Design considerations for the sides of multileaf collimator leaves

The use of multileaf collimators (MLCs) in radiation therapy is becoming increasingly widespread as more commercial vendors offer them as options on medical linear accelerators. In most of the commercially available multileaf collimators, the sides of the leaves have a 'tongue and groove' (TG) feature, which is necessary for blocking the straight through leakage when the leaves are staggered together. The dosimetric effects of the TG design have not been fully studied for applications in block replacement or in dynamic intensity modulation. This article analytically explains the TG effect by calculating a deficit in photon flux when the side of a leaf is exposed. It is shown that the maximum photon deficit occurs when the leaf thickness is shared equally by the tongue and groove of adjacent leaves. New designs are proposed based on the theoretical analysis and improvements of the new design in leakage and the TG effect are verified with Monte-Carlo simulations. Measurements of the TG effect on an MLC from one manufacturer are also conducted. The results of the measurements confirm the analytical explanations of the TG effect.