title: A modified VMAT adaptive radiotherapy for nasopharyngeal cancer patients based on CT-CT image fusion.
abstract: BACKGROUND
  To investigate the feasibility and benefits of a modified adaptive radiotherapy (ART) by replanning in the initial CT (iCT) with new contours from a repeat CT (rCT) based on CT-CT image fusion for nasopharyngeal cancer (NPC) patients underwent volumetric modulated arc radiotherapy (VMAT).
  
  
  METHODS
  Nine NPC patients underwent VMAT treatment with a rCT at 23rd fraction were enrolled in this study. Dosimetric differences for replanning VMAT plans in the iCT and in the rCT were compared. Volumetric and dosimetric changes of gross tumor volume (GTV) and organs at risk (OARs) of this modified ART were also investigated.
  
  
  RESULTS
  No dosimetric differences between replanning in the iCT and in the rCT were observed. The average volume of GTV decreased from 78.83 ± 38.42 cm3 in the iCT to 71.44 ± 37.46 cm3 in the rCT, but with no significant difference (p = 0.42).The average volume of the left and right parotid decreased from 19.91 ± 4.89 cm3 and 21.58 ± 6.16 cm3 in the iCT to 11.80 ± 2.79 cm3 and 13.29 ± 4.17 cm3 in the rCT (both p < 0.01), respectively. The volume of other OARs did not shrink very much. No significant differences on PTVGTV and PTVCTV coverage were observed for replanning with this modified ART. Compared to the initial plans, the average mean dose of the left and right parotid after re-optimization were decreased by 62.5 cGy (p = 0.05) and 67.3 cGy (p = 0.02), respectively, and the V5 (the volume receiving 5 Gy) of the left and right parotids were decreased by 7.8% (p = 0.01) and 11.2% (p = 0.001), respectively. There was no significant difference on the dose delivered to other OARs.
  
  
  CONCLUSIONS
  Patients with NPC undergoing VMAT have significant anatomic and dosimetric changes to parotids. Repeat CT as an anatomic changes reference and re-optimization in the iCT based on CT-CT image fusion was accurate enough to identify the volume changes and to ensure safe dose to parotids.
authors.affiliation: Radiotherapy and Chemotherapy, Department of the 1st Affiliated Hospital of Wenzhou Medical University, No,2 Fuxue Lane, Wenzhou 325000, China. billy07@wzhospital.cn.
authors.forename: Xiance
authors.initials: X
authors.lastname: Jin
authors.forename: Ce
authors.initials: C
authors.lastname: Han
authors.forename: Yongqiang
authors.initials: Y
authors.lastname: Zhou
authors.forename: Jinling
authors.initials: J
authors.lastname: Yi
authors.forename: Huawei
authors.initials: H
authors.lastname: Yan
authors.forename: Congying
authors.initials: C
authors.lastname: Xie
mesh_terms: D006801:Humans; D007091:Image Processing, Computer-Assisted; D009303:Nasopharyngeal Neoplasms / Q000532:radiotherapy*; D058958:Organs at Risk; D010306:Parotid Gland / Q000528:radiation effects; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016896:Treatment Outcome