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Hippocampal Dosimetry and Implication for Treatment Planning in Patients Undergoing Stereotactic Radiosurgery for
N S Iqbal1, M C Williams2, M G J Williams2
1School of Medicine, Cardiff University, Cardiff, United Kingdom; Velindre University NHS Trust, Cardiff, United Kingdom.
Aims:
Stereotactic radiosurgery (SRS) is recommended for patients with limited brain metastases, offering survival benefits while minimising neurocognitive function (NCF) decline compared to whole brain radiotherapy. However, despite its precision, SRS can still cause hippocampal radiation exposure, with the potential to adversely impact NCF.
Materials And Methods:
This study retrospectively examined hippocampal dosimetry in 30 patients with 1-3 brain metastases treated using linear accelerator-based SRS using a dynamic conformal arc (DCA) technique. We then conducted a planning study in 10 patients who received the highest hippocampal doses to assess the feasibility of hippocampal-sparing SRS planning. Two hippocampal sparing SRS techniques were evaluated and compared to the standard technique, namely hippocampal-sparing DCA and hippocampal-sparing volumetric modulated arc therapy (VMAT).
Results:
Retrospective hippocampal dosimetric analysis revealed inter-individual variation in hippocampal dose received: 25% of patients received >5 Gy and 50% received >2 Gy to 0.1 cc of the closest hippocampus. Proximity of planning target volume (PTV) to the hippocampus, brainstem, and optic chiasm, as well as PTV volume and metastasis location in the temporal lobe and cerebellum, significantly influenced hippocampal dose. The number of metastases did not correlate with increased hippocampal exposure. Both hippocampal sparing techniques significantly reduced hippocampal dose without compromising PTV coverage or organ-at-risk (OAR) constraints. Hippocampal-sparing DCA achieved the lowest doses to the hippocampus, while VMAT plans delivered slightly higher low-dose volumes (eg, V1 Gy) to the brain.
Conclusion:
This study demonstrates that SRS can result in significant hippocampal irradiation, particularly for metastases within 2 cm of the hippocampus. Delineating the hippocampus during planning enables meaningful dose reductions without affecting plan quality. These findings support incorporating hippocampal delineation into standard SRS planning for patients with limited brain metastases. Further prospective studies are needed to establish clinical dose constraints and correlate hippocampal dose with NCF outcomes to refine radiotherapy strategies and reduce cognitive toxicity.
