A comparison of the dosimetric effects of systematic MLC leaf position errors on flattening filter and flattening filter-free IMRT for nasopharyngeal carcinoma

Objective: To compare the dosimetric effects of systematic MLC leaf position errors on flattening filter (FF) and flattening filter-free (FFF) IMRT for nasopharyngeal carcinoma (NPC). Methods: In totaly of ten patients with NPC were enrolled in the current study. FF IMRT and FFF IMRT plans were designed for each patient, and in-house software was developed to modify the corresponding MLC files. Briefly, three kinds of systematic MLC error presented with 0.5 and 1 mm magnitudes error were simulated. The modified MLC files were re-imported into IMRT plans for dose recalculation, and differences in the dosimetry trends between FF-IMRT and FFF-IMRT plans were analyzed using a dose-volume histogram. Results: Upon closed MLC, the average changes in D_95% of PTV and D_mean of parotid glands in FFF-IMRT plans were more sensitive than in FF-IMRT plans(t = 3.298 - 4.793, P < 0.05). Otherwise, when MLC are shifted in the same direction, changes in D_95% of PTV, D_mean of PGTV, PTV as well as D_mean of left parotid gland in FFF-IMRT plans were also more sensitive (t = 2.372 - 6.844, P < 0.05), whereas average changes in D_mean of right parotid gland presented with less sensitivity (t = -4.049, -3.378, P < 0.05). Conclusions: For out-of-field or large organs, FFF-IMRT plans are more sensitive to leaf position error than FF-IMRT plans. (authors)