Dosimetric Analysis Of Replanning During IMRT For Locally Advanced Nasopharyngeal Carcinoma

Objective:During IMRT,patients with nasopharyngeal carcinoma may have obvious anatomical changes due to weight loss or tumor regression,which may lead to significant dosimetric changes.In this study,the necessity of radiotherapy replanning was discussed identified analyzing the volumetric and dosimetric changes in the targets and the OARs during IMRT for locally advanced nasopharyngeal carcinoma.And the improvement of dosimetry in the targets and the OARs by replanning was identified.Methods:25 patients with locally advanced nasopharyngeal carcinoma received IMRT and concurrent chemotherapy.The whole course of radiotherapy was 33 fraction.After the10 th and 20 th fraction of IMRT,all patients were re-scanned with CT,and the target volumes was re-delineated,and the IMRT plan was re-formulated.The volume of GTVnx,GTVnd,CTV1 and bilateral parotid glands in the initial CT(CT-0)and the two subsequent CT(CT-10 and CT-20)of each patient were counted to determine the volume changes in tumor and parotid gland during radiotherapy.CT-10 and CT-20 were respectively fused with CT-0 based on the bone markers,and then CT-10 and CT-20 were superimposing with the initial radiotherapy Plan(Plan-0)to form two mixed plans,Plan-10x and Plan-20x,which represented the actual dose delivered to the target volumes and OARs.Plan-0,Plan-10x and Plan-20x were compared to analyze the changes of actual exposure dose in the target volumes and OARs due to anatomical changes after the 10 th and 20 th fraction of radiotherapy,so as to judge the necessity of replanning.The two replanning Plan-10 and Plan-20,which were actually used for treatment,were compared with Plan-10 X and Plan-20 X respectively to analyze the improvement of dosimetry by replanning.Results:Compared with CT-0,the volume of GTVnx in CT-10 and CT-20 decreased by2.16% and 10.42%,respectively.The volume of GTVnd decreased by 6.69% and31.47%,respectively.The volume of CTV1 decreased by 2.78% and 9.86%,respectively.The volume of the left parotid gland was reduced by 11.50% and 24.13%,respectively.The volume of the right parotid gland was reduced by 10.81% and23.80%,respectively.Comparing Plan-10x with Plan-0,the dose-volume parameters of PTV1,PGTVnx and PGTVnd showed no statistically significant difference.Dmean of left parotid gland was increased by 2.11 Gy(P < 0.001)and D50 by 3.25Gy(P=0.004).Dmean of the right parotid gland was increased by 2.45Gy(P=0.003)and D50 by 3.05Gy(P=0.004).The dose parameters of spinal cord,brainstem and optic nerve had no statistical significance.The results indicated that dose in the target volumes in did not change significantly after the 10 th fraction of radiotherapy,while the dose in parotid gland dose increased.Comparing Plan-20x with Plan-0,there was no statistically significant difference in the dose-volume parameters of PGTVnd and PTV1,while the Dmean of PGTVnx decreased by 0.83Gy(P=0.001)and the D95 decreased by 1.06Gy(P=0.002).Dmean of left parotid gland was increased by 4.26Gy(P < 0.001)and D50 by 6.54Gy(P=0.003).Dmean of the right parotid gland was increased by 6.14Gy(P < 0.001)and D50 by 9.26Gy(P=0.001).Dmax of Brain stem Dmax increased by 0.83Gy(P=0.002).Dmax of spinal cord increased by 1.89Gy(P=0.001).The results suggested that after the 20 th fraction of radiotherapy,the dose in PGTVnx decreased significantly,while the parotid gland,brainstem and spinal cord dose increased significantly,and it was necessary to replan.Comparing Plan-10 with Plan-10x,the dose-volume parameters of PTV1,PGTVnx and PGTVnd had no statistical significance.Dmean of the left parotid gland was decreased by 4.69Gy(P < 0.001)and D50 by 5.35Gy(P < 0.001).Dmean of the right parotid gland was decreased by 3.47Gy(P < 0.001)and D50 by 2.69Gy(P <0.001).There was no significant difference in the dose parameters of spinal cord,brainstem and optic nerve.The results suggested that the replanning after the 10 th fraction of radiotherapy had little effect on the target dose,and only reduced the parotid dose.Comparing Plan-20 with Plan-20x,Dmean of PGTVnx was increased by0.85Gy(P=0.048)and D95 by 1.09Gy(P=0.002).There was no significant difference in dose-volume parameters between PTV1 and PGTVnd.Dmean of the left parotid gland was decreased by 7.16Gy(P < 0.001)and D50 by 8.33Gy(P < 0.001).Dmean of the right parotid gland was decreased by 7.88Gy(P < 0.001)and D50 was decreased by 9.42Gy(P < 0.001).Dmax of spinal cord decreased by 2.14Gy(P=0.001);The dose parameters of brainstem and optic nerve had no significant difference.The results suggested that the replanning after the 20 th fraction of radiotherapy increased the dose of PGTVnx,and decreased the dose of parotid gland and spinal cord.Conclusion:1.The volume of tumor and parotid gland in patients with locally advanced nasopharyngeal cancer decreased significantly during IMRT.2.In patients with locally advanced nasopharyngeal carcinoma,the actual dose of PGTVnx was significantly lower than the planned dose after the 20 th fraction of radiotherapy,while the dose of parotid gland,spinal cord and brain stem was significantly increased,so it was necessary to replan.3.Replanning after the 20 th fraction of IMRT significantly improved dosimetric parameters of PGTVnx and reduced the dose to the parotid and spinal cords.4.Rrplanning after the 20 th fraction of IMRT is more appropriate than replanning after the 10 th fraction.