P03.09.B LOW DOSE BATH FROM RADIOSURGERY TO SURROUNDING NORMAL BRAIN IN TREATMENT OF BRAIN METASTASES COULD STERILIZE BRAIN AND INHIBIT RECURRENCES

Abstract

Abstract BACKGROUND Stereotactic radiosurgery and fractionated stereotactic radiotherapy (SRS/fSRT) are standard treatments for brain metastases. Each SRS/fSRT plan has a low-dose penumbra outside the high dose area, which multiplies if multiple metastases are irradiated and results in a low-dose bath in healthy brain tissue. We hypothesize that a certain dose of the low-dose bath can sterilize brain tissue and prevent local recurrences. We present first data of an ongoing project. MATERIAL AND METHODS Data of fifty-three patients with brain metastases treated with single fraction SRS or fractionated fSRT at our institute between 2020 and 2022 were reviewed retrospectively. All patients with at least one cranial follow-up MRI after 3 months were evaluated. Follow-up MRI scans were fused to the original treatment plans. New metastases in the isodose-volumes of 10 Gy-, 8 Gy-, 6 Gy-, 4 Gy-, and 2 - Gy were identified and evaluated. Mean doses, on the site of the new lesions, received from SRS/fSRT were tabulated summarized and analyzed. RESULTS Six of the 53 patients met the inclusion criteria. Five patients developed recurrent brain metastases out-of-field within 2-4 months of follow-up. One metastasis was observed outside the irradiation field. One patient developed no new metastases. With decreasing isodose levels an increasing number of new metastases were observed: In the reconstructed volume of 10Gy (V10Gy), V8Gy and V6Gy two metastases each, no metastases in the V4Gy and 5 metastases in the V2Gy. No toxicities were observed. CONCLUSION Our data suggest a trend towards a benefit that the low-dose bath as part of the penumbra of stereotactic radiotherapy sterilizes brain tissue. Especially, brain regions receiving doses of 4 Gy or more had fewer new lesions compared to regions that received less than 4 Gy. This should be further validated prospectively in larger patient populations, in clinical trials with combination of CNS-accessible immunotherapy like Osimertinib. Indications for whole brain radiotherapy could be further reduced. Low dose spillage with manageable risks might be sufficient to treat microscopic brain metastases in the future.