Impacts of MLCerror on the Different Beam Area of Volumetric Arc Therapy for Nasopharyngeal Carcinoma
Objective To analyze the influence between the mean aperture area(MAA)and the multi-leaf collimator error(MLCerror)for nasopharyngeal carcinoma volumetric modulated arc therapy.Methods 30 patients diagnosed with postoperative nasopharyngeal carcinoma were selected as the study objects.Every original plan for each patient was exported and compiled using Python.Six modified plans were generated by different MLCerror(±0.2 mm,±0.5 mm,±1.0 mm).The dosimetry parameters and passing rate of the original and modified plan were compared and the relationship among MLCerror,and MAA was analyzed.Among them,the planned target volume(PTV)of nasopharyngeal tumor and the target area of cervical metastatic lymph node tumor were integrated into PTV69 by Boolean operation.Results The corresponding variation of PTV69,planning clinical target volume 1 and planning clinical target volume 2 due to MLCerror(-1-1 mm)were 6.1%/mm,6.0%/mm and 6.3%/mm(P<0.05).The dose gradient of the organs at risk(OAR)was higher than that of the target area.The dose fall-off were 10.6%/mm,10.5%/mm,12.3%/mm,12.0%/mm,8.1%/mm and 11.2%/mm in spinal-cord,brainstem,parotid-L,parotid-R,oral and larynx(P<0.05).With the increase of MLCerror,the passing rate difference of different MAA plan showed an increasing trend.When the MLCerror reached±1.0 mm,the discrepancy of different MAA plan was largest(51.7%-90.2%).Conclusion It is more beneficial for limiting MLCerror value to protect OAR than target.For the complicated nasopharyngeal carcinoma,less than 0.5 mm MLCerror is suggested to be the operation tolerance and decreasing the the small segment is recommended to protect OAR and ensure the dose accuracy.
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