title: Biological image-guided radiotherapy in rectal cancer: challenges and pitfalls.
abstract: OBJECTIVE
  To investigate the feasibility of integrating multiple imaging modalities for image-guided radiotherapy in rectal cancer.
  
  
  METHODS
  Magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) were performed before, during, and after preoperative chemoradiotherapy (CRT) in patients with resectable rectal cancer. The FDG-PET signals were segmented with an adaptive threshold-based and a gradient-based method. Magnetic resonance tumor volumes (TVs) were manually delineated. A nonrigid registration algorithm was applied to register the images, and mismatch analyses were carried out between MR and FDG-PET TVs and between TVs over time. Tumor volumes delineated on the images after CRT were compared with the pathologic TV.
  
  
  RESULTS
  Forty-five FDG-PET/CT and 45 MR images were analyzed from 15 patients. The mean MRI and FDG-PET TVs showed a tendency to shrink during and after CRT. In general, MRI showed larger TVs than FDG-PET. There was an approximately 50% mismatch between the FDG-PET TV and the MRI TV at baseline and during CRT. Sixty-one percent of the FDG-PET TV and 76% of the MRI TV obtained after 10 fractions of CRT remained inside the corresponding baseline TV. On MRI, residual tumor was still suspected in all 6 patients with a pathologic complete response, whereas FDG-PET showed a metabolic complete response in 3 of them. The FDG-PET TVs delineated with the gradient-based method matched closest with pathologic findings.
  
  
  CONCLUSIONS
  Integration of MRI and FDG-PET into radiotherapy seems feasible. Gradient-based segmentation is recommended for FDG-PET. Spatial variance between MRI and FDG-PET TVs should be taken into account for target definition.
authors.affiliation: Department of Radiation Oncology, Leuven Cancer Institute, University Hospital Leuven, Herestraat 49, Leuven, Belgium.
authors.forename: Sarah
authors.initials: S
authors.lastname: Roels
authors.forename: Pieter
authors.initials: P
authors.lastname: Slagmolen
authors.forename: Johan
authors.initials: J
authors.lastname: Nuyts
authors.forename: John A
authors.initials: JA
authors.lastname: Lee
authors.forename: Dirk
authors.initials: D
authors.lastname: Loeckx
authors.forename: Frederik
authors.initials: F
authors.lastname: Maes
authors.forename: Vincent
authors.initials: V
authors.lastname: Vandecaveye
authors.forename: Sigrid
authors.initials: S
authors.lastname: Stroobants
authors.forename: Nadine
authors.initials: N
authors.lastname: Ectors
authors.forename: Freddy
authors.initials: F
authors.lastname: Penninckx
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D000964:Antimetabolites, Antineoplastic / Q000627:therapeutic use; D003131:Combined Modality Therapy / Q000379:methods; D005240:Feasibility Studies; D005260:Female; D019788:Fluorodeoxyglucose F18; D005472:Fluorouracil / Q000627:therapeutic use; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals; D011879:Radiotherapy Dosage; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy*; D012074:Remission Induction; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D000964:Antimetabolites, Antineoplastic; D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18; D005472:Fluorouracil