A retrospective review of clinical outcome and hippocampus dosimetry in patients treated with stereotactic radiosurgery for brain metastases in South Wales

Stereotactic radiosurgery(SRS) has demonstrated improved survival, quality of life and neurocognitive function(NCF) for patients with 1–3 brain metastases compared to whole brain radiotherapy. Despite its precision, a significant proportion of patients experience neurocognitive decline: 63.5% of patients undergoing SRS alone had neurocognitive decline[1]. The hippocampus has been implicated in NCF impairment following radiation however, the tolerance dose of hippocampus remains unclear for SRS. In a study of fractionated radiotherapy, a dose of more than 7.3Gy delivered to >40% of the bilateral hippocampus is associated with significantly worse NCF[2]. In animal studies, doses as low as 2Gy has demonstrated increased cell apoptosis in hippocampus[3]. We performed a retrospective review of all patients treated with SRS without WBRT over 1year at Velindre Cancer Centre. Patients with 1–3 brain metastases and WHO performance status 0–2 were included. We studied dose delivered to hippocampi in our patient population. Bilateral hippocampi were outlined manually according to the RTOG0933 atlas[4] and dose volume histograms were recreated using BrainLab iPlan RT Dose4.5. We identified 30 patients between January 2015 and January 2016. Mean age was 61. The most common primary sites were lung, kidney and melanoma. 19 patients had a single metastasis. Median overall survival was 7.5months. Dose to 0.1cc of the hippocampus was >5Gy in 8 and 2–4.9Gy in 12 patients. Dose to 50% of the hippocampus was >5Gy in 6 patients and 2–4.9Gy in 8 patients. Objective neurocognitive assessment was not attempted in this study due to the challenges of collecting such data retrospectively. We have identified a considerable proportion of patients receiving significant radiation to the hippocampus. Overall survival of patients is in line with previously published studies. We will now conduct a prospective study in patients undergoing SRS to correlate detailed radiation dosimetry, NCF and functional MRI measurements of organs at risk