25. CyberKnife SBRT using a new MLC: Advantages, limitations and new treatment opportunities

Purpose The CyberKnife (CK) M6 series is equipped with a MLC intended to achieve higher delivery efficiency than the circular IRIS variable collimator and to expand the feasibility of CK treatments to larger tumors. This study aimed to evaluate the CK-MLC performance for SBRT plans focusing on (i) comparison with IRIS (ii) large volume tumor treatments (iii) dosimetric delivery accuracy. Methods (i) IRIS and MLC plans in 3–5 fractions were created for 30 liver, 15 pancreas and 10 prostate cases and compared in terms of coverage, conformity (nCI), dose gradient (R50), homogeneity (HI), OAR doses, PTV gEUD, MU normalized to prescription dose (MU/PD), treatment time both estimated by TPS (tTPS) and measured (tmeas). Prostate plans MU/PD and tTPS were compared with similar published data [1] , [2] . (ii) SBRT MLC plans (30 Gy/5 fractions) were optimized for large PTVs (240 cm 3  ÷ 1140 cm 3 , diameters 8 cm ÷ 14 cm) in one pancreas and three liver cases. Plan quality, treatment time and feasibility to meet OAR constraints were evaluated. (iii) Delivery QA was performed for 50 MLC plans using different techniques. Results (i) Compared to IRIS ( Table 1 ), MLC achieved equivalent coverage, conformity, homogeneity and EUD without compromising OAR sparing and improving (p  Conclusions MLC improves dose fall-off and delivery efficiency compared to IRIS. Time reduction is partially lost when measured and is highly variable among different studies. MLC permits to extend CK SBRT treatments to larger PTVs maintaining high dosimetric accuracy.