Leakage reduction for the Siemens ModuLeaf

The Moduleaf, an add-on miniature multileaf collimator (MMLC) for the Siemens Oncor linear accelerator, provides high resolution field shaping with a maximum inter-leaf leakage dose of 1.50% at 6 MV. However, beyond the maximum treatment field size, the distribution of leakage and scatter along the y axis is different from that of the x axis, with maximum leakage values of 1.53% and 0.39%, respectively. Such differences can not be modeled in the Pinnacle treatment planning system. Also, within the 10 cm x 12 cm treatment region, leakage from the crack between closed leaf ends was 3.76%. To resolve these issues, gaps in the Moduleaf frame were filled with lead sheets, and the Siemens MLC was operated in MLC mode, rather than bank mode, so that a rectangle of 10.4 cm x 11 cm was formed with the MLC leaves closed behind the Y jaws, whose opening was 10.4 cm. This significantly reduced the difference between the leakage patterns in the x and y directions, with maximum leakage doses of 0.43% outside the treatment region and 1.67% near the crack between abutting Moduleaf leaves. The modification also reduced the mean square error between Pinnacle profiles and measured profiles in the tail region.