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An analytical dose calculation algorithm was developed and commissioned to calculate dose delivered with both static and dynamic multileaf collimator (MLC) in a homogenous phantom. The algorithm is general; however, it was designed specifically to accurately model dose for large and complex IMRT fields. For such fields the delivered dose may have a considerable contribution from MLC transmission, which is dependent upon spatial considerations. Specifically, the algorithm models different MLC effects, such as interleaf transmission, the tongue-and-groove effect, rounded leaf ends, MLC scatter, beam hardening and divergence of the beam, which results in a gradual MLC transmission fall-off with increasing off-axis distance. The calculated dose distributions were compared to measured dose using different methods (film, ionization chamber array, single ionization chamber), and the differences among the treatment planning system, the measurements and the developed algorithm were analysed for static MLC and dynamic IMRT fields. It was found that the calculated dose from the developed algorithm agrees very well with the measurements (mostly within 1.5%) and that a constant value for MLC transmission is insufficient to accurately predict dose for large targets and complex IMRT plans with many monitor units.

Intensity-modulated radiation therapy (IMRT) plans for the treatment of large and complex volumes may contain a relatively large contribution from multileaf collimator (MLC) transmission. In such cases, comprehensive characterization of direct and scatter MLC transmission is important. We designed a set of tests (open beam, closed static MLC, and dynamic MLC gap) to determine dosimetric MLC properties as a function of field size and depth at the central axis. We developed a generalized model of MLC transmission to account for direct MLC transmission, MLC scatter, beam hardening, and leaf-end transmission (dosimetric gap). The model is consistent with the beam model used in IMRT optimization. We tested the model for extreme asymmetric fields relevant for large targets and for split IMRT fields. We applied our MLC scatter estimation formula to clinically relevant cases and showed that MLC scatter is contributing an undesired background dose. This contribution is relatively large, especially in low-dose regions. (For instance, a uniform extra dose may dramatically increase normallung toxicity in thorax treatment.) For complex IMRT of large-volume targets, we found direct MLC transmission dose to be as high as 30%, and MLC scatter, up to 10% within the target volume for the selected cases. We identified that the dose discrepancies between the IMRT planning system [Eclipse (Varian Medical Systems, Palo Alto, CA)] and ionization chamber measurements (inside and outside of the field) are attributable to an inadequate model of MLC transmission in the planning system (constant-value model). In the present study, we measured MLC transmission properties for Varian 6EX (6 MV) and 21EXs (6 and 10 MV) linear accelerators; however, the experimental method and theoretical model are more general.

In complex intensity-modulated radiation therapy cases, a considerable amount of the total dose may be delivered through closed leaves. In such cases an accurate knowledge of spatial characteristics of multileaf collimator (MLC) transmission is crucial, especially for the treatment of large targets with split fields. Measurements with an ionization chamber, radiographic films (EDR2, EBT) and EPID are taken to characterize all relevant effects related to MLC transmission for various field sizes and depths. Here we present a phenomenological model to describe MLC transmission, whereby the main focus is the off-axis decrease of transmission for symmetric and asymmetric fields as well as on effects due to the tongue and groove design of the leaves, such as interleaf transmission and the tongue and groove effect. Data obtained with the four different methods are presented, and the utility of each measurement method to determine the necessary model parameters is discussed. With the developed model, it is possible to predict the relevant MLC effects at any point in the phantom for arbitrary jaw settings and depths.