Previously in a Monte Carlo (MC) study an increase in MLC leakage with increase in field size was reported by Kim et al. In dynamic IMRT treatment large numbers of monitor units are used due to which the MLC leakage can exceed above 10% of the maximum in field dose. The contribution of MLC leakage to a point in an IMRT field can be calculated by the static field leakage multiplied by the product of the number of monitor units delivered for the IMRT field and the fraction of time the point is blocked by the MLC. In these blocked segments significant portion of dose can be delivered due to radiation leakage from MLC. To treat the desire section of a treatment field rest portion is blocked by MLC in an IMRT treatment. The dose distributions for a given complex intensity pattern are very sensitive to detailed structure of MLC. Introduction The planning aspects of intensity modulated radiation therapy (IMRT) treatment delivery rely on the use of multileaf collimators (MLC) to produce desired intensity pattern. These results suggest that use of MLC to define treatment field increases surface dose.ฤก. Clear increments in surface dose and electron fluence spectra for MLC define fields have been observed. In our study, we have also calculated surface dose and electron fluence spectra for MLC and jaw define fields. Photon spectra and photon average energy distributions are found to be substantially modified by MLC as it removes lower-energy photons resulting in increase of PDDs for MLC blocked fields in comparison to the jaw define open fields. A significant increase in MLC leakage with increase in field size has been observed in our study. We also calculated the effect of MLC on percentage depth dose characteristics, photon spectra and photon average energy distributions. We have used Monte Carlo simulation model to calculate radiation leakage as a function of field size for Varian 120-leaf MLC by accurate modelling of the complex geometry of MLC. #Purpose of multileaf collimator in radiotherapy code#In this study, the BEAMnrc Monte Carlo code is used to investigate the characteristics of radiation transported through multileaf collimators (MLC) for 6 MV photon beam produced by Varian linear accelerator.
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