Disseminated iodine-avid lung metastases in differentiated thyroid cancer: a challenge to 124I PET

This study assessed the ability of visual and quantitative 124-iodine positron emission tomography ((124)I PET) data to detect disseminated iodine-avid lung metastases (DILM) of differentiated thyroid cancer (DTC).

Using "post-therapy" 131-iodine ((131)I) whole-body scintigraphy (TxWBS) and thoracic computed tomography (CT), we retrospectively divided 70 consecutive DTC patients undergoing (124)I PET dosimetry approximately 1 week before (131)I therapy into subgroups positive (n = 7) or negative (n = 63) for DILM, defined as lung metastases visible on TxWBS but not thoracic CT. To determine whether (124)I PET data could distinguish patients with versus without DILM, we compared these data with the TxWBS findings. The (124)I PET data were acquired via whole-body PET scans approximately 24 h after oral administration of (124)I, 24 +/- 2 MBq. Quantitative data comprised absolute lung (124)I activity concentrations and lung-to-background (L/B) (124)I uptake ratios.

Only 1/7 DILM-positive patients had visible disseminated lung uptake. Absolute (124)I lung uptake activities overlapped considerably between both groups and did not differ significantly (P = 0.150). Mean L/B ratios, however, differed significantly between the DILM-positive and negative groups (P < 0.001). Nevertheless, L/B ratios overlapped between the groups (0.62-1.37 versus 0.13-0.69).

Quantitative analysis of (124)I PET data using the L/B ratio is a promising tool to detect patients suspicious for DILM. However, L/B ratios overlapped between the groups to an extent that an unequivocal diagnosis based solely on this criterion was impossible in some patients. In those cases, additional diagnostic tests are necessary for diagnosis.