title: The impact of 18 F-FET PET-CT on target definition in image-guided stereotactic radiotherapy in patients with skull base lesions.
abstract: BACKGROUND
  18 F-fluoro-ethyl-tyrosine PET is gaining more indications in the field of oncology. We investigated the potentials of usage of FET-PET/CT in addition to MRI for definition of gross tumor volume (GTV) in stereotactic radiotherapy of lesions of skull base.
  
  
  METHODS
  We included in a prospective setting 21 cases. An MRI was performed, completed by FET PET/CT. Different GTV's were defined based on respective imaging tools: 1. GTVMRI, 2. GTV MRI /CT, 3. GTV composit (1 + 2), and GTVPET = GTV Boost. Lesions could be visualised by MRI and FET-PET/CT in all patients.
  
  
  RESULTS
  FET tracer enhancement was found in all cases. Skull base infiltration by these lesions was observed by MRI, CT (PET/CT) and FET-PET (PET/CT) in all patients. Totally, brain tissue infiltration was seen in 10 patients. While, in 7 (out 10) cases, MRI and CT (from PET/CT) were indicating brain infiltration, FET-PET could add additional information regarding infiltrative behaviour: in 3 (out 10) patients, infiltration of the brain was displayed merely in FET-PET. An enlargement of GTVMRI/CT due to the FET-PET driven information, which revealed GTVcomposite , was necessary in 7 cases,. This enlargement was significant by definition (> 10% of GTVMRI/CT). The mean PET-effect on GTV counted for 1 ± 4 cm3. The restricted boost fields were based mainly on the GTVPET volume. In mean, about 8.5 cm3 of GTVMRI/CT, which showed no FET uptake, were excluded from target volume. GTV boost driven by only-PET-activity, was in mean by 33% smaller than the initial large treatment field, GTV composite, for those cases received boost treatment. FET-PET lead to significant (>10%) changes in the initial treatment fields in 11/21 patients and showed additional tumour volume relevant for radiation planning in 6/21 cases, and led to a subsequent decrease of more than 10% of the initial volumes for the boost fields.
  
  
  CONCLUSIONS
  The implementation of FET PET into the planning procedures showed a benefit in terms of accurate definition of skull base lesions as targets for Image-guided stereotactic Radiotherapy. This has to be investigated prospectively in larger cohorts.
authors.forename: Harun
authors.initials: H
authors.lastname: Badakhshi
authors.forename: Reinhold
authors.initials: R
authors.lastname: Graf
authors.forename: Vikas
authors.initials: V
authors.lastname: Prasad
authors.forename: Volker
authors.initials: V
authors.lastname: Budach
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D064847:Multimodal Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D011446:Prospective Studies; D019275:Radiopharmaceuticals; D016634:Radiosurgery / Q000379:methods*; D061089:Radiotherapy, Image-Guided; D019292:Skull Base Neoplasms / Q000175:diagnosis* / Q000473:pathology / Q000532:radiotherapy; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18