A Volumetric Analysis of Stereotactic Radiosurgery Low Dose Volumes in Preventing Future Brain Metastases

Abstract

Stereotactic radiosurgery (SRS) is now commonly used to treat brain metastases. A primary limitation, compared to whole brain radiotherapy (WBRT) is its inability to prevent development of brain metastases in untreated areas. The purpose of this study is to assess whether low-dose volumes delivered to uninvolved regions of the brain during standard SRS can reduce the risk of developing brain metastases in those regions. Clinical data were collected for 69 patients with brain metastases who were treated with SRS on two or more occasions. Patients who had WBRT prior to or in between rounds of SRS were not included. Tumor volume and dosimetric data were collected from the first round of SRS. The regions of uninvolved brain receiving a high, intermediate, and low dose of incidental radiotherapy were identified. These doses were defined as the absolute volume receiving at least 10, 5, and 2.5 Gy if the prescribed dose was < 25 Gy (1-3 fraction plans) or the absolute volume receiving at least 15, 7.5, and 5 Gy if the prescribed dose was ≥ 25 Gy (5 fraction plans). Tumor volume and location in reference to these three dose levels were collected for metastases that were treated in the second round of SRS. A second-round metastasis was considered to occur within a given dose level if 20% or more of the volume of the tumor was found within that dose level. Progression of treated metastases were not included. Volumetric probabilities were calculated based on the volume of each dose level as a percentage of total brain volume. The sum of these probabilities across all cases was used to calculate the expected number of cases with at least one metastasis occurring in each dose level. P-values were calculated using the chi-square test statistic. Of the 69 patients included in this study, the primary cancer was non-small cell lung cancer for 35%, breast cancer for 32%, and melanoma for 16%. The average number of metastases treated in both the first and second rounds of SRS was two. The mean total clinical target volume (the sum of individual clinical target volumes) was 7.5 cc in the first round and 3.0 cc in the second round. The expected and observed number of cases with at least one second round metastasis occurring in high, intermediate, and low dose levels are displayed in the table below. We observed fewer than expected new metastases within prior SRS low dose levels based on volumetric probabilities, though this difference was only significant for the intermediate dose level. This suggests that low dose volumes from SRS may provide benefit in preventing future regional metastases. Further research is warranted to assess the effects of low dose volumes on future metastases based on cancer type and region of the brain.Abstract 3618; TableNumber of cases with new metastases within prior dose levelsExpectedObservedP-valueHigh dose level200.151Intermediate dose level730.018Low dose level17110.094 Open table in a new tab