The dosimetric and delivery advantages of a new 160-leaf MLC

The purpose of this study was to conduct a measurement and treatment planning study on the dosimetric and delivery advantages of a new 160-leaf multileaf collimator (MLC). Recently, a new 160-leaf multileaf collimator (Siemens 160 MLC(TM)) was introduced. The 160-MLC is a single focused design that consists of 160-leafs (80 pairs), each 95 mm thick with a projected leaf width of 5 mm at the machine isocenter. Compared to its double focused predecessors, the 82-leaf MLC (Siemens OPTIVIEW((tm)) MLC) and 58-leaf MLC (Siemens 3-D MLC((tm))), the 160-MLC has leaf widths of half the size. The most notable difference is the new slanted leaf design that replaced the tongue and groove system and allows for complete interdigitation. A systematic study that compared the dosimetric and delivery differences among the 160-MLC, 58-MLC, and divergent Cerrobend blocks was performed. Dosimetric conformity for each collimator type was determined by conforming each to circular targets of various diameters. The effective penumbra for each collimator type was calculated by conforming each, at various collimator angles, to a square stationary target. The quality of 3D conformal radiotherapy treatment (3D-CRT) plans and the quality intensity modulated radiation treatment (IMRT) plans were respectively compared with each collimator type. The 160-MLC was found to have improved dosimetric conformity over the 58-MLC. The divergent Cerrobend block showed marginal dosimetric conformity improvement over the 160-LMC. Overall, the 160-MLC had a 45% and 29% reduction in the 20/80 and 30/90 effective penumbra over the 58-MLC, respectively, while exhibiting only a slightly larger effective penumbra over the divergent Cerrobend block. Comparing 3D-CRT plans generated for small lesions of the head and neck, the V100 for the PTV of the plans generated with the Cerrobend blocks, the 58-MLC, and the 160-MLC were 97.78%, 92.51%, and 99.18%, respectively, while with regards to the OARs, the three produced similar DVHs. IMRT plans generated with the 160-MLC were found to significantly reduce the total delivered monitor units by up to 14.7% and the number of segments by as much as 10.7% compared to the 58-MLC. The average delivery time for the direct aperture optimized (DAO) IMRT plans generated with the 160-MLC was approximately 5 minutes. Overall, compared to the 58-MLC, the new 160-MLC was found to improve dosimetric conformity and IMRT delivery efficiency.