Original research
The power of FDG-PET to detect treatment effects is increased by glucose correction using a Michaelis constant
1 Department of Biomedical Imaging, Genentech, Inc., 1 DNA Way, South San Francisco, CA, 94080, USA
2 Department of Pharmacokinetics and Pharmacodynamics, Genentech, Inc., South San Francisco, CA, 94080, USA
3 Department of Biostatistics, Genentech, Inc., South San Francisco, CA, 94080, USA
EJNMMI Research 2012, 2:35 doi:10.1186/2191-219X-2-35
Published: 27 June 2012Abstract
Background
We recently showed improved between-subject variability in our [18F]fluorodeoxyglucose positron emission tomography (FDG-PET) experiments using a Michaelis-Menten
transport model to calculate the metabolic tumor glucose uptake rate extrapolated
to the hypothetical condition of glucose saturation:
, where Ki is the image-derived FDG uptake rate constant, KM is the half-saturation Michaelis constant, and [glc] is the blood glucose concentration.
Compared to measurements of Ki alone, or calculations of the scan-time metabolic glucose uptake rate (MRgluc = Ki * [glc]) or the glucose-normalized uptake rate (MRgluc = Ki*[glc]/(100 mg/dL), we suggested that
could offer increased statistical power in treatment studies; here, we confirm this
in theory and practice.
Methods
We compared Ki, MRgluc (both with and without glucose normalization), and
as FDG-PET measures of treatment-induced changes in tumor glucose uptake independent
of any systemic changes in blood glucose caused either by natural variation or by
side effects of drug action. Data from three xenograft models with independent evidence
of altered tumor cell glucose uptake were studied and generalized with statistical
simulations and mathematical derivations. To obtain representative simulation parameters,
we studied the distributions of Ki from FDG-PET scans and blood [glucose] values in 66 cohorts of mice (665 individual
mice). Treatment effects were simulated by varying
and back-calculating the mean Ki under the Michaelis-Menten model with KM = 130 mg/dL. This was repeated to represent cases of low, average, and high variability
in Ki (at a given glucose level) observed among the 66 PET cohorts.
Results
There was excellent agreement between derivations, simulations, and experiments. Even
modestly different (20%) blood glucose levels caused Ki and especially MRgluc to become unreliable through false positive results while
remained unbiased. The greatest benefit occurred when Ki measurements (at a given glucose level) had low variability. Even when the power
benefit was negligible, the use of
carried no statistical penalty. Congruent with theory and simulations,
showed in our experiments an average 21% statistical power improvement with respect
to MRgluc and 10% with respect to Ki (approximately 20% savings in sample size). The results were robust in the face of
imprecise blood glucose measurements and KM values.
Conclusions
When evaluating the direct effects of treatment on tumor tissue with FDG-PET, employing a Michaelis-Menten glucose correction factor gives the most statistically powerful results. The well-known alternative ‘correction’, multiplying Ki by blood glucose (or normalized blood glucose), appears to be counter-productive in this setting and should be avoided.
