Room: Exhibit Hall
Purpose: To investigate single isocenter multiple target SRS planning with dynamic conformal arcs using simple off-axis fields and complex patient plans. The dose contribution from MLC leakage was considered.
Methods: Point measurements and calculations were performed on a water phantom for 5cm off-axis rectangular fields ranging in size from 0.6cm to 10cm utilizing and SFD diode and pencil beam algorithm. The field geometry used was 100cm SAD and 7cm depth. MLC leakage was measured both with standard closed fields and with clinical plans. Patient plans were created for four patients and up to five targets. PTV volumes, maximum treated distance from isocenter, CI, and brain dose were recorded. Leakage from clinical plans was measured with a farmer chamber in solid water. Measurements were compared to TPS calculations in the phantom averaged over several points where the Farmer chamber would be located. Patient plans were also projected onto ArcCheck with planned table angles moved to zero degrees in the planning system. Plans were measured on ArcCheck. Measurements were compared to plan calculations with two different MLC leakage values.
Results: SFD measurements showed a 3% agreement for off axis points with treatment plans for all field sizes. ArcCheck measurements showed better agreement with TPS calculations with MLC leakage value of 1.4% as opposed to 1.1% which is measured with standard geometry. QA passing rates with the 1.1% leakage parameter were as low as 61.5% as opposed to 96.6% passing rate with the 1.4% leakage parameter.
Conclusion: The Brainlab Elements TPS was deemed ready for clinical implementation. A 1.4% MLC leakage parameter was used in the TPS and validated by Brainlab. Patient treatment plans accomplished good conformity and adequate sparing of normal structures.