Study of the dose-volume parameters variation in tumor target volumes and organs at risk during nasopharyngeal carcinoma radiotherapy applying deformation registration

Background: During intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC), the volume of the target volume and the organs at risk (OARs) will change constantly, which may lead to differences between the actual dose received and the initial planned dose. In this study, the cumulative dose of the two plans was obtained by deformable registration. This study provides an approach to evaluate the dose volume of IMRT for the NPC objective. Methods: From July 2014 to May 2018, eighteen NPC patients who accepted simultaneous integrated boost IMRT were enrolled. All patients underwent simulation CT (CT1) and replanning CT (CT2) scans after 20–25 fractions of radiation therapy. The treatment plans were designed on CT1 and CT2 with the name of Plan1 and Plan2, respectively. The Planreg and Plandef were obtained after registering from CT2 to CT1 using rigidity and deformation technology by Velocity. Then the dose-volume indices of the tumor target volumes and OARs at Plan1, Plan2, Planrig and Plandef were compared. Results: The gross tumor volume (GTV) and the left and right parotid gland volumes decreased by 20.8% (P<0.001), 36.8% (P<0.001) and 37.5% (P<0.001), respectively, from CT1 to CT2. There was no significant difference in the dose-volume index on the GTV and plan gross tumor volume (PGTV) between Plan1 and Plan2. The V30 of the left and right parotid gland and the Dmax of the brainstem, left and right eyeballs, left and right lens, and left and right optic nerves were all lower in Plan2 than in Plan1 (the average decrease was 17.0% to 60.1%). The differences in some dose-volume parameters (including Dmean, D99 of the GTV and PGTV, Dmean of the parotid glands, Dmax of the lens and optic nerves) between Plandef and Plan1 were less than 5%. The differences in some dose-volume parameters (including Dmean, D95 of the GTV and PGTV, Dmean, D50 and V30 of the parotid glands, Dmax of lens and optic nerves) between Planrig and Plan1 were less than 10%. The Dyce Similarity Coefficient of the target volume and OARs after deformation registration were higher than that after rigid registration. Conclusions: The volume of the GTV and parotid glands were decreased during the IMRT for NPC. The dose-volume indices of the GTV and the OARs in Plandef were similar to those in Plan1. Therefore, the dose-volume indices of Plan1 can be used to evaluate the efficacy of radiotherapy and to predict radioactive damage.