AbstractID: 7461 Title: Multileaf Collimator (MLC) for Field Matching

ID: 7461 Title: Multileaf Collimator (MLC) for Field Matching Photon field matching at the beam isocenter using independent jaws is the method of choice in many radiation treatments that require abutted fields. This study evaluated the suitability of using Multileaf Collimator (MLC) for such matching. The study employed a treatment planning system to model the field matching and the modeling was verified using x-ray film dosimetry. In this study, it was found that, for a double focused MLC system the MLC profile characteristics, especially the characteristics at the penumbra regions, were essentially the same as that of independent jaws. As such double focused MLC beam matching closely resembled that of independent jaws. In addition, it was found that the dose change varied linearly with the gap overlap (from 3 mm gap to 3 mm overlap) with a coefficient of 12 ± 2 %/mm. Using an auto field-sequencing machine operation the junction could be offset once or twice by ± 1 cm within an irradiation. Such intra-fraction ‘feathering’ of junction resulted in a reduction of the slope of the curve (from 12 ± 2 %/mm to 6 ± 2 %/mm when the junction was moved once and to 4 ± 2 %/mm when moved twice). A major difference between independent and MLC jaw field matching was that the former resulted in a uniform match line but the latter a line of underdoses/overdoses depending on the formation of gaps/overlaps by individual leaf pairs. Thus the intra-fraction ‘feathering’ would be the key to a successful application of MLC for isocentric field matching.