Beam characteristics of a retrofitted double-focused multileaf collimator

Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.