RADT-08. A VOLUMETRIC ANALYSIS OF STEREOTACTIC RADIOSURGERY LOW DOSE VOLUMES IN PREVENTING FUTURE BRAIN METASTASES

PURPOSE/OBJECTIVE(S) Stereotactic radiosurgery (SRS), compared to whole brain radiotherapy is limited in its ability 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 SRS can reduce the risk of developing brain metastases in those regions. MATERIALS AND METHODS Data were collected for 69 patients with brain metastases who were treated with SRS at least two occasions. The regions of uninvolved brain receiving a high, intermediate, and low dose of incidental radiotherapy were defined as the volume receiving at least 10, 5, and 2.5 Gy if the prescribed dose was < 25 Gy (1-3 fraction plans) or the volume receiving at least 15, 7.5, and 5 Gy if the prescribed dose was ≥ 25 Gy (5 fraction plans). A second round metastasis was considered to occur within a given dose level if 20% or more of the tumor was found within that dose level. Probabilities were calculated based on the volume of each dose level as a percentage of total brain volume and were used to estimate the expected number of cases with at least one metastasis occurring in each dose level. RESULTS The average number of metastases treated in both rounds of SRS was two. The expected and observed number of cases with at least one second round metastasis were 0 and 2 for the high dose level (p=0.151), 7 and 3 for the intermediate dose level (p=0.018), and 17 and 11 for the low dose level (p=0.094). CONCLUSION 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.