Comparison of stereotactic plans for brain tumors with two different multileaf collimating systems

Linac-based stereotactic radiosurgery (SRS) has been widely used for treating small intracranial lesions. This technique allows conforming the dose distribution to the planning target volume (PTV), providing a steep dose gradient with the surrounding normal tissues. This is realized through dedicated collimation systems. The present study aims to compare SRS plans with two collimating systems: the beam modulator (BM) of the Elekta Synergy linac and the DirexGroup micromultileaf collimator (mu MLC). Seventeen patients (25 PTVs) were planned both with BM and mu MLC (mounted on an Elekta Precise linac) using the Odyssey (PerMedics) treatment planning system (TPS). Plans were compared in terms of dose-volume histograms (DVH), minimum dose to the PTV, conformity index (CI), and homogeneity index (HI), as defined by the TPS, and doses to relevant organs at risk (OAR). The mean difference between the mu MLC and the BM plans in minimum PTV dose was 5.7% +/- 4.2% in favor of the mu MLC plans. No statistically significant difference was found between the distributions of the CI values for the two planning modalities (p = 0.54), while the difference between the distributions of the HI values was statistically significant (p = 0.018). For both BM and mu MLC plans, no differences were observed in CI and HI, depending on lesion size and shape. The PTV homogeneity achieved by BM plans was 15.1% +/- 6.8% compared to 10.4% +/- 6.6% with mu MLC. Higher maximum and mean doses to OAR were observed in the BM plans; however, for both plans, dose constraints were respected. The comparison between the two collimating systems showed no substantial differences in terms of PTV coverage or OAR sparing. The improvements obtained by using mu MLC are relatively small, and both systems turned out to be adequate for SRS treatments.