Determination of depth and field size dependence of multileaf collimator transmission in intensity‐modulated radiation therapy beams

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

Eight years of IMRT quality assurance with ionization chambers and film dosimetry: experience of the montpellier comprehensive cancer center

“…Many publications relate methods to determine the DLS parameters but the transmission factor is defined in a unique value [27]. The effect of this value certainly varies with the size of the primary jaws opening and with the amount of time a specific point is irradiated under the leaves [28], leading to different accuracy of measurements inside the same patient for the same energy between large fields and small fields [29]. The degree of complexity of the modulation also explains the different results for various tumor localizations.…”

Reducing the low-dose lung radiation for central lung tumors by restricting the IMRT beams and arc arrangement

“…Suggestions on how to include the dose generated by photons scattering off MLC leaves in pencil-beam algorithms have also been made. 13 We used a step-and-shot IMRT delivery in which we limited the minimum segment size to 1.5 cm for improved MU efficiency to minimize these dose errors for our delivered plans.…”

An EPID response calculation algorithm using spatial beam characteristics of primary, head scattered and MLC transmitted radiation

“…We also model direct and scattered MLC transmission, 27 as well as tongue and grove ͑T&G͒ and interleaf transmission, 26 which have been shown to be significant for planning complex IMRT for large tumor volumes. 27 We validate the method by comparing measurements, our independent prediction, and the prediction generated by the Eclipse Treatment Planning System ͑TPS͒. The presented method is applicable to the calibration of any planar detector with spatial resolution better than 1 mm ͑Ͻ1 mm͒.…”

“…The modeling of MLC transmission and dosimetric gap are affected by the introduction of the MLC scatter parameter. 27 The dosimetric gap accounts for the transmitted radiation through the rounded MLC leaf end by increasing the nominal distance between the tips of MLC leaves. While MLC transmission consists of photons that penetrate through the leaves without interactions, MLC scatter consists of photons that penetrate after interacting ͑scattering͒ with the MLC leaves.…”

“…Independent determination of these parameters, along with the MLC scatter coefficient, is possible. 27 The method consists of a series of ion chamber measurements for open beam, closed MLC, and sweeping gap irradiation. Performing the same measurements using the EPID, the three mentioned parameters are determined specifically for the EPID.…”