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Development of Monte Carlo Dose Calculation System for a New Dual Layer Multi-Leaf Collimator

Y Ishihara1*, M Nakamura2 , T Mizowaki2 , M Hiraoka1 , (1) Japanese Red Cross Wakayama Medical Center, Wakayama, Wakayama, (2) Kyoto University, Graduate School of Medicine, Kyoto


(Sunday, 7/29/2018) 3:00 PM - 3:30 PM

Room: Exhibit Hall | Forum 7

Purpose: Recently, an innovative dual layer MLC system with 6MV flattening filter-free (FFF) X-ray has been announced. The purposes of this study were to propose specific Monte Carlo (MC) models of the new dual layer MLC and to validate their dose characteristics.

Methods: 6MV FFF X-ray delivered by accelerator was implemented by EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc. Subsequently, the X-ray head composed of a target, a primary collimator, and a new dual layer MLC was simulated based on the specification. The dual layer MLC consisted of proximal MLC (upper bank, 29 leaf pairs) and distal MLC (Lower bank, 28 leaf pairs and 2 pair of outboard leaves). Leaf thickness was 1.0 cm at isocenter. Proximal and distal banks were offset by 0.5 cm.Next, the central axis depth doses and the lateral doses in the water at 13, 100, and 200 mm depth were simulated under SSD of 900 mm. Then, each of them was compared with the corresponding representative beam data supplied by Varian. Finally, Tongue-and-Groove, leaf leakage, and round leaf effects or proximal and distal MLC were simulated.

Results: For the 6 MV FFF X-ray, depth doses beyond the buildup region and lateral doses within the region of flatness showed agreement of within 2.3%. Tongue-and-Groove effect with proximal or distal MLC caused 11.2% dose reduction between each leaves, but, there was no Tongue-and-Groove dose reduction using the both proximal and distal MLC. Leaf leakage was about 0.6% with closing proximal or distal MLC. Then, in the case of closing proximal and distal MLC, leaf leakage was less than 0.01%.

Conclusion: We have demonstrated that the proposed MC models of the 6MV FFF X-ray have reasonable accuracy. We also simulated dose characteristic features using the new dual layer MLC.


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