FDG-PET and FDG-PET/CT can help identify metastatic thyroid cancer in patients with recurrent or persistent PTC who have increased serum thyroglobulin levels and negative or equivocal whole-body iodine scans
]. Metastatic PTCs that lose the ability to concentrate radioactive iodine are believed to be clinically more aggressive and have a worse outcome, probably because they are less well-differentiated and cannot be treated successfully with radioactive iodine therapy. Papillary thyroid cancers that are radioactive iodine negative take up FDG-PET more readily because of their higher metabolic activity
When we compared the outcome of patients with recurrent or persistent PTC who had FDG-PET-positive or FDG-PET-negative metastatic tumors, we found that patients who had positive lesions had larger tumors at the time of initial treatment. Furthermore, in these patients the initial TNM stage was higher than in patients who had a negative lesion. These patients also had higher serum thyroglobulin levels at the time of the FDG-PET scan. In our cohort study, only patients with FDG-PET-positive lesions died. Although our sample size is relatively small, this observation supports previous reports that PET-positive lesions are associated with a worse prognosis
]. One of those previous studies showed that patients with PET-positive lesions were also older, had a higher initial cancer stage and higher thyroglobulin levels
]. These risk factors indicate that PET-positive patients have extensive and more aggressive PTC and are more likely to have either recurrent or persistent thyroid cancer. Another study found that most patients with PET-positive metastases also had high-risk clinical characteristics like extensive extrathyroidal extension of the primary tumor, and that 70% of patients with PTC had less well-differentiated metastases than found in their primary tumors
In our study, in only two of seven patients with PET-positive neck lesions who underwent reoperation loco-regional control was obtained without evidence of residual disease. One of these patients had a mediastinal lymph node metastasis resected. The other patient had two lymph nodes resected and had no evidence of residual disease on imaging studies four years after surgery, but the thyroglobulin level remained elevated. Although one might question why palliative reoperations were done in patients with distant metastasis, we believe that surgical resection of PET-positive lesions in the neck is indicated to control loco-regional disease. One might anticipate that PET-positive lesions would grow faster and be more likely to cause local morbidity. We realize that there could be a bias in our series. It is known that patients with FDG-PET-positive lesions have more aggressive disease and, therefore, the FDG-PET scans may have been performed more readily in patients suspected of more aggressive disease. Although we failed to show a clear benefit of surgical resection in this study, we believe that resection of recurrent or persistent neck lesions should usually be performed especially for recurrent tumors that are growing or are larger than 1
In our study, all FDG-PET-positive lesions that were resected were either palpable or seen on an ultrasound of the neck or magnetic resonance image (MRI) of the mediastinum. Ultrasound is generally more sensitive for identifying nodal metastases in the neck than FDG-PET and FDG-PET/CT
]. Ultrasound-guided needle biopsy can provide a definitive diagnosis. In two patients, the decision to operate was made regardless of the FDG-PET scan result.
In a previous study, the impact of FDG-PET/CT for recurrent PTC on the clinical management changed the treatment plan in 40% of 33 patients, supported the treatment plan made before the PET scan in 27%, and failed to contribute to the management in 33%
]. FDG-PET/CT scanning was most useful when the thyroglobulin levels were greater than 10
ng/ml. In that study, 22 out of 30 patients were treated surgically. However, they did not show the outcome of these patients. Another study that evaluated the clinical value of FDG-PET scans in 37 patients who had an increased thyroglobulin level and a negative whole-body iodine scan found that 28 patients had a positive PET scan. In 29 patients, the management of these patients appears to have changed from the initial treatment plan (23 underwent surgery, 14 of whom were disease-free after a mean of six months
]. FDG-PET/CT scan may contribute to the surgical strategy in patients with persistent or recurrent PTC.
In our study, 23% of patients with PET-positive lesions had metastatic lesions that took up radioactive iodine. This is a lower percentage than the 37.5% reported previously
]. In that study, prognosis was surprisingly worse in patients who had both PET-positive lesions and radioactive iodine uptake, and survival was similar, whether their lesions did or did not take up radioactive iodine
]. We were surprised to find that among our patients who had PET- positive lesions that were less likely to take up radioactive iodine, yet more were treated with radioactive iodine than patients with PET-negative lesions. In a report of the Memorial Sloan Kettering Cancer Center experience, patients with PET-positive lesions were treated with higher doses of radioactive iodine than those with PET-negative lesions. Despite this higher dose of radioactive iodine, subsequent FDG-PET uptake was similar after one year, the PET volume of the lesions was larger, and the thyroglobulin level was also higher after one year of follow-up
The results of reoperations of patients with PET-positive lesions have been studied before by Mirallie et al.
In that series, patients with recurrent differentiated thyroid cancer and FDG-PET-positive lesions underwent surgical resection. Half of these patients had no evidence of residual disease. A subgroup of these patients even had no detectable thyroglobulin levels indicating cure
]. However, the prognosis of patients with FDG-PET-positive lesions and PDG-PET-negative lesions after surgery remains unclear. Nevertheless, our study had several limitations: its retrospective design, relatively small sample size, and inability to obtain histological confirmation for all FDG-PET and FDG-PET/CT lesions. Moreover, using FDG-PET scanning to localize recurrent or persistent PTC has some established disadvantages. First, it has relatively low sensitivity, specificity and positive predictive value. Second, it is expensive. Third, the decision to operate does not always depend on the scan results, which are certainly of most value in patients who have elevated thyroglobulin levels, but are radioactive iodine negative and also have other negative scans. However, FDG-PET scanning can document patients with multiple distant metastases who are not candidates for reoperation.