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Impact of the MLC Leaf-Tip Model: TPS Dose Calculation Deficiencies and IROC-H Phantom Failures

B Koger*, R Price, D Wang, D Toomeh, S Geneser, E Ford, University of Washington, Seattle, WA


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: Radiotherapy treatment planning system (TPS) dose calculation is sensitive to MLC modeling, especially when treating with IMRT or VMAT. AAPM recommendations such as TG-119 and MPPG-5a suggest that patient-specific IMRT QA can be used to detect modelling errors. This study investigates the dosimetric impact of two MLC model parameters (leaf-tip width and leaf-tip offset) in a commercial TPS. In addition, the detectability of introduced errors and the relationship with IROC-H head-and-neck phantom failures is assessed.

Methods: An Agility MLC (Elekta Inc.) was commissioned clinically in RayStation v.6.1. Nine IMRT and VMAT plans were optimized to treat the IROC-H phantom. Dose distributions were re-calculated on 27 different beam models, varying the leaf-tip width (ranging from 2.0 mm to 6.5 mm) and leaf-tip offset (ranging from -2.0 mm to +2.0 mm), and doses were compared to IROC-H TLD and ArcCheck measurements.

Results: Leaf-tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord, respectively. Dose calculations were more sensitive to leaf-tip offset. Offsets of 1.0 mm caused differences up to 10% in the PTV and 15% in the spinal cord. Offsets of 2.0 mm caused dose deviations up to 50% in the spinal cord. These errors were not apparent during the modeling process, and IMRT QA was unable to reliably detect these dose deviations; ROC AUCs were 0.513 - 0.756 for detecting 10-20% dose differences.

Conclusion: Small errors in MLC model parameters can cause large changes in the calculated dose that may be unapparent both in dose curves and through standard IMRT QA methods. This may, in part, explain the high failure rate of IROC-H phantom tests reported by the community. It is recommended that external validation be performed as part of the commissioning process of IMRT treatment planning.


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