title: Characterization of positron emission tomography hypoxia tracer uptake and tissue oxygenation via electrochemical modeling.
abstract: OBJECTIVE
  Unique uptake and retention mechanisms of positron emission tomography (PET) hypoxia tracers make in vivo comparison between them challenging. Differences in imaged uptake of two common hypoxia radiotracers, [(61)Cu]Cu-ATSM and [(18)F]FMISO, were characterized via computational modeling to address these challenges.
  
  
  METHODS
  An electrochemical formalism describing bioreductive retention mechanisms of these tracers under steady-state conditions was adopted to relate time-averaged activity concentration to tissue partial oxygen tension (PO(2)), a common metric of hypoxia. Chemical equilibrium constants of product concentration to reactant concentration ratios were determined from free energy changes and reduction potentials of pertinent reactions reported in the literature. Resulting transformation functions between tracer uptake and PO(2) were compared against measured values in preclinical models. Additionally, calculated PO(2) distributions from imaged Cu-ATSM tracer activity concentrations of 12 head and neck squamous cell carcinoma (HNSCC) patients were validated against microelectrode PO(2) measurements in 69 HNSCC patients.
  
  
  RESULTS
  Both Cu-ASTM- and FMISO-modeled PO(2) transformation functions were in agreement with preclinical measured values within single-deviation confidence intervals. High correlation (r(2)=0.94, P<.05) was achieved between modeled PO(2) distributions and measured distributions in the patient populations. On average, microelectrode hypoxia thresholds (2.5 and 5.0 mmHg) corresponded to higher Cu-ATSM uptake [2.5 and 2.0 standardized uptake value (SUV)] and lower FMISO uptake (2.0 and 1.4 SUV). Uncertainties in the models were dominated by variations in the estimated specific activity and intracellular acidity.
  
  
  CONCLUSIONS
  Results indicated that the high dynamic range of Cu-ATSM uptake was representative of a narrow range of low oxygen tension whose values were dependent on microenvironment acidity, while FMISO uptake was representative of a wide range of PO(2) values that were independent of acidity. The models shed light on possible causes of these discrepancies, particularly as it pertains to image contrast, and may prove to be a useful methodology in quantifying relationships between other hypoxia tracers. Comprehensive and robust assessment of tumor hypoxia prior to as well as in response to therapy may be best provided by imaging of multiple hypoxia markers that provide complementary rather than interchangeable information.
authors.affiliation: University of Wisconsin School of Medicine and Public Health, Department of Medical Physics, Madison, WI 53706, USA. srbowen@wisc.edu
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Bowen
authors.forename: Albert J
authors.initials: AJ
authors.lastname: van der Kogel
authors.forename: Marianne
authors.initials: M
authors.lastname: Nordsmark
authors.forename: Søren M
authors.initials: SM
authors.lastname: Bentzen
authors.forename: Robert
authors.initials: R
authors.lastname: Jeraj
mesh_terms: D001692:Biological Transport; D015687:Cell Hypoxia; D056831:Coordination Complexes; D003301:Copper Radioisotopes; D004563:Electrochemistry; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology; D006801:Humans; D008920:Misonidazole / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D008954:Models, Biological; D009942:Organometallic Compounds / Q000737:chemistry / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D049268:Positron-Emission Tomography; D011849:Radioactive Tracers; D015203:Reproducibility of Results; D013882:Thiosemicarbazones / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors
chemical_list: D056831:Coordination Complexes; D003301:Copper Radioisotopes; D009942:Organometallic Compounds; D011849:Radioactive Tracers; D013882:Thiosemicarbazones; C089548:copper (II) diacetyl-di(N(4)-methylthiosemicarbazone); C031843:fluoromisonidazole; D008920:Misonidazole; D010100:Oxygen