Is Dosimetric Effect of Leaf Width of MLC Clinically Significant in IMRT

Abstract

It is generally believed that the quality of intensity modulated radiation therapy (IMRT) treatment plans with finer multi-leaf collimator (MLC) leaf resolution is better. We tested this hypothesis by analyzing treatment plans optimized using Philips Pinnacle3 treatment planning system (TPS) for multiple patients with tumor sites located in Head and Neck (H&N), Prostate and Spine. For each patient, two IMRT plans are created using the same objective functions, optimization algorithm (Direct Machine Parameter Optimization), and other radiation therapy planning parameters: one with 0.5 cm leaf width and the other with 1.0 cm leaf width. The beam modeling of the two virtual machines in the TPS was exactly the same except for the MLC leaf width. The dose volume histogram (DVH) is used in the planning process. A criterion of 95% of the target volume receiving the prescribed dose was satisfied for all plans. Dose verification using a diode array (Mapcheck, Sun Nuclear) was also made to quantify the dosimetric accuracy of each plan in terms of average pass rate of total dose points using various evaluation criteria. The analysis of DVHs revealed clinically minor dosimetric differences between 0.5 cm and 1 cm width MLCs. Dosimetric indices, i.e., Planed Target Volume (PTV) coverage, conformity and inhomogeneity, presented simple quantitative measures of the plans width different MLC leaf width. Almost identical target coverage and similar dose distributions among the critical structures were observed in both H&N, prostate and spine patients. Interestingly, it was found that the dosimetric accuracy of plans generated with 1.0 cm leaf width MLC was superior than those with 0.5 cm leaf width.