Guidelines For Safe Prescription Doses For Lgk Srs Of Multiple Brain Lesions

Paula Petti1, Sandeep Kunwar2, David Larson3

1Fremont, CA United States 2Washington Hospital Healthcare System, Fremont, CA, United States 3San Francisco, CA United States

Keywords: Dose, gamma knife, dose planning, Dose Prescription, brain metastasis

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     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.


Project Roles:

P. Petti (), S. Kunwar (), D. Larson ()