Purpose: This study evaluated the absolute quantification of iodine-124 ((124)I) activity concentration with respect to the use of this isotope for dosimetry before therapies with (131)I or (131)I-labeled radiotherapeuticals. The recovery coefficients of positron emission tomography(/computed tomography) PET(/CT) systems using (124)I 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 (124)I and with fluorine-18 ((18)F) 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 (18)F 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 (124)I PET(/CT) protocol for DTC patients.
Results: Compared with that of (18)F, transversal spatial resolution of (124)I was slightly, but statistically significantly degraded (7.4 mm vs. 8.3 mm, P<0.002). Using the body phantom, recovery was lower for (124)I than for (18)F in both 2D and 3D modes. The (124)I recovery coefficient of the largest sphere was significantly higher in 2D than in 3D mode (81% vs. 75%, P=0.03). Remarkably, the (18)F 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 (124)I activity concentration under in vivo conditions after applying partial volume correction was +/-10% for spheres > or =12.6 mm in diameter.
Conclusions: Recovery correction is mandatory for (124)I 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.