S887
Clinical - Mixed sites & palliation
ESTRO 2026
Keywords: Lattice Radiotherapy, Lattice, LRT
Distance between PTV and the ipsilateral hippocampus was <1cm in patients who received >5Gy to D0.1cc and between 1-2 cm for patients who received 2 - 4.9Gy. 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) guidance. Hippocampal-sparing DCA achieved the lowest doses to the hippocampus, while VMAT plans delivered slightly higher low-dose volumes (e.g., V1Gy) 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 guidance and correlate hippocampal dose with NCF outcomes to refine radiotherapy strategies and reduce neurocognitive side effects.
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Hippocampal dosimetry and implication for treatment planning in patients undergoing stereotactic radiosurgery for limited brain metastases. Sahar Iqbal 1 , Maeve WiIliams 2 , Matthew Williams 2 , Ceri Doherty 2 , Andrew Bryant 2 , Michael Chu 2 , Jillian Maclean 1 , Tony Millin 2 , Nachi Palaniappan 1 , John Staffurth 1,3 , James R Powell 1,3 1 Clinical Oncology, Velindre University NHS Trust, Cardiff, United Kingdom. 2 Medical Physics, Velindre University NHS Trust, Cardiff, United Kingdom. 3 Medicine, Cardiff University, Cardiff, United Kingdom Purpose/Objective: Stereotactic radiosurgery (SRS) is an established treatment for patients with limited brain metastases, providing local control and survival benefits while reducing neurocognitive function (NCF) decline compared to whole brain radiotherapy. Despite its precision, SRS treatment alone can still result in NCF impairment, affecting up to 60% of patients[1]. Whilst mechanisms underlying this side effect are not fully understood, irradiation of the hippocampi - bilateral medial temporal lobe structures, important for memory formation and rich in mitotically active neural stem cells - has been identified as an important contributing factor[2,3]. This study aimed to identify factors influencing hippocampal dose and to evaluate two alternative hippocampal-sparing SRS techniques designed to reduce hippocampal dose in linac-based SRS for patients with limited brain metastases. Material/Methods: A retrospective dosimetric review was performed in 30 patients with 1-3 brain metastases treated using linear accelerator-based dynamic conformal arc (DCA) SRS. Hippocampal doses were analysed, and the 10 patients receiving the highest doses were selected for
a planning study to assess the feasibility of hippocampal-sparing SRS. Two alternative hippocampal-sparing techniques, namely
hippocampal-sparing DCA and hippocampal-sparing volumetric modulated arc therapy (VMAT), were evaluated and compared to the standard technique. Results: Retrospective hippocampal dosimetric analysis revealed inter-individual variation in hippocampal dose received: 25% of patients received >5Gy and 50% received >2Gy to 0.1 cc of the closest hippocampus.
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