FEASIBILITY OF HIPPOCAMPUS-SPARING VMAT IN LOCOREGIONALLY ADVANCED OROPHARYNGEAL CARCINOMA: A RETROSPECTIVE DOSIMETRIC EVALUATION

Abstract

Background: Radiotherapy generates incidental dose to non-target neural structures including the hippocampus, which is highly radiosensitive and linked to neurocognitive decline even at low doses. While hippocampal sparing is evolving in neuro-oncology, its feasibility in head-and-neck radiotherapy, particularly oropharyngeal carcinoma treated with volumetric-modulated arc therapy (VMAT), remains underexplored. Methods: This retrospective dosimetric study included 20 patients with locoregionally advanced oropharyngeal carcinoma (LA-OPC) treated with radical VMAT between 2021–2022. Bilateral hippocampi were retrospectively contoured per RTOG atlas. Original clinical plans (non-hippocampus-sparing) were compared to re-optimized hippocampus-sparing VMAT plans using identical arc geometry. Dose–volume histogram parameters were analyzed for hippocampi (Dmean, Dmax), planning target volume (PTV) coverage (D90, D95, D98), plan quality indices, and 32 additional organs-at-risk (OARs). Constraints followed the Radiation Therapy Oncology Group (RTOG) 0933 protocol (Dmax <16 Gy, D100 <9 Gy). Statistical comparisons were performed using paired tests, with p<0.05 considered significant. Results: Mean ipsilateral hippocampal dose reduced from 2.00 to 1.89 Gy (p=0.001) and maximum dose from 3.44 to 2.96 Gy (p=0.001). Bilateral hippocampal sparing met all constraint thresholds without compromising high-risk PTV coverage (p>0.05). Significant dose reductions were also observed in spinal cord (p=0.001), brainstem (p=0.046) and parotids (p<0.01), while conformity and homogeneity indices remained preserved. Intermediate and low-risk PTVs demonstrated improved dose uniformity (p≤0.041). No clinically relevant increase was seen in any other OAR. Conclusion: Hippocampus-sparing VMAT is dosimetrically feasible for LA-OPC, achieving significant neural dose reduction without detriment to target coverage or OAR safety. These findings support prospective evaluation of cognitive-preserving RT strategies in head-and-neck cancer survivors.