title: Quantitative SPECT/CT Imaging of (177)Lu with In Vivo Validation in Patients Undergoing Peptide Receptor Radionuclide Therapy.
abstract: OBJECTIVE
  The purpose of this study is to extend an established SPECT/CT quantitation protocol to (177)Lu and validate it in vivo using urine samples, thus providing a basis for 3D dosimetry of (177)Lu radiotherapy and improvement over current planar methods which improperly account for anatomical variations, attenuation, and overlapping organs.
  
  
  METHODS
  In our quantitation protocol, counts in images reconstructed using an ordered subset-expectation maximization algorithm are converted to kilobecquerels per milliliter using a calibration factor derived from a phantom experiment. While varying reconstruction parameters, we tracked the ratio of image to true activity concentration (recovery coefficient, RC) in hot spheres and a noise measure in a homogeneous region. The optimal parameter set was selected as the point where recovery in the largest three spheres (16, 8, and 4 ml) stagnated, while the noise continued to increase. Urine samples were collected following 12 SPECT/CT acquisitions of patients undergoing [(177)Lu]DOTATATE therapy, and activity concentrations were measured in a well counter. Data was reconstructed using parameters chosen in the phantom experiment, and estimated activity concentration from the images was compared to the urine values to derive RCs.
  
  
  RESULTS
  In phantom data, our chosen parameter set yielded RCs in 16, 8, and 4 ml spheres of 80.0, 74.1, and 64.5 %, respectively. For patients, the mean bladder RC was 96.1 ± 13.2% (range, 80.6-122.4 %), with a 95 % confidence interval between 88.6 and 103.6 %. The mean error of SPECT/CT concentrations was 10.1 ± 8.3% (range, -19.4-22.4 %).
  
  
  CONCLUSIONS
  Our results show that quantitative (177)Lu SPECT/CT in vivo is feasible but could benefit from improved reconstruction methods. Quantifying bladder activity is analogous to determining the amount of activity in the kidneys, an important task in dosimetry, and our results provide a useful benchmark for future efforts.
authors.affiliation: Pattern Recognition Lab, Department of Computer Science, University of Erlangen-Nuremberg, Martensstr. 3, 91058, Erlangen, Germany, James.Sanders@uk-erlangen.de.
authors.forename: J C
authors.initials: JC
authors.lastname: Sanders
authors.forename: T
authors.initials: T
authors.lastname: Kuwert
authors.forename: J
authors.initials: J
authors.lastname: Hornegger
authors.forename: P
authors.initials: P
authors.lastname: Ritt
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D004067:Digestive System Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018358:Neuroendocrine Tumors / Q000000981:diagnostic imaging / Q000532:radiotherapy; D015282:Octreotide / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics / Q000627:therapeutic use / Q000652:urine; D009942:Organometallic Compounds / Q000737:chemistry* / Q000493:pharmacokinetics / Q000627:therapeutic use* / Q000652:urine; D019047:Phantoms, Imaging; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D001743:Urinary Bladder / Q000378:metabolism
chemical_list: D009942:Organometallic Compounds; C447941:lutetium Lu 177 dotatate; D015282:Octreotide