The widespread adoption of high-resolution ultrasound over the past 3 decades has led to a large increase in detection of small asymptomatic thyroid nodules in patients, but according to the American Cancer Society, the rate of deaths due to thyroid cancer has not been declining.1 By some estimates, the majority of thyroidectomies performed to evaluate suspicious or indeterminate thyroid nodules have been performed on patients without a malignancy.2
Using F18-Fluorodeoxyglucose (FDG) positron emission computed tomography (PET) when combined with sonographic features may improve identification of cytologically indeterminate thyroid nodules that are benign and can rule out the need for invasive lobectomy, according to findings from a new study published in Thyroid.2 The authors combined an original retrospective study at a single site with 8 previous published prospective studies to show that indeterminate nodules with little or no FDG uptake by PET have a low risk of malignancy, and the test has a negative predictive value of 94%.2
“In patients with cytologically defined indeterminate thyroid nodules in whom diagnostic lobectomy is considered, the combination of benign, low, or intermediate sonographic appearance and negative results by FDG-PET identified patients at low risk for clinically significant thyroid cancers,” said study author, Michele M. Merten, APRN, CNP, from the Division of Endocrinology at the Mayo Clinic in Rochester, Minnesota. “Therefore, observation with surveillance may be a reasonable alternative to immediate diagnostic lobectomy.”
A non-invasive clinical tool that would allow the safe and accurate identification of nodules with minimal risk for malignant growth is lacking, noted Joshua D. Safer, MD, an associate professor of medicine and molecular medicine at Boston University School of Medicine in Boston, Massachusetts, who was not involved in the study but commented as a member of the EndocrineWeb editorial board. “Opportunities to reassure more patients that their nodules are safe to be followed would be attractive.”
Cytologically defined indeterminate thyroid nodules typically have been evaluated using surgical lobectomy or molecular profiling, which test for the presence of gene expression patterns or genetic mutations commonly associated with different types of thyroid cancers. However, surgery poses undesirable, increased risk to patients, and molecular profiling is known to perform inconsistently between sites and cannot rule out certain types of tumors, such as follicular thyroid carcinoma and Hürthle cell carcinoma.
The study authors examined FDG-PET--a technology already used for the detection and staging of various solid tumors--for its utility as a non-invasive technique for evaluating indeterminate thyroid nodules. Cancerous cells accumulate the radiolabeled glucose analog, FDG, at an increased rate compared to normal cells due to alterations in metabolism that result from malignant transformation. The result is higher standardized uptake values (SUVs) detected by PET in malignant nodules compared to benign growths.
The use of this diagnostic technique may allow clinicians to discriminate between benign and malignant tumors. Low-risk nodules could then be followed over time, instead of being surgically removed.
“Observation, rather than surgery, could be a reasonable and safe option for those patients with low SUVs given the high NPV of such findings,” Ms. Merten told EndocrineWeb.
However, the results of this study may not be generalizable in the near-term, according to Dr. Safer. “Unfortunately, FDG-PET is expensive and suffers from a great degree of variation in results among operators and centers,” he said. “It seems likely that in many centers, negative studies would not be sufficiently reassuring to avoid surgery.”
A total of 51 patients with indeterminate thyroid nodules suspicious for follicular neoplasm or Hürthle cell neoplasm were examined in this retrospective, single-site study. Additionally, the authors evaluated findings from similar prospective studies using FDG-PET to evaluate indeterminate thyroid nodules from the medical literature.
Results from the treated group suggested that FDG-PET had an NPV of 90% in all indeterminate thyroid modules and 95% in nodules larger than 1 cm. The 8 prospective studies identified by the authors reported NPVs ranging from 57% to 100%, which is how the authors arrived at a value of 94%.
Caveats to this study, identified by the authors, included an enrichment of FDG-avid nodules due to referral bias in the study participants. They also noted that a large majority of the participants in this retrospective study received FDG-PET due to the presence of other malignant lesions, which may have reduced the amount of FDG available for uptake by the patients’ thyroid nodules.
1. American Cancer Society. Key statistics for thyroid cancer. Available at: http://www.cancer.org/cancer/thyroidcancer/detailedguide/thyroid-cancer-key-statistics. Accessed on November 4, 2016.
2. Bahl M, Sosa JA, Nelson RC, Hoang JK. Imaging-detected incidental thyroid nodules that undergo surgery: a single-center experience over 1 year. AJNR Am J Neuroradiol. 2014 Nov-Dec;35(11):2176-80.
3. Merten MM, Castro MR, Zhang J, Durski JM, Ryder M. Examining the Role of Preoperative Positron Emission Tomography/Computerized Tomography (PET/CT) in Combination with Ultrasonography in Discriminating Benign from Malignant Cytologically Indeterminate Thyroid Nodules. Thyroid. 2017;27(1): 95-102.