False positive FDG-uptake in neck and mediastinum in recurrent differentiated thyroid cancer (DTC)

Objectives Evaluate the diagnostic accuracy of FDG-uptake in the diagnostic workup recurrent DTC.

Methods 49 consecutive patients (pts.; age 57 ± 16 years; 43 % men; 86 % papillary TC, 14 % folliculary) after previous ablative therapy were examined between 7/2009 and 12/2011 with: (1) ultrasound (US), (2) I-131-SPECT-CT with ≥ 3 GBq, (3) F-18-FDG PET-CT with a dedicated PET series with contast-enhanced CT of the neck, (4) post-PET US, (6) US-guided fine-needle biopsy (FNB) of at least 1 index lesion. hTg, I-131 and PET were taken under TSH-stimulation.

Results Imaging was positive in 31 pts. (median hTg 20 ng/ml; range 0 - 2674) and negative in 18 (hTg 2.2; 0 - 86; p < 0.05, Wilcoxon test). Of 47 prominent lesions in the neck and mediastinum, 34 lesions in 21 pts. were malignant: 29 lymph nodes (LN), 5 thyroid bed recurrences (confirmed by histology in 23, cytology in 6, follow-up in 2, not yet in 2). 13 lesions in 11 pts. were benign: 11 inflammatory LN (histology in 2 pts., cytology 5, follow-up imaging 3, not yet 1), 1 thymus (histology in 34-year old female), and 1 pyramidal lobe (I-131 uptake). All malignant lesions were FDG-positive except 2 histologically proven LN metastases. Sensitivity of FDG-PET is 94 %. Positive predictive value was 80 % due to FDG-uptake in 8 benign lesions. 2 out of 24 patients underwent unnecessary surgery due to a false positive preoperative diagnosis (1 false pos. cytology, 1 thymic uptake). Median SUV max. for FDG-uptake in malignant lesions in neck and mediastinum was 7 (range: 0 - 40) versus 4 in benign lesions (0 - 11; n. s., Wilcoxon test). - Outside neck and mediastinum, 17 distant metastases were found: 10 lung, 4 bone, 2 hilar LN, 1 adrenal (histologically confirmed), 13 of which were FDG-positive.

Conclusions The positive predictive value of FDG-PET in neck and mediastinum is limited to 80 %. Ultrasound-guided fine needle biopsy helps to avoid unnecessary surgery.

Research Support Western Norwegian Health Care grant 91149