Evaluation of dosimetric effect of leaf position in a radiation field of an 80 leaf multileaf collimator fitted to the LINAC head as tertiary collimator

Abstract

This study evaluates changes in the dosimetric characteristics of a Varian Millennium 80-leaf multileaf collimator (MLC) in a radiation field. In this study, dose rate, scatter factor, percentage depth dose, surface dose and dose in the buildup region, beam profile, flatness and symmetry, and penumbra width measurements were made for 6-MV and 15-MV photon beams. Analysis of widths between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement shows a significant difference between the fields defined by MLC and/or jaws and MLC (zero gap) and the fields defined by jaws only. The position of the MLC leaves in the radiation field also significantly affects scatter factors. A new relationship has, therefore, been established between the scatter factors and the position of the MLC, 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 and MLC (zero gap) by 1.5 mm to 4.2 mm and 3.8 mm to 5.0 mm, for 6-MV, and 1.5 mm to 2.4 mm and 3.0 mm to 5.6 mm, for 15-MV, at 20% to 80% and 10% to 90% levels, respectively. The surface dose and the dose in the buildup region were smaller for fields defined by jaws and MLC (zero gap) than the fields defined by jaws and/or MLC for both photon energies. No significant differences were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness and symmetry for both energies. The results of this study suggest that while one collects linear accelerator beam data with a MLC, the effects of the positions of the MLC leaves play an important role in dosimetric characteristics of 3D conformal radiation therapy as well as intensity-modulated radiotherapy.

Figure 1. Dose rates (DR) for the 6‐MV photon beam (a) in air and (b) in water for field‐defining methods described in section B.1(1), (2), and (3)

Figure 2. Dose rates (DR) for the 15‐MV photon beam (a) in air and (b) in water for field sizes defined by jaw only, MLC only, and jaw and MLC (matching)

Figure 3. Plot between scatter factors Sj, Sm, and Sjm and field size for (a) the 6‐MV photon beam and (b) the 15‐MV photon beam for air dosimetry. Curves for the product of Sj and Sm, and Sjm for (c) 6‐MV photon beams and (d) 15‐MV photon beams

Figure 4. Curves between scatter factors in water and field size for (a) 6‐MV photon beams and (b) 15‐MV photon beams. (c) Phantom scatter factors for 6‐MV photon beams and (d) phantom scatter factors for 15‐MV photon beams determined using scatter factors for field sizes defined by jaw only, MLC only, and jaw and MLC (matching) in air and in water.

Figure 5. Plots of percentage depth dose (PDD) versus depth for 4×4 cm2, 10×10 cm2, and 20×20 cm2 field sizes defined by jaw only, MLC only, and jaw and MLC (matching) for (a) 6‐MV photon beam and (b) 15‐MV photon beam

Figure 6. Plot for penumbra width versus field size (a) between 20% and 80% dose levels at dmax, (b) between 10% and 90% dose levels at dmax, (c) between 20% and 80% dose levels at 10 cm, and (d) between 10% and 90% dose levels at 10 cm for the 6‐MV photon beam

Figure 7. Curves shown for plots of penumbra width versus field size (a) between 20% and 80% dose levels at dmax, (b) between 10% and 90% dose levels at dmax, (c) between 20% and 80% dose levels at 10 cm, and (d) between 10% and 90%