Suitability of using multileaf collimator (MLC) for photon field matching.

This study evaluated and presented the results on the suitability of using a Siemens double-focused multileaf collimator (MLC) for photon field matching. The study was conducted using film-based dosimetry for measurements and a Theraplan Plus (version 3.8) treatment planning system (TPS) for planning comparisons and additional field matching investigations. In this study, it was found that the double-focused MLC field matching closely resembled that of the independent jaws, with the exception that the MLC had a nonuniform match line-a--match line formed by a different size leaf pair gap--overlap at a different join-up position. In addition, it was found that the dose change varied linearly with the MLC leaf pair gap-overlap (from 3-mm gap to 3-mm overlap) by 11 +/- 1%/mm. Using intrafraction "feathering" of the junction, a reduction of the dose change from 11 +/- 1%/mm to 6.5 +/- 1%/mm occurred when the junction was moved once and from 11 +/- 1%/mm to 4 +/- 1%/mm when moved twice. Based on the results, the study suggested that MLC positioning error tolerances of +/-2 mm might be too large for photon field matching. On the other hand, a combination of reducing the tolerance to +/-1 mm and intrafraction junction moves (twice by +/-1 cm off matching axis) could reduce the maximum peak height--trough low at the junction to within a required range of -5 to +7% in planning target volume (PTV) dose heterogeneity.