Dosimetric sensitivity with MU and MLC errors in IMRT versus VMAT plan for nasopharyngeal carcinoma

Objective: To simulate the possible systematic delivery errors introduced by monitor units (MU) and multi-leaf collimator (MLC) in radiotherapy plans for nasopharyngeal carcinoma (NPC), and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) with these errors. Methods: Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 1.25%, 2.50%, and 5.00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (0.25 mm, 0.50 mm, 1.00 mm, 1.50 mm, and 2.00 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows: (1) the MLC banks moved in the same direction; (2) the MLC banks moved in opposing directions (expand or contract the MLC gaps). The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results: With the increase in MU errors, the doses to target and organs at risk (OARs) of IMRT and VMAT plans increased in a linear way,and met R² = 0.992-1.000(P < 0.05). For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were -0.26%/mm and -0.65%/mm in case of offset errors, 4.87%/mm and 8.68%/mm in case of expansion errors, and -6.04%/mm and -9.88%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. Conclusions: MU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy. (authors)