Room: AAPM ePoster Library
Purpose: stereotactic radiosurgery (SRS) for multiple brain metastases is gaining popularity because of volumetric modulated arc therapy (VMAT). The purpose of this study is to investigate whether there is a volume limit at which Linac-based SRS may not be suitable.
Methods: patients with two or three brain metastases (30 targets total) previously treated with Gamma Knife (GK) were re-planned using either a single- or two-isocenter, multiple non-coplanar dynamic arcs using a 6MV flattening filter free beam and with high-definition multileaf collimator (HD-120 MLC). The prescription dose varies from 11-24 Gy based on tumor size. All plans had >99% of each tumor volume receiving the prescription dose with an average of 99.53% and 100% tumor coverage for VMAT and GK respectively. The median target volume was 1.68 cc (range: 0.09-32.52 cc), which is used as a cutoff value between small and large targets. Plan quality was evaluated based on RTOG conformity index (CI), Paddick gradient index (GI), V12Gy and V4.5Gy.
Results: median CI for VMAT plans for both small and large targets were better than that of GK plans (1.54 vs. 1.68, p<0.05 for small targets, and 1.29 vs. 1.51, p<0.05 for large targets). The median V12Gy from VMAT plans were lower than GK plans for large targets (9.74 cc vs. 12.40 cc, p<0.05), and similar for small targets (4.70 cc vs. 4.76 cc, p>0.05). There was no statistically significant difference in the median GI and the median V4.5Gy for both groups with slightly lower GI and V4.5Gy in GK plans for small targets and the reverse trend for large targets.
Conclusion: volumes of brain metastases greater than 1.68 cc should be suitable for Linac-based SRS with improved conformity and normal brain sparing while for volumes smaller than 1.68 cc, GK plans can be considered for improved low-dose normal brain sparing.
Funding Support, Disclosures, and Conflict of Interest: CWL is supported by AVO and JS and PX received a research grant from AVO.