Impact of the Number of Metastatic Tumors Treated by Stereotactic Radiosurgery on the Dose to Normal Brain: Implications for Brain Protection

Abstract

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.

How many brain metastases can be treated with stereotactic radiosurgery before the radiation dose delivered to normal brain tissue rivals that associated with standard whole brain radiotherapy?

“…One recent analysis suggests acceptable safety and efficacy profiles associated with administration of SRS to ≤15 metastases 19 . Another retrospective study estimated the radiation dose delivered to normal brain as a function of the total tumor volume treated with SRS 20 . To complement these emerging data, we used a novel approach to estimate that treating 12–13 tumors per day over 10 days would deliver the dose of radiation to healthy brain tissue typically associated with a standard course of WBRT (3 Gy x 10 fractions).…”

A Comprehensive Evaluation of Single Isocenter Multiple Target SRS Plans and the Analytical Relationship between Plan Quality Indices with the Number and Volume of Targets

“…Studies by Xue et al, (2015) and Rivers et al, (2017) in Gamma Knife treatment found that the total PTV volume was the influencing factor of the normal brain mean dose, and the number of targets did not influence the same. We observed in Linac-based SIMT planning that the normal brain dose tends to increase with the number of targets.…”

A multidisciplinary management algorithm for brain metastases

“… 60 The threshold of >3 J was not dependent on the number of lesions treated, but was met when total tumor volume exceeded 25 cc. 61 Importantly, cognitive outcomes have not been rigorously evaluated in patients receiving SRS for >10 metastases, although this is being actively examined in multiple open phase III trials. 52 …”