Purpose: The Gamma Knife (GK) platform was traditionally considered to be the only safe way to treat cases of multiple brain metastasis with stereotactic radiosurgery (SRS), particularly those with numerous (> 10) targets. We herein report the advancement of treatment planning that enables similarly safe treatment on a linear accelerator based platform. In this work, we provide a case study showcasing the feasibility of fast, linac-based SRS to a similarly large number of brain metastases at a safe background dose. As there are currently no operational GK units in the VA, these data facilitate the treatment of cases in the system wherein it was not previously possible.
Methods: A patient with 15 small metastases was treated on our GK Model C with 18Gy prescribed to each target. The patient was replanned for RapidArc SRS on a TrueBeam STx with a high-definition multileaf collimator, according to the University of Alabama at Birmingham volumetric modulated arc therapy (VMAT) method with an identical prescription. Plan quality metrics for conformity (RTOG CI), gradient (AUC-DVH), moderate isodose spill (V12Gy to healthy brain), and low isodose spill (V4.5Gy to healthy brain) were calculated and compared with those of the delivered GK plan.
Results: Replan of the GK treatment with RapidArc on TrueBeam STx linear accelerator resulted in a clinically equivalent plan. The RapidArc plan had improved conformity (CIGK = 2.48, CIVMAT = 1.46) with higher, but still safe and inconsequential background dose to healthy brain (V4.5GK = 80.7 cc, V4.5VMAT = 228.4 cc). Moderate isodose spill was essentially equivalent (V12GyGK = 8.4 cc, V12GyVMAT = 13.0 cc). In accordance with its improved conformity, high isodose spill to healthy tissue was slightly lower with RapidArc (V18GyGK = 3.29cc, V18GyVMAT = 0.96cc).
Conclusions: In this work, we demonstrated the feasibility of safely treating numerous brain metastases with RapidArc radiosurgery on the TrueBeam STx linear accelerator. Because such treatments can be delivered very quickly (< 20 min) and with high accuracy, some providers may find the technique a desirable surrogate to whole brain radiation therapy or GK SRS.
