Abstract
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 normal‐lung 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.PACS number: 87.53.‐j
Highlights
Zygmanski et al.: Determination of depth and field size dependence of...the optimization objectives
Measurements were made in air with and without cones to show that multileaf collimator (MLC) scatter is responsible for the field-size dependence of RcMLC
Only about 21% of the field size dependence of MLC transmission is attributable to scatter from the MLC
Summary
Zygmanski et al.: Determination of depth and field size dependence of...the optimization objectives (multiple targets with different dose levels, up to a dozen organs at risk). Zygmanski et al.: Determination of depth and field size dependence of. The result may be highly complex and noisy fluence patterns with low monitor unit (MU) efficiency and small average multileaf collimator (MLC) gap. The MU efficiency of dynamically delivered IMRT fields can be defined as a ratio of the MUs needed to deliver an open beam and IMRT for the same field size, depth, and average dose per field. The MU efficiency for an open beam would be 100%. 20% efficiency corresponds to about 81% of the delivered radiation being attenuated and only about 19% being transmitted through MLC gaps. Low MU efficiency of this kind implies an increased contribution of MLC transmission (mid-leaf, interleaf, and tongue-and-groove), MLC-induced beam hardening, and MLC scatter to the total dose
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