title: Three-dimensional Gaussian model to define brain metastasis limits on 11C-methionine PET.
abstract: OBJECTIVE
  Since 11C-methionine (MET) heavily accumulates in brain tumors, PET with MET (MET-PET) is proposed for the image-guided planning of their targeted therapy. Determination of bulk tumor limits is therefore a crucial component of MET-PET image analysis. We aimed at validating a Gaussian model of tumor delineation on MET-PET. We choose MET-PET and MRI data obtained in brain metastases to adjust the model. Indeed, MRI limits of these non-infiltrative hypermetabolic brain lesions are efficiently used for their curative treatment.
  
  
  METHODS
  We developed a three-dimensional (3D) Gaussian model that relates the tumor-limit-defining threshold to maximum and mean count values in the defined tumor volume and to mean count values in a reference region. To adjust the model to experimental data, we selected 25 brain metastases following these criteria: (i) no surgery or classical radiotherapy within 6 months, (ii) no previous radiosurgery, (iii) MET-PET and MRI acquired within a 48-h interval, (vi) necrosis representing less than 25% of tumor volume on MRI. We applied a progressive thresholding procedure on MET-PET so as to match tumor limits on contrast-enhanced co-registered MRI.
  
  
  RESULTS
  In 22 tumors, a match could be reached between tumor margins on MET-PET and MRI. The relation between mean, maximum and threshold values closely fits the 3D-Gaussian model function. We found a quadratic relation between the mean-to-threshold ratio and the maximum-to-cerebellum activity ratio.
  
  
  CONCLUSIONS
  A 3D-Gaussian model may describe the limits of MET uptake distribution within brain metastases, providing a simple method for metabolic tumor delineation.
authors.affiliation: Department of Nuclear Medicine and PET/Biomedical Cyclotron Unit, Brussels, Belgium.
authors.forename: Bich-Ngoc-Thanh
authors.initials: BN
authors.lastname: Tang
authors.forename: Gaetan
authors.initials: G
authors.lastname: Van Simaeys
authors.forename: Daniel
authors.initials: D
authors.lastname: Devriendt
authors.forename: Niloufar
authors.initials: N
authors.lastname: Sadeghi
authors.forename: Olivier
authors.initials: O
authors.lastname: Dewitte
authors.forename: Nicolas
authors.initials: N
authors.lastname: Massager
authors.forename: Philippe
authors.initials: P
authors.lastname: David
authors.forename: Marc
authors.initials: M
authors.lastname: Levivier
authors.forename: Serge
authors.initials: S
authors.lastname: Goldman