Impact of prompt gamma coincidence correction on absorbed dose estimation in differentiated thyroid cancer using 124I PET/CT imaging

Introduction Iodine-124 positron emission tomography/computed tomography (I-124 PET/CT) is increasingly being used in the absorbed dose estimation in the radioiodine treatment of differentiated thyroid cancer (DTC). However, the produced prompt gamma coincidences (PGCs) associated with the I-124 decay result in a bias in the absorbed dose estimation. The impact of a sinogram-based PGC correction approach on the absorbed dose estimation in I-124 PET/CT DTC imaging is investigated. Methods I-124 phantom and patient measurements were performed on a Siemens Biograph mCT PET/CT system. All images were reconstructed with (PGC(on)) and without PGC correction (PGC(off)). The phantom contained seven spheres (diameters: 6.6-37 mm). The spheres and background compartment were filled with a I-124 solution, resulting in a low (9.4 : 1) and a high sphere-to-background activity concentration ratio (750 : 1). Sphere recovery coefficient (RC) values were determined. In addition, the impact of PGC correction on measured lesion uptake and calculated lesion-absorbed dose was assessed for 66 lesions identified in 24 DTC patients. Results PGC correction systematically increased sphere RC values up to 71% for the smallest spheres. For the patient data, PGC correction significantly increased both the measured I-124 uptake (P<0.005) and the calculated lesion-absorbed dose (P=0.008) by similar to 3%. The percentage difference in the calculated lesion-absorbed dose ranged from -19% to 50%, showing that PGC correction had a variable and large impact for a few lesions. Conclusion PGC correction resulted in significantly higher sphere RC values, I-124 lesion uptake values and estimated lesion-absorbed doses.