/42 (19%) vs 20/61 (33%) for photons vs protons. 86 pts (38 photon, 48 proton) with bilateral hippocampi contoured were eligible for dosimetric analysis. Pts receiving photons were older (45 vs 40, p=0.045) and trended toward larger CTV (187 cc vs 157 cc, p=0.16). Hippocampal doses were significantly higher treated with photons vs protons (Dmean: 20 Gy vs 10 Gy, p<0.01; V10Gy: 61% vs 25%, p<0.01; V30Gy: 24% vs 16%, p<0.01) but trend did not meet significance after replanning (Dmean: 13 Gy vs 10 Gy, p=0.24; V10Gy: 34% vs 23%, p=0.11; V30Gy: 15% vs 16%, p=0.97). Doses to surrounding organs will be included in the final report. Degree of potential improvement in hippocampal doses with each modality is shown in the Table.

Conclusion:

Treated hippocampal doses were significantly higher in pts receiving photons vs protons, supporting dosimetric advantages of protons for neurocognitive protection. Lower hippocampal doses in the proton cohort were likely due to inherent physical advantages, given lower impact of proton replanning. However, the difference between photons and protons was significantly mitigated by photon replanning (albeit at unclear cost to surrounding organs), supporting consideration of hippocampal delineation in pts with favorable glioma.

	Photons (n=38)			Protons (n=48)
	Treated	Replanned	Diff	Treated	Replanned	Diff
Mean	19.7 Gy	13.1 Gy	6.6 Gy	10.4 Gy	9.8 Gy	0.6 Gy
V10Gy	61.1%	33.8%	27.3%	24.9%	23.0%	1.9%
V30Gy	24.0%	15.4%	8.6%	16.3%	15.6%	0.7%