J Nucl Med 2001 Jan;42(1):71-6
Impact of FDG PET on patients with differentiated thyroid cancer who present
with elevated thyroglobulin and negative 131I scan.
Schluter B, Bohuslavizki KH, Beyer W, Plotkin M, Buchert R, Clausen M.
FDG PET is increasingly performed in patients with differentiated thyroid cancer
who present with elevated human thyroglobulin (hTG) levels and negative 131I
scan. The aim of this study was to evaluate the impact of FDG PET on treatment
in these patients. METHODS: A total of 118 FDG PET studies were performed on 64
patients, and follow-up data were available for all patients. Whole-body images
were acquired 1 h after intravenous injection of 370 MBq (10 mCi) FDG using a
PET scanner with an axial field of view of 16.2 cm. Tumor-suspicious FDG PET
studies were evaluated by histology, cytology, 131I uptake, CT or MRI, and
follow-up of hTg levels. The therapeutic consequence was noted for each patient.
Moreover, results of FDG PET were correlated with hTg levels. RESULTS:
Forty-four patients had positive scans, which were proven to be true-positive in
34 patients, whereas 7 patients had false-positive findings. Two patients
exhibited a secondary malignancy. One patient did not fit in any category,
having true-positive, false-positive, and false-negative findings. On the other
hand, 20 patients had negative scans. These were true-negative findings in 5
patients, whereas the remaining 15 patients had false-negative results.
Accordingly, the positive predictive value of FDG PET was 83% (34/41), whereas
the negative predictive value was 25% (5/20). Treatment was directly changed in
19 of 34 patients with true-positive PET studies: 18 patients had further
surgery, and 4 patients were referred for external irradiation, 3 of them after
incomplete removal of local recurrences. FDG PET showed widespread disease in 7
patients; thus, palliative treatment, rather than curative therapy, was
initiated. True-positive FDG PET findings were correlated positively with
increasing hTg levels (i.e., FDG PET was true-positive in 11%, 50%, and 93% of
patients with hTg levels of <10, 10-20, and >100 microg/L, respectively).
CONCLUSION: FDG PET is a valuable diagnostic tool in patients with
differentiated thyroid cancer who present with increased hTg levels and negative
131I scans because it permits selection of patients for surgery, which may be
curative. FDG PET is most promising at hTg levels of >10 microg/L.
