Dosimetric and Geometric Analysis for MLC Based and Iris Based Treatment Plans: A Retrospective Study for Intracranial Tumors with Cyberknife Device

Purpose In our institute, stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (SRT) were performed by Cyberknife-S7 (CK-S7) which provided the selection of different collimators. This study aimed to compare critical plan qualities including conformality, high-dose area, dose fallout, and treatment efficiency between InCise™ multileaf collimator (MLC) based and Iris™ variable aperture collimators (Iris) based treatment plans. Methods Twenty-five patients with intracranial tumors treated with CK-S7 were retrospectively analyzed. The Precision 3.3.0.0 with VOLO™ optimizer and GPU-accelerated Monte Carlo dose calculation algorithm was used for treatment planning. The new conformality index (nCI), homogeneity index (HI), high-dose ratio (HDR) and distribution inside plan target volume (PTV), dose gradient distance (DGD) outside PTV, organs at risk (OARs) sparing, and treatment efficiency were compared between MLC based and Iris based plans. Results MLC plan achieved higher nCI, higher HDRs from 135% prescription dose (PD) to 110%PD and trended to form more centralized and gathered high-dose distribution inside PTV, while no statistical difference was found in HI. Iris possessed better dose-engraving ability around the target boundary especially when it close to OARs with strict dose constraints. MLC plan showed shorter DGD from 90%PD to 20%PD. MLC plan achieved less MUs (−67.14%), less nodes (−41.5%), less beams (−74.06%) and shorter treatment time (−51.64%). There were positive correlations between the effective radius of PTV (rPTV) and DGD from 90%PD to 10%PD both in MLC and Iris plans. Conclusions MLC plan achieved comparable conformality, higher HDRs, more gathered high-dose distribution, faster dose fallout and more efficient treatment which proved it an excellent SRS/SRT choice for intracranial tumors treated with CK-S7. MLC might take an important advantage for the uncompliant and painful patients. However, Iris showed a better dose-engraving ability, it might be taken into consideration especially when the tumor was close to OARs with strict dose constraints.