RESUMO
UNLABELLED: The presence of (124)I in tissue near the trachea can cause a phenomenon that might be called shine-through. The effect is due to high-energy positrons that cross the air-filled trachea and annihilate at the opposite tracheal wall, incorrectly suggesting uptake at that location. METHODS: We investigated shine-through using (124)I, (68)Ga, and (18)F PET/CT scans of a neck phantom. Additionally, we evaluated (124)I studies of 29 patients with differentiated thyroid cancer who underwent imaging for postsurgical staging. RESULTS: In the phantom studies with a 0.1-mL (124)I source, the relative intensity of shine-through decreased from 7% to nearly zero when the thickness of the positron-stopping layer was increased from 0.3 to 3.85 mm. In patients, shine-through was observed in 5 of the 29 studies, with intensities between 0.7% and 14%. CONCLUSION: Shine-through appears rather common in differentiated thyroid cancer. Recognition is important for identification of real lesions, calculation of uptake, and dosimetry.
