Purpose/Objectives: The purpose of this study was to evaluate the effect of the number of brain lesions for which stereotactic radiosurgery (SRS) was performed on the dose volume relationships in normal brain. Materials and Methods: Brain tissue was segmented using the patient's pre-SRS MRI. For each plan, the following data points were recorded: total brain volume, number of lesions treated, volume of brain receiving 8 Gy (V8), V10, V12, and V15. Results: A total of 225 Gamma Knife® treatments were included in this retrospective analysis. The number of lesions treated ranged from 1 to 29. The isodose for prescription ranged from 40 to 95% (mean 55%). The mean prescription dose to tumor edge was 18 Gy. The mean coverage, selectivity, conformity, and gradient index were 97.5%, 0.63, 0.56, and 3.5, respectively. The mean V12 was 9.5 cm³ (ranging from 0.5 to 59.29). There was no correlation between the number of lesions and brain V8, V12, V10, or V15. There was a direct and statistically significant relationship between the brain volume treated (V8, V10, V12, and V15) and total volume of tumors treated (p < 0.001). In our study, the integral dose to the brain exceeded 3 J when the total tumor volume exceeded 25 cm³. Conclusions: The number of metastatic brain lesions treated bears no significant relationship to total brain tissue volume treated when using SRS. The fact that the integral dose to the brain exceeded 3 J when the total tumor volume exceeded 25 cm³ is useful for establishing guidelines. Although standard practice has favored using whole brain radiation therapy in patients with more than 4 lesions, a significant amount of normal brain tissue may be spared by treating these patients with SRS. SRS should be carefully considered in patients with multiple brain lesions, with the emphasis on total brain volume involved rather than the number of lesions to be treated.

Purpose The study was designed to evaluate the cumulative dose exposure of brain and hippocampus following multiple stereotactic radiosurgery (SRS) procedures on a single patient. Materials and methods Ninety-four patients who received at least two SRS treatments were evaluated. Their SRS dose plans were summated to obtain dose volume parameters. Hippocampal structures were delineated, in addition to segmentation of the brain. Results A total of 267 treatments were delivered to 94 patients. The number of SRS procedures per patient varied from 2 to 7 (mean = 2.8), and the total number of lesions treated in a given patient over multiple treatments varied from 2 to 70 (mean = 11). The interval between the first and last SRS treatments varied from 1 to 79 months (mean = 16.2 months). The mean prescription dose was 16.7 Gy prescribed to isodose lines from 40 to 95%. The mean coverage, selectivity, Paddick conformity, and gradient index were 99.6%, 79%, 0.57, and 2.74, respectively. The mean summated dose delivered to the whole brain over the course of multiple treatments was 5.11 Gy. The hippocampal mean dose after summation was 2.95 Gy. Patients with plan PCIs averaging below the mean value of 0.57 had higher V8, V10, V12, and V15 values for a given total tumor volume when compared with more conformal plans (PCI > 0.57).Conclusions Repeat SRS delivered low cumulative doses to the brain and hippocampus with the lowest normal brain volumes irradiated in highly conformal plans.

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