Model-based measurements of MLC leaf-offset and transmission factors for IMRT dose calculations

Absorbed dose calculations for IMRT require an incident photon intensity distribution. The leaf transmission factor, ε, and the effective leaf offset, δ, are required to accurately compute the incident fluence distribution from leaf motion data. Currently, ε and δ are independently measured. We present a method to obtain these, simultaneously. As opposed to a linear equation used by other investigators, we found that a non-linear equation best represents the relation between the delivered point dose and the leaf parameters, i.e., the width of leaf opening of the sliding window and the total travel length of the leaves in addition to ε and δ. We considered the delivery of uniform dose over a 10x10 cm2 field. The absorbed dose was measured with cylindrical ionization chambers at the depth of dose maximum in a phantom for 6 and 10 MV photon beams of Varian 21EX with a standard 80 leave MLC and 6 and 18 MV beams of Varian 2100C with a Millennium 120 leaf MLC. The values of ε and δ were estimated by applying a non-linear regression analysis method to the measured data. The measured transmission factors are 2.62, 2.70, 2.32, and 2.47 % for 6MVX (21EX), 10MVX, 6MVX (2100C), 18MVX, respectively. The effective leaf offsets are 0.91, 0.95, 0.76, and 0.83 mm, respectively. The non-linear equation fits the experimental data very well. This suggests the validity of our model. The measured data are being used with our in-house point dose calculation program for IMRT plan check. Bibliography Y. Watanabe, “Point dose calculations using an analytical pencil beam kernel for IMRT plan checking, ” Phys Med Biol 46 (4), 1031-8. (2001). T. LoSasso, C. S. Chui, and C. C. Ling, “Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy, ” Med Phys 25 (10), 1919