Guidelines For Safe Prescription Doses For Lgk Srs Of Multiple Brain Lesions Keywords: Dose, gamma knife, dose planning, Dose Prescription, brain metastasis ## Interactive ManuscriptAsk Questions of this Manuscript: Abstract Although RTOG study 90-05 has reported maximum safe doses for single-fraction stereotactic radiosurgery (SRS) in patients with recurrent, previously-irradiated single lesions, there are few clinical data regarding safe prescription doses for SRS of multiple brain metastases. The purpose of this work is to suggest a method of prescribing SRS doses for multiple brain lesions based on the RTOG criteria for single lesions, the total target volume and target/dose conformity. We retrospectively applied it to 20 patients (number of lesions ranging from 1 to 41) who received SRS at our institution using LGK PFX. To infer safe dose criteria from the RTOG data, we calculated normal-brain dose volume histograms (DVHs) for spherical volumes having the maximum diameter in each RTOG category (i.e., 2, 3 and 4 cm). We observed that the three DVHs crossed at a dose of about 15-Gy corresponding to a normal-brain volume of about 5 cc. This implies that regardless of the tumor size, no more than approximately 5 cc of normal brain should receive in excess of 15 Gy. We used this criterion, along with a dose-volume relationship for peripheral isodose volumes relative to the prescription isodose volume, to develop a formula to calculate safe SRS prescription doses. To evaluate if the formula accurately predicted prescription doses that resulted in 15-Gy normal-brain volumes < 5 cc. Using doses calculated with the formula, the average 15-Gy normal-brain volume for the 20 patients was 5.3 ± 0.5 cc, indicating that the formula works reasonably well. Variables in the formula include the total target volume, SVT, and the total prescription-isodose volume, SV0. Letting C = SV0/SVT, where C is a measure of the overall conformity of the treatment plan, we wrote the formula in terms of SVT and C. We then derived a family of curves relating safe prescription doses to the total target volume for different values of C. This is a retrospective study. The proposed formula and family of curves could be used as a guideline in prescribing doses for LGK SRS of multiple brain metastases. More follow-up data are needed to confirm or refine this model. Acknowledgements Project Roles: |

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