Dose calculation for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations.

A dose calculation algorithm has been developed for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations. First, an in-air fluence distribution is constructed based on the dynamic motion of the jaws or leaves, taking into account the variation of output with field size defined by the jaws. The fluence distribution is then convolved with the appropriate pencil beam kernel to give correction factors which are used to calculate the dose distribution for an intensity-modulated photon field. The proposed algorithm is strictly valid in homogeneous media only, patient heterogeneity correction is accounted for in an approximate manner. Dose distributions at several depths and for several field sizes were calculated for 6- and 15-MV x-ray beams for a set of standard wedges produced by dynamic jaws. Measurements were made with film and an ion chamber. Comparisons between calculated and measured data show good agreement (within 2%) for both dose profiles and wedge factors. Similar calculations and measurements were also made for a 25-MV intensity-modulated photon field produced by dynamic motion of a multileaf collimator. Agreement between calculations and measurements is also good (within 3%). The "tongue-and-groove" effect associated with a multileaf collimator design is also examined using a ring-shaped field produced by matching two component fields. The computation time for a dynamic-collimated field is the same as that for an irregular field shaped by conventional blocks. The algorithm is applicable to any pattern of jaw or multileaf motions. The strengths and remaining problems of the algorithm are discussed.