How Does the Number of Brain Metastases Correlate With Normal Brain Exposure in Single-Isocenter Multitarget Multifraction Stereotactic Radiosurgery

Abstract

PurposeTo investigate the relationship between normal brain exposure in linac-based single-isocenter multi-target multi-fraction stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) and the number or volume of treated brain metastases, especially for high numbers of metastases. Methods and MaterialsA cohort of 44 SRT patients with 709 brain metastases was studied. Renormalizing to a uniform prescription of 27 Gy in 3 fractions, normal brain dose volume indices, including V23Gy (volume receiving >23Gy), V18Gy (volume receiving >18Gy), and mean dose, were evaluated on these plans against the number and the total volume of targets for each plan. To compare with exposures from whole-brain radiotherapy (WBRT), the SRT dose distributions were converted to equivalent dose in 3 Gy fractions (EQD3) using an alpha-beta ratio of 2 Gy. ResultsWith increasing number of targets and increasing total target volume, normal brain exposures to dose ≥18 Gy increases, and so does the mean normal brain dose. The factors of number of targets and total target volume are both significant, though the number of targets has a larger effect on the mean normal brain dose and the total target volume has a larger effect on V23Gy and V18Gy. The EQD3 mean normal brain dose with SRT planning is lower than conventional WBRT. On the other hand, SRT results in higher hot spot (i.e. maximum dose outside of tumor) EQD3 dose than WBRT. ConclusionsBased on clinical SRT plans, our study provides information on correlations between normal brain exposure and the number and total volume of targets. As SRT becomes more greatly utilized for patients with increasingly extensive brain metastases, more clinical data on outcomes and toxicities is necessary to better define the normal brain dose constraints for high-exposure cases and to optimize the SRT management for those patients.