Abstract
Purpose. An analytical and experimental study of split shape dose calculation correction by adjusting the position of the on-axis round leaf end position is presented. We use on-axis corrected results to predict off-axis penumbra region dosimetric performance in an intensity-modulated radiation therapy treatment planning system. Materials and Methods. The precise light-field edge position (X tang.p) was derived from the on-axis 50% dose position created by using the nominal light field for geometric and mathematical manipulation. Leaf position (X mlc.p) could be derived from X tang.p by defining in the treatment planning system for monitor unit calculation. On-axis offset (correction) could be obtained from the position corresponding to 50% of the central axis dose minus the Xmlc.p position. The off-axis 50% dose position can then be derived from the on-axis 50% dose position. Results. The monitor unit calculation of the split shape using the on-axis rounded leaf end MLC penumbra region could provide an under-or overdose of 7.5% per millimeter without an offset correction. When using the on-axis rounded leaf end offset correction to predict the off-axis dose, the difference between the off- and on-axis 50% dose position is within ±1.5 mm. Conclusions. It is possible to achieve a dose calculation within 0.5% error for an adjusted MLC leaf edge location in the treatment planning system with careful measurement and an accurate on-axis offset correction. Dose calculations located at an off-axis spilt shape region should be used carefully due to noncorrectable errors which were found to be up to 10%.
Highlights
Multileaf collimator (MLC) systems are available on most commercial linear accelerators, for intensity-modulated radiation therapy (IMRT) treatment techniques, and many of these MLC systems utilize designs with rounded leaf ends to improve the dose profile of the geometric and transmission penumbra
The projection of the nominal light-field at source-to-axis distance (SAD) 100 cm was adopted for dose profile measurements, but the dose profile from the nominal light-field edge could not quantitatively determine the geometry of the tangential edge (Xtang,p) for the derivation of Xmlc,p; the precise light-field edge (Xtang,p) was derived from the point corresponding to 50% of the central axis dose by geometrical and mathematical methods using (1) in this study
The precise leaf edge position of the tangential split field (Xtang,p) could be derived using the measured on-axis 50% dose position from the mathematical model and can be used to obtain the planning system defined by leaf position (Xmlc,p)
Summary
Multileaf collimator (MLC) systems are available on most commercial linear accelerators, for intensity-modulated radiation therapy (IMRT) treatment techniques, and many of these MLC systems utilize designs with rounded leaf ends to improve the dose profile of the geometric and transmission penumbra. The general designs of rounded leaf end MLC systems have already been described in detail by many researchers [1,2,3,4,5,6,7,8] These MLC design considerations result in differences between the MLC 50% isodose points and the projected light-field edge locations. Radiation field size is defined as the lateral distance between the 50% isodose lines at a reference depth. This definition is practically achieved [10] by a procedure called beam alignment. The field-defining light is made to coincide with the 50%
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