A Light-Field-Based Method to Adjust On-Axis Rounded Leaf End MLC Position to Predict Off-Axis MLC Penumbra Region Dosimetric Performance in a Radiation Therapy Planning System

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 X(mlc.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%.