Abstract

Maximilian Grohmann¹, Manuel Todorovic¹, Cordula Petersen¹

¹University Medical Center Hamburg- Eppendorf, Department of Radiotherapy and Radiation Oncology, Hamburg, Germany

In recent publications several popular treatment options for stereotactic radiosurgery (SRS) of multiple brain metastases were compared. Given the high degree of complexity of these cases, the inter-planner variability can vary to a considerable extent and affect the results more than the technique itself. In addition, parameters such as radiation device, dose grid resolution and evaluation platform can play an important role.

The purpose of this study is to compare the plan quality of two modern auto-planning approaches while minimizing possible misleading differences as much as possible.

The employed treatment planning solutions were Brainlab Elements - Multiple Brain Mets SRS - version 3 (MBM) and Varian - HyperArc - Eclipse 16.1 (HA).

The main difference between both systems is the treatment technique. HA utilizes RapidArc-VMAT and MBM depends on multi-aperture dynamic conformal arcs. Similarities include jaw tracking and using a single isocenter at the centroid of all targets.

Furthermore, beam data is the same and origins from a Varian TrueBeam STx LINAC.

Both systems can operate highly automated but were influenced by the same planner to ensure that prescription coverage was tolerable (98.5% ± 1%) and no SRS dose constraints (QUANTEC) were violated.

Ten cases with a total of 70 metastases (mean volume 1.1cc ± 0.9cc) were compared. The prescription of 20Gy was the same for all metastases.

The following table summarizes some of the most important SRS plan quality parameters. These include the Paddick conformity index (CI), the Paddick gradient index (GI), and volume dose metrics for the normal brain (whole brain minus targets). V5Gy is used to quantify the low dose spread and V12Gy is a well-established predictor for radio-necrosis.

To get a better overview of all cases, a DVH-comparison for both techniques is plotted.

Caption: DVH comparison for HA- (blue) and MBM-results (red) of all ten multiple metastases cases. In favor of better overview, only whole brain and targets are plotted.

We conclude that both solutions are able to generate acceptable SRS plan quality without significant planner interactions. Still, there are differences. HA achieves better conformity, with MBM providing the better gradient, which is also reflected in better V12. Due to the fact that the planning technique of MBM has fewer degrees of freedom than HA, this must be compensated for by the number of arcs and MUs, which is at the expense of the V5.