Detection of unsuspected metachronous second primary malignancy giving rise to supposed "non-iodine avid metastasis" in differentiated thyroid carcinoma.
We, herein, explore the added aspect of FDG-PET to investigate an I-131 scan negative for differentiated thyroid carcinomas (DTCs), namely the identification of previously unsuspected second primary malignancies. We present 2 cases of DTC, showing metastatic lesions in the liver and the lungs but showing no I-131 uptake and hence was initially thought to be due to dedifferentiation. FDG-PET was performed as a part of a study to prospectively evaluate its usefulness in "noniodine concentrating metastases" of DTC and to look into the validity of the traditionally described "flip-flop" between I-131 whole-body scan (reflecting the sodium-iodide symporter status in the tumor) and FDG-PET (reflecting the glucose transporter status in the tumor). In addition to the uptake in the metastatic sites, FDG-PET demonstrated unusually intense foci of hypermetabolism in the gut and the right kidney. These were subsequently found to harbor clinically silent coexisting second primary malignancies at those sites giving rise to hepatic and pulmonary metastases. Thus FDG-PET, in both these cases, provided the correct explanation for the absence of radioiodine uptake in the metastatic sites, which were otherwise thought to be due to the loss of differentiation of DTC. This role of FDG-PET in incidentally detecting a coexisting additional primary malignancy giving rise to extensive metastases is relatively unexplored and adds a new dimension to its routine application of a metastatic survey in so-called noniodine avid thyroid carcinoma, which can have a significant bearing on subsequent patient management.
Radiation Medicine Centre, Bhabha Atomic Research Centre, Bombay, India. drsanb@yahoo.com
Sandip
S
Basu
Narendra
N
Nair
Tanuja
T
Shet
D000368:Aged; D002291:Carcinoma, Papillary / Q000000981:diagnostic imaging* / Q000556:secondary*; D005188:False Negative Reactions; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D033162:Incidental Findings; D007457:Iodine Radioisotopes; D008875:Middle Aged; D016609:Neoplasms, Second Primary / Q000000981:diagnostic imaging* / Q000556:secondary*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging*
D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18