Study on dosimetric marginal effects in intensity-modulated radiotherapy for nasopharyngeal carcinoma

Abstract

Objective To analyze the dosimetric marginal effects of the physical parameters in the intensity-modulated radiotherapy (IMRT) planning for nasopharyngeal carcinoma (NPC):number of segments,segment area,and number of monitor units (MUs) per segment.Methods The IMRT planning for 17 patients with NPC who were hospitalized from November 2011 to March 2012 were redesigned and optimized by changing one of the physical parameters;a marginal effect function between the technical input of IMRT planning and dosimetric output was established from the aspect of marginal effect evaluation ; the dosimetric results of three optimization methods were statistically analyzed.Results When optimizing the number of segments,segment area,and number of MUs per segment,the mean conformity indices of the planning gross tumor volumes (PGTVs) of the 17 patients were 0.02,0.74,and 0.55,respectively,and the dose homogeneity indices were 0.56,0.89,and 0.73,respectively.The corresponding coefficients of organs at risk (OARs) were as follows:parotid_D (L/R) =0.11/0.02,-0.71/1.62,and-0.40/ 0.71,respectively; parotid_V30 (L/R) =0.01/0.01,0.08/0.05,and 0.02/0.03,respectively ; brain stem _Dmax =0.13,1.20,and 1.72,respectively;spinal cord_Dmax =0.09,1.46,and 1.30,respectively; optic chiasm_Dmax =0.03,0.22,and 0.01,respectively;lens Dmax (L/R) =0.01/0.02,0.28/0.34,and 0.02/0.04,respectively.Conclusions For the IMRT planning for NPC,the dosimetric effects on PGTVs and OARs are much greater when optimizing the segment area and number of MUs per segment than when optimizing number of segments.Therefore,reducing the number of segments and giving priority to a better setting of segment area and number of MUs per segment are helpful for shortening treatment time and improving radiobiological effect. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Marginal effect; Segment number; Segment area; Segment monit unit