An experimental investigation of the tongue and groove effect for the Philips multileaf collimator

The tongue and groove effect is an underdosing effect which can occur in certain applications of multileaf collimators. It results from the need to overlap adjacent leaves of a multileaf collimator in order to limit leakage between leaves. The applications in which the effect can occur are the abutment of fields where the beam edges are defined by the leaf edge and the production of intensity-modulated fields by dynamic collimation. The effect has been measured for the 'worst case' when just two MLC fields are matched along leaf edges which have overlapping steps. Measurements of the dose have been made at d(max) and also at a more clinically relevant depth of 87 mm in Perspex for beam energies of 6 MV, 8 MV and 20 MV on two Philips SL series accelerators. Dose distributions were recorded on radiographic film which was subsequently digitized for analysis. The dose reduction of the tongue and groove effect was found to be 15-28% and spread over a width of 3.8 to 4.2 mm. This is somewhat shallower and wider than would be expected from a simple, idealized model of the effect which would predict a dose reduction of 80% over a width of 1 mm.