Evaluation of PTV margins and plan robustness for single isocentre multiple target stereotactic radiosurgery

Robustness to residual setup errors and linac delivery errors of BrainLab Elements single-isocentre-multiple-target stereotactic radiosurgery was evaluated.

Residual setup errors of 13 patients were evaluated. Linac delivery error was quantified through multi-metastases-Winston-Lutz measurements. PTV margins were calculated using the van Herk recipe. Patient scans were translated and rotated by the median and 95th percentile of the combined uncertainties, and plans were recalculated subsequently. Previous patients' plans were then replanned with the derived margins, effects on GTV coverage and normal brain doses were assessed.

Mean (±stdev) coverage of all targets in the original plans were 99.4% (±0.9%) and 98.9% (±1.0%) for 1 and 3-fraction patients respectively. Median geometrical errors did not result in significant differences. A statistically significant reduction in coverage to 91.4% (±10.4%) and 93.0% (±9.6%) was seen under 95th percentile errors. Applying the derived optimal margin of 0.5 mm resulted in 78% of the GTVs retaining a coverage of 98% or above even in the presence of 95th percentile errors, compared to only 30% if no margins were applied. Replanning with margins also caused no significant increase to local normal brain doses, however global dose increases varied according to the number of metastases.

Plans were shown to be robust to average geometrical uncertainties despite targets having no margins, however occurrence of GTV under-coverage increased under 95th percentile scenarios. The margin was proven to substantially improve the target dose coverage with limited change to local normal brain doses, although not all sources of geometrical uncertainty were considered.