Impact of the MLC leaf-tip model in a commercial TPS: Dose calculation limitations and IROC-H phantom failures

Purpose Treatment planning system (TPS) dose calculation is sensitive to multileaf collimator (MLC) modeling, especially when treating with intensity-modulated radiation therapy (IMRT) or VMAT. This study investigates the dosimetric impact of the MLC leaf-tip model in a commercial TPS (RayStation v.6.1). The detectability of modeling errors was assessed through both measurements with an anthropomorphic head-and-neck phantom and patient-specific IMRT QA using a 3D diode array. Methods and Materials An Agility MLC (Elekta Inc.) was commissioned in RayStation. Nine IMRT and VMAT plans were optimized to treat the head-and-neck phantom from the Imaging and Radiation Oncology Core Houston branch (IROC-H). Dose distributions for each plan were re-calculated on 27 beam models, varying leaf-tip width (2.0, 4.5, and 6.5 mm) and leaf-tip offset (-2.0 to +2.0 mm) values. Doses were compared to phantom TLD measurements. Patient-specific IMRT QA was performed, and receiver-operating characteristic (ROC) analysis was performed to determine the detectability of modeling errors. Results Dose calculations were very sensitive to leaf-tip offset values. Offsets of +/- 1.0 mm resulted in dose differences up to 10% and 15% in the PTV and spinal cord TLDs respectively. Offsets of +/- 2.0 mm caused dose deviations up to 50% in the spinal cord TLD. Patient-specific IMRT QA could not reliably detect these deviations, with an ROC area under the curve (AUC) value of 0.537 for a +/- 1.0 mm change in leaf-tip offset, corresponding to >7% dose deviation. Leaf-tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord TLDs respectively. Conclusions Small changes in the MLC leaf-tip offset in this TPS model can cause large changes in the calculated dose for IMRT and VMAT plans that are difficult to identify through either dose curves or standard patient-specific IMRT QA. These results may, in part, explain the reported high failure rate of IROC-H phantom tests.