Multileaf collimation versus alloy blocks: analysis of geometric accuracy

Purpose To compare the misalignment error due to the fabrication of custom lead alloy blocks with the displacement error introduced by the finite resolution of a multileaf collimator relative to the prescribed smooth apertures. Methods and materials Treatment field apertures for randomly selected patients for four clinical sites were obtained at various stages of the block fabrication process. These apertures and the corresponding multileaf collimator (MLC) apertures for each field were superimposed with the smooth apertures prescribed by physicians. The deviations from the prescribed apertures were measured at 10 degrees intervals. Comparisons of the magnitude and frequency of errors from block fabrication with those from the geometric displacements introduced by the finite leaf width of the multileaf collimator were made. Results The degree of conformity of the multileaf collimator is treatment-site dependent as, in general, are the shapes of fields. For three of the four sites examined, the multileaf collimator apertures track the prescribed apertures at least as accurately as custom blocking when the block design, construction, mounting, and alignment on the treatment machine are considered. Conclusions The geometric conformality of multileaf collimation is comparable to, and in some cases superior to, that of custom blocks.