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.

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.

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.

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.