Abstract
The aim of the present study was to evaluate the effect of various specific dosimetric leaf gaps on the multileaf collimator (MLC)–based small‐beam dose distribution. The dosimetric static leaf gap was determined by comparing the profiles of small MLC‐based beams with those of small collimated fields (square fields of 1, 2, 3, and 4cm). The results showed that an approximately 2‐mm gap was optimal with the Millennium 120‐leaf MLC (Varian Medical Systems, Palo Alto, CA) and a Varian 21EX 6‐MV photon beam. We also investigated how much the leaf gap affects the planning results and the actual dose distribution. A doughnut‐shaped planning target volume (PTV, 6.1 cm3) and inner organ at risk (OAR, 0.3 cm3) were delineated for delicate intensity‐modulated radiosurgery test planning. The applied leaf gaps were 0, 1, and 2 mm. The measured dose distributions were compared with the dose distribution in the treatment planning system. The maximum dose differences at inside PTV, outside PTV, and inner OAR were, respectively, 22.3%, 20.2%, and 35.2% for the 0‐mm leaf gap; 17.8%, 22.8%, and 30.8% for the 1‐mm leaf gap; and 5.5%, 8.5%, and 6.3% for the 2‐mm leaf gap. In a human head phantom (model 605: CIRS, Norfolk, VA) study, large dose differences of 1.3% – 12.7% were noted for the measurements made using the MLC files generated by the three different leaf gaps. The planned results were similar, and measurements showed a large dose difference associated with the various leaf gaps. These results strongly suggest that plans generated by a commercial inverse planning system commissioned using general collimated field data will probably demonstrate discrepancies between the planned treatments and the measured results.PACS number: 87.53.Dq
Highlights
55 Lee et al.: Effects of static dosimetric leaf gap...because the small beamlets used in multileaf collimator (MLC)–based intensity-modulated fields present complex interactions such as rounded leaf-end effect,(1) tongue-and-groove effect,(2) and lateral disequilibrium,(3) the field of radiation therapy physics requires that considerable attention be paid to planning and dosimetry
The penumbra width might change as a function of the distance of the leaf end from the field midline.[4]. The dose fluence generated from the MLC-based small beam is highly sensitive to inaccuracies in the MLC calibration,(5) and an undesired dose distribution to the planning target volume (PTV) and the organ at risk (OAR) might result
The final dose calculation can contain errors if it is based on commissioning measurements that use a standard ion chamber.[1]. We addressed that problem by comparing the dose distribution from the Eclipse planning system with the direct measurements made using various static dosimetric leaf gaps
Summary
Because the small beamlets used in multileaf collimator (MLC)–based intensity-modulated fields present complex interactions such as rounded leaf-end effect,(1) tongue-and-groove effect,(2) and lateral disequilibrium,(3) the field of radiation therapy physics requires that considerable attention be paid to planning and dosimetry. The IMRT commissioning process requires specification of a dosimetric leaf gap, which affects the leaf motion calculation (LMC) based on the optimized results[1] and on basic beam data from the jawcollimated fields, not MLC fields. The penumbra width might change as a function of the distance of the leaf end from the field midline.[4] The dose fluence generated from the MLC-based small beam is highly sensitive to inaccuracies in the MLC calibration,(5) and an undesired dose distribution to the planning target volume (PTV) and the organ at risk (OAR) might result
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