title: Respiratory gated [18F]FDG PET/CT for target volume delineation in stereotactic radiation treatment of liver metastases.
abstract: OBJECTIVE
  The use of 4D-[(18)F]fluorodeoxyglucose (FDG) PET/CT in combination with respiratory gated magnet resonance imaging (MRI) in target volume definition for stereotactic radiation of liver metastases was investigated.
  
  
  METHODS
  A total of 18 patients received respiration gated FDG-PET/CT and MRI. Data were fused using a rigid co-registration algorithm. The quality of the co-registration was rated on a scale from 1 (excellent) to 5 (poor) for co-registration of MRI with gated PET and ungated PET. Gross tumor volume (GTV) was delineated in CT (GTV (CT)), MRI (GTV(MRI)), and PET (GTV(PET)). MRI- and PET-based GTVs were defined by three observers each. Interobserver variability was calculated for all patients as well as for subgroups with and without previous treatment of liver metastases. All GTVs were compared for all patients and separately for patients with previous local therapy. In addition, a semiautomatic segmentation algorithm was applied on the PET images.
  
  
  RESULTS
  Co-registration  between MR and PET images was rated with 3.3 in average when non-gated PET was used and improved significantly (p < 0.01) to 2.1 using gated PET. The average GTV(CT)  was 51.5 ml, GTV(MRI)  51.8 ml, and the average GTV(PET)  48.1 ml. Volumes delineated in MRI were 9.9% larger compared to those delineated in CT. Volumes delineated in PET were 13.8% larger than in MRI. The differences between the GTVs were more pronounced in patients with previous treatment. The GTVs defined in MRI showed an interobserver variability of 47.9% (84.1% with previous treatment and 26.2% without previous treatment). The PET-defined GTVs showed an interobserver variability of 21% regardless of previous treatment. Semiautomatic segmentation did not provide satisfying results.
  
  
  CONCLUSIONS
  FDG-PET can distinguish vital tumor tissue and scar tissue, and therefore alters the GTV especially in patients with previous local treatment. In addition, it reduces the interobserver variability significantly compared to MRI. However, respiratory gated PET is necessary for good co-registration of PET and MRI.
authors.affiliation: Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar der Technischen Universität München, Ismaninger Str. 22, 81675, Munich, Germany. Bundschuh_R@klinik.uni-wuerzburg.de
authors.forename: R A
authors.initials: RA
authors.lastname: Bundschuh
authors.forename: N
authors.initials: N
authors.lastname: Andratschke
authors.forename: J
authors.initials: J
authors.lastname: Dinges
authors.forename: M N
authors.initials: MN
authors.lastname: Duma
authors.forename: S T
authors.initials: ST
authors.lastname: Astner
authors.forename: M
authors.initials: M
authors.lastname: Brügel
authors.forename: S I
authors.initials: SI
authors.lastname: Ziegler
authors.forename: M
authors.initials: M
authors.lastname: Molls
authors.forename: M
authors.initials: M
authors.lastname: Schwaiger
authors.forename: M
authors.initials: M
authors.lastname: Essler
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008113:Liver Neoplasms / Q000175:diagnosis / Q000556:secondary* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals; D016634:Radiosurgery / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D055425:Respiratory-Gated Imaging Techniques / Q000379:methods*; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016896:Treatment Outcome
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18