An evaluation of Pencil Beam vs Monte Carlo calculations for intracranial stereotactic radiosurgery

Purpose: To compare the accuracy of two separate models when calculating dose distributions in patients undergoing stereotactic radiosurgery (SRS) treatment for brain cancer. Methods: For this comparison, two dose calculation algorithms were evaluated on two different treatment planning systems (TPS): Elekta’s Monaco Version 5.11.00 Monte Carlo Gold Standard XVMC algorithm and Brainlab’s iPlan Pencil Beam algorithm. The DICOM files of 11 patients with a total of 19 targets were exported from iPlan and then imported into Monaco to be recalculated. Using the dose distributions of the original (pencil beam/PB) and recalculated (Monte Carlo/MC) plans, four indices for plan quality were evaluated: coverage (Q), conformity index (CI_RTOG), homogeneity index (HI), and gradient index (GI). Results: There was a significant difference in the CI_RTOG and HI between the two TPS calculations. However, the magnitude of these differences is often not substantial enough to cause the plan to fall outside of RTOG protocol deviation limits. Only 3 of the 19 targets had CI_RTOG values which moved to a new level of deviation, and these targets were unique in terms of size (<0.1 cm³). Conclusion: It was found that the difference between systems is often not enough to cause the plan to fall outside of RTOG protocol deviation limits. This is an indication that a PB-based treatment planning system is sufficient for the mostly homogeneous conditions of intracranial SRS planning when the target is larger than 0.1 cm³. If below 0.1 cm³, the prescribing physician may need to evaluate TPS differences.