title: Voxel-based dual-time 18F-FDG parametric imaging for rectal cancer: differentiation of residual tumor from postchemoradiotherapy changes.
abstract: BACKGROUND
  18F-Fluorodeoxyglucose (18F-FDG) PET/computed tomography (CT) has been used for evaluation of the response of rectal cancer to neoadjuvant chemoradiotherapy (CRT), but differentiating residual tumor from post-treatment changes remains a problem. We propose a voxel-based dual-time 18F-FDG PET parametric imaging technique for the evaluation of residual rectal cancer after CRT.
  
  
  METHODS
  Eighty-six patients with locally advanced rectal cancer who underwent neoadjuvant CRT between March 2009 and February 2011 were selected retrospectively. Standard 60-min postinjection PET/CT scans followed by 90-min delayed images were coregistered by rigid-body transformation. A dual-time parametric image was generated, which divided delayed standardized uptake value (SUV) by 60-min SUV on a voxel-by-voxel basis. Maximum delayed-to-standard SUV ratios (DSR) measured on the parametric images as well as the percentage of SUV decrease from pre-CRT to post-CRT scans (pre/post-CRT response index) were obtained for each tumor and correlated with pathologic response classified by the Dworak tumor regression grade (TRG).
  
  
  RESULTS
  With respect to the false-positive lesions in the nine post-CRT patients with false-positive standard 18F-FDG scans in case groups who responded to therapy (TRG 3 or 4 tumors), eight were undetectable on dual-time parametric images (P<0.05). The maximum DSR showed significantly higher accuracy for identification of tumor regression compared with the pre/post-CRT response index in receiver-operating characteristic analysis (P<0.01). With a 1.25 cutoff value for the maximum DSR, 85.0% sensitivity, 95.5% specificity, and 93.0% overall accuracy were obtained for identification of good response.
  
  
  CONCLUSIONS
  A voxel-based dual-time parametric imaging technique for evaluation of post-CRT rectal cancer holds promise for differentiating residual tumor from treatment-related nonspecific 18F-FDG uptake.
authors.affiliation: aDepartment of Nuclear Medicine bCenter for Colorectal Cancer, National Cancer Center, Goyang, Korea.
authors.forename: Hongyoon
authors.initials: H
authors.lastname: Choi
authors.forename: Hai-jeon
authors.initials: HJ
authors.lastname: Yoon
authors.forename: Tae Sung
authors.initials: TS
authors.lastname: Kim
authors.forename: Jae Hwan
authors.initials: JH
authors.lastname: Oh
authors.forename: Dae Yong
authors.initials: DY
authors.lastname: Kim
authors.forename: Seok-ki
authors.initials: SK
authors.lastname: Kim
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D059248:Chemoradiotherapy; D003937:Diagnosis, Differential; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D008297:Male; D008875:Middle Aged; D020360:Neoadjuvant Therapy; D018365:Neoplasm, Residual / Q000000981:diagnostic imaging / Q000628:therapy; D049268:Positron-Emission Tomography; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging* / Q000628:therapy*; D013997:Time Factors; D016896:Treatment Outcome
chemical_list: D019788:Fluorodeoxyglucose F18