Purpose: Finding your way to an excellent cranial stereotactic plan can be easy if one knows how to use a “Map” and “Compass”. The Map tells you how best to get to your destination (great plan). The Compass tells you how to orient the map, what direction to go. In stereotactic treatment the Compass is represented by R50%Analytic, the theoretical minimum value of R50% (Volume of 50% isodose cloud / Volume of PTV). The Map is represented by the AFI Strategy optimization, which gives you an efficient path to a treatment plan with intermediate dose spill very close to R50%Analytic.
Methods: This work combines concepts of previously published works in a direct exposition of this Map and Compass philosophy for cranial SRS/SRT. We apply the Map and Compass to a broad sample of cases: a single PTV, a single PTV abutting the brainstem, and single isocenter multiple target cases with 5, 9 and 10 PTVs. All plans were done in Eclipse v 15.6. and validated for deliverability on a Varian Trubeam with 6xFFF beams with a Millennium MLC by direct measurement with an ArcCheck detector and passed a minimum criteria of 3%/2mm with >95% pass rate.
Results: R50%, CI-RTOG, CI-Paddick, and Organ at Risk doses are reported for all plans. The CI-RTOG values for all PTVs are greater than 1.0, and CI-Paddick values are between 0.68 and 0.94 indicating that all PTVs are well covered by the prescription dose. The R50% values achieved are on average within 1.02 ± 0.89 of the R50%Analytic. All R50% values are within recently proposed universal R50% guidelines for each PTV.
Conclusion: The Map (AFI Strategy optimization) and Compass (R50%Analytic) provides a useful technique for SRS/SRT planning and provides excellent plans while minimizing optimization struggles. This technique can be scripted or built into autoplanning solutions.