Two-dimensional dynamic multileaf collimator (2DDMLC) technique for improved treatment quality in intensity modulated radiation therapy.

This study aims to evaluate feasibility of the two-dimensional dynamic MLC (2DDMLC) technique through analytical and experimental investigations, focusing on its potential to improve intensity-modulated radiation therapy (IMRT). The leaf motion calculator (LMC), which calculates the leaf motion of the MLC during the treatment, was developed to obtain the MLC sequence including the bank's movement and anticipate the actual fluence delivery. The effect of the y-axis MLC motion was evaluated by calculating the actual fluence distributions from the optimal fluence maps. The actual fluence maps were compared to those of the conventional MLC. The conformity index (CI) and average difference from the optimal fluence map were calculated. Subsequently, prototype of the 2DDMLC was manufactured by using the Varian Millennium 120 MLC to measure dose distributions according to presence of MLC's y-axis movement. The dose distributions were compared to their optimal fluence maps according to different MLCs in terms of dose sparing to the volume outside the tumor. The beam fluences produced by the 2DDMLC technique showed better conformity to their optimal fluence distributions than the conventional MLC. The 2DDMLC technique enhanced the beam conformity compared to the conventional MLC, as indicated by a CI closer to 1, indicating better target conformity. In addition, difference between the actual and the optimal fluence maps was reduced by a factor of 2. The dose distributions were improved by the 2DDMLC as the irradiation area outside the target region was reduced by 50% in maximum. For example, the normal tissue irradiation area was reduced by 49% and 24% in the lung and head & neck cases, respectively. The 2DDMLC technique can contribute enhanced normal tissue sparing with less probability of side effects. As extension of this study, the suggested device can be employed to the volumetric modulation arc treatment.