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 (CT(1)) and replanning CT (CT(2)) scans after 20–25 fractions of radiation therapy. The treatment plans were designed on CT(1) and CT(2) with the name of Plan(1) and Plan(2), respectively. The Plan(reg) and Plan(def) were obtained after registering from CT(2) to CT(1) using rigidity and deformation technology by Velocity. Then the dose-volume indices of the tumor target volumes and OARs at Plan(1), Plan(2), Plan(rig) and Plan(def) 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 CT(1) to CT(2). There was no significant difference in the dose-volume index on the GTV and plan gross tumor volume (PGTV) between Plan(1) and Plan(2). The V(30) of the left and right parotid gland and the D(max) of the brainstem, left and right eyeballs, left and right lens, and left and right optic nerves were all lower in Plan(2) than in Plan(1) (the average decrease was 17.0% to 60.1%). The differences in some dose-volume parameters (including D(mean), D(99) of the GTV and PGTV, D(mean) of the parotid glands, D(max) of the lens and optic nerves) between Plan(def) and Plan(1) were less than 5%. The differences in some dose-volume parameters (including D(mean), D(95) of the GTV and PGTV, D(mean), D(50) and V(30) of the parotid glands, D(max) of lens and optic nerves) between Plan(rig) and Plan(1) 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 Plan(def) were similar to those in Plan(1). Therefore, the dose-volume indices of Plan(1) can be used to evaluate the efficacy of radiotherapy and to predict radioactive damage.