Iododeoxyuridine (
IUdR) uptake and retention was imaged by positron emission tomography (PET) at 0-48 min and 24 h after administration of 28.0-64.4 MBq (0.76-1.74 mCi) of [124I]
IUdR in 20 patients with
brain tumors, including
meningiomas and
gliomas. The PET images were directly compared with
gadolinium contrast-enhanced or T2-weighted magnetic resonance images. Estimates for
IUdR-
DNA incorporation in
tumor tissue (Ki) required pharmacokinetic modeling and fitting of the 0-48 min dynamically acquired data to correct the 24-h image data for residual, nonincorporated radioactivity that did not clear from the tissue during the 24-h period after
IUdR injection. Standard uptake values (SUVs) and
tumor:brain activity ratios (Tm:Br) were also calculated from the 24-h image data. The Ki, SUV, and Tm/Br values were related to
tumor type and grade,
tumor labeling index, and survival after the PET scan. The plasma half-life of [124I]
IUdR was short (2-3 min), and the arterial plasma input function was similar between patients (48 +/- 12 SUV*min). Plasma clearance of the major radiolabeled metabolite ([124I]
iodide) varied somewhat between patients and was markedly prolonged in one patient with
renal insufficiency. It was apparent from our analysis that a sizable fraction (15-93%) of residual nonincorporated radioactivity (largely [124I]
iodide) remained in the
tumors after the 24-h washout period, and this fraction varied between the different
tumor groups. Because the SUV and Tm:Br ratio values reflect both
IUdR-
DNA incorporated and exchangeable nonincorporated radioactivity, any residual nonincorporated radioactivity will amplify their values and distort their significance and interpretation. This was particularly apparent in the
meningioma and
glioblastoma multiforme groups of
tumors. Mean
tumor Ki values ranged between 0.5 +/- 0.9 (
meningiomas) and 3.9 +/- 2.3 microl/min/g (peak value for
glioblastoma multiforme, GBM). Comparable SUV and Tm:Br values at 24 h ranged from 0.13 +/- 0.03 to 0.29 +/- 0.19 and from 2.0 +/- 0.6 to 6.1 +/- 1.5 for
meningiomas and peak GBMs, respectively. Thus, the range of values was much greater for Ki (approximately 8-fold) compared with that for SUV (approximately 2.2-fold) and Tm:Br (approximately 3-fold). The expected relationships between Ki, SUV, and Tm:Br and other measures of
tumor proliferation (
tumor type and grade, labeling index, and patient survival) were observed. However, greater image specificity and significance of the SUV and Tm:Br values would be obtained by achieving greater washout and clearance of the exchangeable fraction of residual (background) radioactivity in the
tumors, i.e., by increased hydration and urinary clearance and possibly by imaging later than 24 h after [124I]
IUdR administration.