Lay Summary: PET scan can detect non-iodine avid thyroid cancers and is standard of care.
PET scan has a defined albeit limited role in thyroid cancer. In the setting of suspected recurrent thyroid cancer, PET scans may be used to perform functional imaging when radioactive iodine scans have proven to be unreliable or difficult to interpret.
Differentiated thyroid cancer cells may undergo a process of transformation whereby they lose some or all of the ability to take up and retain 131-Iodine. Such cells may still retain the ability to absorb a different radioactive tracer called 18-Fluorodeoxyglucose (abbreviated FDG). If a PET scan is performed after a patient receives a tracer dose of FDG, the images could reveal abnormal areas of increased uptake that may indicate the presence of thyroid cancer cells. Images from a survey of the body could reveal abnormal areas of uptake indicating the spread (or metastasis) of thyroid cancer to lymph nodes, lungs, bones, or central nervous system.
However, a common general principle is that it is desirable to identify persistent or recurrent thyroid cancer as soon as possible so that appropriate therapy can be initiated. For differentiated thyroid cancer, 131-I scans are useful, and, for all types of thyroid cancer, radiologic studies such as MRI and CT scans as well as neck sonograms are proving to be increasingly useful. When to utilize PET scans is more controversial.
Golger and colleagues studied 25 patients with elevated serum thyroglobulin levels (average 19 ng/mL). PET scans performed during thyroid hormone withdrawal (average TSH 53 mcU/mL) were found to be more sensitive in detecting metastatic differentiated thyroid cancer than either PET scans performed on the same patients when taking L-thyroxine therapy or when compared with 131-I withdrawal scans.
Robbins and colleagues retrospectively studied 403 patients with differentiated thyroid cancer. Most of these patients had known or suspected metastasis, an elevated serum thyroglobulin level with a negative 131-I scan, or poorly differentiated histology. Of these 403 patients, 61% had positive 131-I withdrawal scans, 55% had positive lesions on FDG-PET scans, and 67% had papillary thyroid cancer. Seventy-one patients had died of their disease. Multivariate analysis showed that age, intensity of PET uptake, and the number of lesions identified on FDG-PET scan correlated with survival. Each increasing decade of age increased the risk of death by 28%; a positive FDG-PET scan was associated with an 8-fold higher risk of death, and each additional lesion identified by FDG-PET scan increased the risk of death by 9%. Of the 179 patients with a negative FDG PET scan, only 3 died.
These abstracts give further information about the importance and utility of performing FDG-PET scans. It appears that PET scans are useful in identifying recurrent or persistent metastatic thyroid cancer that may not be visualized by other radiologic techniques. Furthermore, it now seems that PET scans may contribute significant information relevant to prognosis in thyroid cancer patients. The precise role of PET scan in patients with known thyroid cancer of all types requires further definition, and cost-effectiveness studies would also be important. However, based on the studies reported to date, PET scans are increasingly being recognized as an important diagnostic tool in selected patients with recurrent or persistent thyroid cancer.
There is one (referenced) guideline that recommends PET scanning(1). On the other hand, a recent textbook(5) says: “The use of CT, MRI, and FDG-PET in the routine evaluation of patients with PTC(papillary thyroid cancer), however, is not recommended.” Note that this statement applies to papillary cancer.
2.Golger A, Freedman T, Gulenchyn K, et al. Efficacy of 18F-fluorodeoxyglucose positron emission tomography for diagnosis and long-term monitoring of patients with well-differentiated thyroid cancer. Program and abstracts of the 75th Annual Meeting of the American Thyroid Association; September 16-21, 2003; Palm Beach, Florida. Abstract 7.
3.Robbins R, Reibke R, Drucker W. Real-time prognosis in metastatic thyroid cancer based on FDG-PET scanning. Program and abstracts of the 75th Annual Meeting of the American Thyroid Association; September 16-21, 2003; Palm Beach, Florida. Abstract 99.
4. C Marcus et al, PET/CT in the Management of Thyroid Cancers, AJR Volume 202, Issue 6 > PET/CT In The Management Of Thyroid Cancers. June 2014, VOLUME 202
5. Bryan R. Haugen et al, 2015 American Thyroid Association ManagementGuidelines for Adult Patients with Thyroid Nodulesand Differentiated Thyroid CancerThe American Thyroid Association Guidelines Task Forceon Thyroid Nodules and Differentiated Thyroid Cancer THYROID Volume 26, Number 1, 2016