Evaluation of dosimetric effect of leaf position in a radiation field of an 80‐leaf multileaf collimator fitted to the LINAC head as a 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.PACS number: 87.53.Dq

Dosimetric effects of jaw tracking in step‐and‐shoot intensity‐modulated radiation therapy

“…Comparison of jaw, MLC, and combined fields for various dosimetric parameters have also been explored. ( 14 , 15 ) All these studies were performed using Varian linear accelerators.…”

Design of a modulated orthovoltage stereotactic radiosurgery system

“…As examples, CyberKnife (Accuracy Incorporated, Sunnyvale, CA, USA) equipment specifications define flatness using a 40 mm secondary collimator . Kehwar et al . and Huq et al .…”

“…As examples, CyberKnife (Accuracy Incorporated, Sunnyvale, CA, USA) equipment specifications define flatness using a 40 mm secondary collimator. 65,66 Kehwar et al 67 and Huq et al 68 examined flatness for linear accelerator beam profiles down to (4 9 4) cm 2 in size. Similarly, Azimi et al 69 measured flatness, also down to a (4 9 4) cm 2 square field size, for an orthovoltage irradiator.…”

Investigation of Dose Falloff for Intact Brain Metastases and Surgical Cavities Using Hypofractionated Volumetric Modulated Arc Radiotherapy

“…4,24 The parameters R50 and GI were statistically different between cavities and intact metastases, but the curves of best fit in Figure 2 exhibit similar power law coefficients, indicating that this difference between intact metastases and cavity treatment plans is due to the difference in overall TV. The fact that the R50 increases slowly with TV is logical as beam penumbra slowly increases with field size, 25 and field size varies in proportion to TV. Based on an increasing R50, one can simulate/predict a decrease in GI as a function of PTV with reasonable agreement in power law exponent coefficients: À0.22 for simulation versus À0.18 and À0.14 for actual data in cavities and intact, respectively.…”