Evaluation of dosimetric effect of leaf position in radiation field of an 80 leaf Multileaf Collimator fitted to the LINAC head as tertiary collimator.

Abstract

This study is designed to evaluate any change in dosimetric characteristics of Varian Millennium 80 leaf MLC in radiation field. In this study, measurements were done for dose rates, scatter factors, percentage depth dose, relative surface dose and dose in the buildup region, beam profiles, flatness & symmetry, and penumbra width, for 6 MV and 15 MV photon beams. Analysis of width between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement, penumbra widths - shows significant difference for the fields defined by MLC and/or jaws & MLC (zero gap) than that of the fields defined by jaws only. The scatter factors have also been influenced significantly due to MLC leaf position in the radiation field. A new relationship has therefore, been established between the scatter factors and MLC positions which will indeed be useful in the dose calculation for irregular fields. Penumbra widths increase with field size, and were higher for fields defined by jaws and/or MLC than jaws & MLC (zero gap) by 1.5 ? 4.2 mm & 3.8 ? 5.0 mm, for 6 MV, and 1.5 ? 2.4 mm & 3.0 ? 5.6 mm, for 15 MV, at 20% ?80% & 10% ? 90% levels, respectively. The relative surface dose and dose in buildup region were smaller for fields defined by jaws & MLC (zero gap) than that defined by jaws and/or MLC. No significant difference were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness & symmetry for both energies. The results of this study suggest that while collecting beam data of linear accelerator with MLC, the effects of leaf positions plays an important role on dosimetric characteristics of 3D-CRT as well as IMRT.