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The Elekta Agility MLC is designed with rounded end tip leaves. This MLC design allows to control the penumbra for different field sizes despite the linear MLC motion that does not adapt to the variation of the beam’s divergence. One of the peculiarities of this system is that, in order to avoid possible collisions, the leaves cannot be completely closed and therefore the radiation transmission through the radiation axis is very high. This transmission can be reduced by closing the leaves off axis because the divergent beam passes through a wider portion of the leaf, reducing therefore the transmission. The treatment system planning (TPS) models this effect by physically changing the width of the gap between leaves as the gap moves off axis. The aim of this work is to verify the correct modelling of the radiation transmission through the gap in the Monaco TPS as a function of the distance to the radiation axis.

The radiation transmission through the closed leaf gap was measured in the radiation axis and  5, 10 cm  off axis  in a VersaHD accelerator equipped with an Agility MLC and energies of 6MV and 10MV. Measurements were made with a PTW micro-diomond chamber at DFS=100cm, depth =1. 5 cm (6MV) and depth=2. 5 cm (10MV) . Measurements were normalized to a 10x10 cmxcm field delivered in the same conditions. The results were compared to the calculations made in the TPS Monaco v. 5. 51 with a 2 mm resolution calculation matrix, MonteCarlo algorithm and an uncertainty of 0.5% per calculation.

In the accelerator acceptance, a closed leaf gap of 4.5 mm was physically measured  at the isocenter (1.4 mm at the leaves plane). The radiation transmission measured for 6MV-10MV energies with respect to the 10x10 field was 54-56% on the radiation axis, 41-44% at 5 cm off axis and 7% at 10 cm off the axis. The differences between the measurements and the TPS Monaco for 6MV-10MV energies were 13%, 15-16% and 8-9% at 0, 5 and 10 cm off axis respectively. Technical service was requested to close the gap to 1 mm at the plane of the leaves (3.5 mm at the isocenter), as this is the value recommended by Elekta. Once the gap was closed, the transmission measured at the same positions, for 6MV-10MV energies, was 42-45%, 28% and 3%. The differences to the TPS were 1-2%, 3-1% and 12%.Table 1 and Figure 1 summarize the results obtained.

The TPS Monaco models successfully the reduction in the radiation transmission when the closed gap moves off  axis, provided the MLC Agility gap measures physically 1 mm at the leaves plane. The existence of this minimun gap between adjacent leaves makes it necessary to close the gap off axis in order to reduce the transmission radiation through it. During the accelerator commissioning, the size of this gap should be checked to ensure minimum transmission. In the case of dynamic treatments in which the leaves are closed within the field, the correct modelling of this parameter is particularly important.