Published in:
01-03-2008 | Original Article
Iodine-124 PET dosimetry in differentiated thyroid cancer: recovery coefficient in 2D and 3D modes for PET(/CT) systems
Authors:
Walter Jentzen, Reiner Weise, Jürgen Kupferschläger, Lutz Freudenberg, Wolfgang Brandau, Ronald Bares, Wolfgang Burchert, Andreas Bockisch
Published in:
European Journal of Nuclear Medicine and Molecular Imaging
|
Issue 3/2008
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Abstract
Purpose
This study evaluated the absolute quantification of iodine-124 (124I) activity concentration with respect to the use of this isotope for dosimetry before therapies with 131I or 131I-labeled radiotherapeuticals. The recovery coefficients of positron emission tomography(/computed tomography) PET(/CT) systems using 124I were determined using phantoms and then validated under typical conditions observed in differentiated thyroid cancer (DTC) patients.
Methods
Transversal spatial resolution and recovery measurements with 124I and with fluorine-18 (18F) as the reference were performed using isotope-containing line sources embedded in water and six isotope-containing spheres 9.7 to 37.0 mm in diameter placed in water-containing body and cylinder phantoms. The cylinder phantom spheres were filled with 18F only. Measurements in two-dimensional (2D) and three-dimensional (3D) modes were performed using both stand-alone PET (EXACT HR+) and combined PET/CT (BIOGRAPH EMOTION DUO) systems. Recovery comparison measurements were additionally performed on a GE ADVANCE PET system using the cylinder phantom. The recovery coefficients were directly determined using the activity concentration of circular regions of interest divided by the prepared activity concentration determined by the dose calibrator. The recovery correction method was validated using three consecutive scans of the body phantom under our 124I PET(/CT) protocol for DTC patients.
Results
Compared with that of 18F, transversal spatial resolution of 124I was slightly, but statistically significantly degraded (7.4 mm vs. 8.3 mm, P<0.002). Using the body phantom, recovery was lower for 124I than for 18F in both 2D and 3D modes. The 124I recovery coefficient of the largest sphere was significantly higher in 2D than in 3D mode (81% vs. 75%, P=0.03). Remarkably, the 18F recovery coefficient for the largest sphere significantly deviated from unity (range of 87%–93%, P<0.004) for all scanners but the GE ADVANCE. The maximum range of inaccuracy of the measured 124I activity concentration under in vivo conditions after applying partial volume correction was ±10% for spheres ≥12.6 mm in diameter.
Conclusions
Recovery correction is mandatory for 124I PET quantification, even for large structures. To ensure accurate dosimetry, thorough absolute recovery measurements must be individually established for the particular PET scanner and radionuclide to be used.