SU-E-T-590: Procedure for Verification and Inter-Comparison of IMRT Beam Models

The implementation of an accurate beam model is an integral part of the commissioning of any planning system. This process is especially challenging in the case of IMRT beam models owing to the complexity of small field sizes and MLC leaf-end and tongue-and-groove effects. The question of how to judge the quality of an IMRT beam model in comparison with other versions of the same model is central to this work.

We make an important distinction between evaluation of the beam model and evaluation of the optimization routine that is a part of any IMRT planning system. The H-shaped target used in this work has several important features: it can only be covered by segments with small field size, for which all leaf design effects are important, and it has the overall dimensions of a common IMRT target. The procedure for inter-comparison of two IMRT beam models (old and new) involves the generation of two plans optimized with each beam model using identical IMRT prescriptions. Both plans are subsequently delivered on a solid water phantom with film located in two parallel planes with a small-volume ionization chamber inserted in the center.

Four dose calculations are performed, such that each plan is calculated with either of the two beam models. The four dose distributions are subsequently compared with the two film measurements using gamma analysis. In addition, the absolute dose measured in the center of the dose distribution is compared with the calculated value. A score is assigned to each beam model based on the results.

Using the procedure outlined in this presentation, different versions of an IMRT beam model can be compared and scored for quality. Adoption of a unified strategy for beam model inter-comparison can greatly facilitate the evaluation and commissioning of IMRT beam models.