Impact of multileaf collimator position errors on simultaneous integrated boost intensity-modulated radiotherapy for nasopharyngeal carcinoma

Objective To investigate the impact of multileaf collimator (MLC) position errors on simultaneous integrated boost intensity-modulated radiotherapy (SIB-IMRT) for patients with nasopharyngeal carcinoma. Methods Totally 10 patients with locally advanced nasopharyngeal carcinoma treated with SIB-IMRT were enrolled in this study. By modifying the plan files, different MLC position errors were introduced into the clinical plans. The differences of dosimetries were compared between the original and simulated plans. Results The dosimetric impact of the random and system shift errors of MLC position was insignificant within 2 mm (P>0.05), the maximum changes in D95% of PGTV, PTV1 and PTV2 were (-0.92±0.51)%, (1.00±0.24)% and (0.62±0.17)%, the maximum changes in the D0.1cc of spinal cord and brainstem were (1.90±2.80)% and (-1.78±1.42)%, the maximum changes in the Dmean of left and right parotids were (1.36±1.23)% and (-2.25±2.04)%. For 2 mm leaf extension errors, the average changes in D95% of PGTV, PTV1 and PTV2 were increased significantly(t=8.97, 10.97, 9.74, 7.30, 6.04, 3.04, P<0.05), the averaged value of the D0.1cc to spinal cord and brainstem were increased significantly (t=6.16, 9.22, P<0.05), the averaged value of the mean dose to left and right parotid were increased significantly (t=7.12, 4.25, P<0.05), respectively. Conclusions No significant difference was found for random and shift MLC leaf position errors up to 2 mm for nasopharyngeal carcinoma IMRT patients, respectively. There is a high sensitivity to dose distribution for MLC extension or contraction errors. The quality control of systematic MLC position errors should be paid attention in order to ensure the therapeutic accuracy. Key words: Intensity modulated radiotherapy; Nasopharyngeal carcinoma; Multi-leaf collimator; Position errors