Diffuse and diffuse-plus-focal uptake in the thyroid gland identified by using FDG-PET: prevalence of thyroid cancer and Hashimoto's thyroiditis

Unexpected focal fluorodeoxyglucose uptake in main organs; pass through or pass by?

Since the inception of fluorine-18 fluorodeoxyglucose (F-18 FDG), positron emission tomography/computed tomography (PET/CT) utilizing F-18 FDG has become widely accepted as a valuable imaging modality in the field of oncology, with global prevalence in clinical practice. Given that a single Torso PET/CT scan encompasses the anatomical region from the skull base to the upper thigh, the detection of incidental abnormal focal hypermetabolism in areas of limited clinical interest is both feasible and not uncommon. Numerous investigations have been undertaken to delineate the distinctive features of these findings, yet the outcomes have proven inconclusive. The incongruent results of these studies present a challenge for physicians, leaving them uncertain about the appropriate course of action. This article provides a succinct overview of the characteristics of fluorodeoxyglucose, followed by a comprehensive discussion of the imaging findings and clinical significance associated with incidental focal abnormal F-18 FDG activity in several representative organs. In conclusion, while the prevalence of unrecognized malignancy varies across organs, malignancies account for a substantial proportion, ranging from approximately one-third to over half, of incidental focal uptake. In light of these rates, physicians are urged to exercise vigilance in not disregarding unexpected uptake, facilitating more assured clinical decisions, and advocating for further active evaluation.

Incidentalomas in the head & neck

Incidental findings (IFs) in the head & neck are a frequent challenge to the reporting radiologist. A combination of complex anatomy, widely varied imaging techniques and the high prevalence of benign pathology, makes safe and appropriate management of head & neck IFs problematic. The non-head & neck radiologist is unlikely to have prior personal experience of the relevant specialties or current involvement with the pertinent multidisciplinary teams, creating unfamiliarity with both the clinical aspects of head & neck disease and the value of examination techniques. This triumvirate of complex anatomy, pathology and imaging creates the perfect environment for excessive investigation and overdiagnosis. In this article, the most frequently encountered and clinically relevant head & neck IFs are summarised. To reflect daily clinical practice, we will firstly consider anatomic abnormalities identified on cross-sectional imaging and ultrasound, followed by a review of PET-CT incidental findings.

Global prevalence and epidemiological trends of Hashimoto's thyroiditis in adults: A systematic review and meta-analysis

Objective

Although Hashimoto's thyroiditis is associated with cardiovascular disease and malignancy, the global status of Hashimoto's thyroiditis is not well characterized across regions. Our objective was to evaluate the prevalence and trends of Hashimoto's thyroiditis in adults in regions with different economic income levels around the world.

Methods

For this systematic review and meta-analysis, we searched PubMed, Embase, MEDLINE, Scopus, and Web of Science databases, and 48 random-effects representative studies from the inception to June 2022 were included without language restrictions to obtain the overall prevalence of Hashimoto's thyroiditis in adults worldwide. In addition, we stratified by time of publication, geographic region, economic level of the region of residence, gender, diagnostic method, etc.

Results

A total of 11,399 studies were retrieved, of which 48 met the research criteria: 20 from Europe, 16 from Asia, five from South America, three from North America, and three from Africa. Furthermore, there are two projects involving 19 countries and 22,680,155 participants. The prevalence of Hashimoto's thyroiditis was 7.5 (95%CI 5.7-9.6%), while in the low-middle-income group the prevalence was 11.4 (95%CI 2.5-25.2%). Similarly, the prevalence was 5.6 (95%Cl 3.9-7.4%) in the upper-middle-income group, and in the high-income group, the prevalence was 8.4 (95%Cl 5.6-11.8). The prevalence of Hashimoto's varied by geographic region: Africa (14.2 [95% CI 2.5-32.9%]), Oceania (11.0% [95% CI 7.8-14.7%]), South America and Europe 8.0, 7.8% (95% Cl 0.0-29.5%) in North America, and 5.8 (95% Cl 2.8-9.9%) in Asia. Although our investigator heterogeneity was high (I²), our results using a sensitivity analysis showed robustness and reliability of the findings. People living in low-middle-income areas are more likely to develop Hashimoto's thyroiditis, while the group in high-income areas are more likely to develop Hashimoto's thyroiditis than people in upper-middle-income areas, and women's risk is about four times higher than men's.

Conclusions

Global Hashimoto's thyroiditis patients are about four times as many as males, and there are discrepancies in the regions with different economic levels. In low-middle-income areas with a higher prevalence of Hashimoto's thyroiditis, especially countries in Africa, therefore local health departments should take strategic measures to prevent, detect, and treat Hashimoto's thyroiditis. At the same time, the hidden medical burden other diseases caused by Hashimoto's thyroiditis should also be done well.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier: CRD 42022339839.

The role of PET/CT in thyroid autoimmune diseases

Autoimmune thyroid diseases (AITD) are a heterogeneous group of disorders. They include, in particular, Graves' disease and Hashimoto's thyroiditis with a wide range of different functional status ranging from subclinical biochemical abnormalities to severe hyperthyroidism or severe hypothyroidism respectively. Furthermore, other conditions more frequently infectious or drug related can cause an immune reaction in the thyroid tissue. In AITDs, positron emission tomography/computed tomography (PET/CT) does not play a primary role for disease diagnosis or management, but accidental findings can occur in both symptomatic and asymptomatic patients, and they should be recognized and well interpreted. A comprehensive literature search of the PubMed databases was conducted to identify papers (systematic review, prospective and retrospective study, case report) evaluating the role of PET/CT in thyroid autoimmune diseases. Thyroid diffuse uptake of 18F-fluoro-2-deoxy-2-d-glucose ([18F]FDG) has been shown to be frequently associated with AITDs, but also with immune-induced thyroid disorders related to SARS-CoV-2 or immunotherapy, while malignant lesions more often have a focal aspect. Other radiopharmaceuticals as [68Ga]-DOTA-peptides, [68Ga]-fibroblast activation protein inhibitors (FAPIs) and [68Ga]-prostate specific membrane antigen ([68Ga]-PSMA) showed similar findings. In conclusion, PET/CT scan in AITDs does not play a primary role in the diagnosis, but the occasional finding of a thyroid uptake must always be described in the report and possibly investigated for a better patient's management.

Characteristics of malignant thyroid lesions on [18F] fluorodeoxyglucose (FDG)-Positron emission tomography (PET)/Computed tomography (CT)

Objectives

To determine the imaging variables that can best differentiate malignant from benign thyroid lesions incidentally found on F-18 FDG PET/CT scans.

Methods

All F-18 FDG PET/CT studies starting from 2011 to end of 2016 were reviewed for incidental thyroid lesions or metabolic abnormalities. Only patients who were found to have FNAB or histopathology were included. Patients with known thyroid malignancy were excluded. Patients were analyzed for age, sex, SUVmax, non-enhanced CT tissue density in mean Hounsfield units (HU), uptake pattern (focal or diffuse) and gland morphology (MNG or diffuse). A control group of 15 patients with normal thyroid glands were used to assess the tissue density in HU for normal thyroid tissue. Sensitivity, specificity, PPV, NPV and accuracy to detect malignancy were calculated. Pearson Chi-square test was used to compare categorical variables while unpaired T-test and one way ANOVA test were used to compare means of continuous variables. ROC analysis was used to assess the best cut off points for SUVmax and HU. Regression analysis was used to detect the independent predictors for malignant lesions.

Results

Biopsy was unsatisfactory or indeterminate in 4/48 patients (8%). Only 44 patients (mean age 55.2 ± 14.7; 30 females (68 %)) with unequivocal FNAB or histopathology were included for further analysis. MNG was noted in 17/44 patients (38.6 %). Thyroid malignancy was found in 16/44 (36.4 %), benign thyroid lesions in 28/44 (63.6 %). Thyroid malignancies were 12 papillary, 1 follicular, 1 Hurthle cell neoplasm and 2 lymphoma. Benign lesions were 23 benign follicular or colloid nodules and 5 autoimmune thyroiditis. Focal FDG uptake pattern was more frequently associated with malignant lesions compared to benign lesions (75 % vs. 43 %; p = 0.039). The mean SUVmax and tissue density (HU) were both higher in malignant than benign lesions (8.8 ± 8.3 vs. 3.6 ± 1.9, p = 0.024) and (48.9 ± 12.7 vs. 32.9 ± 17.5, p = 0.003) respectively. The mean HU in the control group with normal thyroid tissue was 90 ± 7.4 significantly higher than in both the benign and malignant lesions (p < 0.001). ROC analysis revealed SUVmax cutoff of >4.7 and HU cutoff of >42 to best differentiate malignant from benign lesions. The sensitivity, specificity, PPV, NPV and accuracy to detect malignancy for SUVmax>4.7 were 68.8 %, 78.6 %, 64.8 %, 81.5 & 75.0 % (p = 0.002), for HU > 42 were 81.3.0 %, 75.0 %, 65.0 %, 87.5 & 77.3 % (p = 0.0003) and for both parameters combined were 87.5 %, 60.7 %, 56.0 %, 89.5 % and accuracy of 70.5 % (p = 0.002) respectively. Only HU > 42 and SUVmax>4.7 were independent predictors for malignancy with odd ratios 8.98 and 4.93 respectively.

Conclusion

A higher tissue density (HU > 42) and SUVmax>4.7 as well as tendency for focal uptake pattern are the most significant characteristics associated with malignant thyroid lesions occasionally detected on PET/CT.

PET/CT Variants and Pitfalls in Head and Neck Cancers Including Thyroid Cancer

PET/CT imaging is a dual-modality diagnostic technology that merges metabolic and structural imaging. There are several currently available radiotracers, but ¹⁸F-FDG is the most commonly utilized due to its widespread availability. ¹⁸F-FDG PET/CT is a cornerstone of head and neck squamous cell carcinoma imaging. ⁶⁸Ga-DOTA-TOC is another widely used radiotracer. It allows for whole-body imaging of cellular somatostatin receptors, commonly expressed by neuroendocrine tumors and is the standard of reference for the characterization and staging of neuroendocrine tumors. The normal biodistribution of these PET radiotracers as well as the technical aspects of image acquisition and inadequate patient preparation affect the quality of PET/CT imaging. In addition, normal variants, artifacts and incidental findings may impede accurate image interpretation and can potentially lead to misdiagnosis. In order to correctly interpret PET/CT imaging, it is necessary to have a comprehensive knowledge of the normal anatomy of the head and neck and to be cognizant of potential imaging pitfalls. The interpreter must be familiar with benign conditions which may accumulate radiotracer potentially mimicking neoplastic processes and also be aware of malignancies which can demonstrate low radiotracer uptake. Appropriate use of structural imaging with either CT, MR or ultrasound can serve a complimentary role in several head and neck pathologies including local tumor staging, detection of bone marrow involvement or perineural spread, and classification of thyroid nodules. It is important to be aware of the role of these complementary modalities to maximize diagnostic accuracy and patient outcomes. The purpose of this article is to outline the basic principles of PET/CT imaging, with a focus on ¹⁸F-FDG PET/CT and ⁶⁸Ga-DOTA PET/CT. Basic physiology, variant imaging appearances and potential pitfalls of image interpretation are presented within the context of common use cases of PET technology in patients with head and neck cancers and other pathologies, benign and malignant.

Detection of thyroiditis on PET/CT imaging: a systematic review

PURPOSE

The incidence and clinical significance of thyroiditis detected by molecular imaging methods is a clinical challenge that is not widely investigated in the literature. The aim of this systematic review was to analyze published data about the detection of thyroiditis on PET or PET/CT using different tracers.

METHODS

A comprehensive computer literature search of the Scopus, PubMed/MEDLINE, Embase, and Cochrane library databases was conducted up to November 2019 to find relevant papers on the detection of thyroiditis by PET/CT, the metabolic appearance, and the clinical significance.

RESULTS

Twenty-six articles were selected and retrieved in full-text version. From the analyses of these studies, the following main findings have been found. Diffuse thyroid uptake of PET tracers is a relatively frequent event, ranging from 0.4 to 46.2%, and it is commonly related to benign disease. Thyroiditis is the most frequent reason for diffuse increased thyroid ¹⁸F-FDG uptake. Cases of malignant disease with a pattern of diffuse ¹⁸F-FDG thyroid uptake are less frequent. Preliminary studies show a possible role of thyroiditis detected by ¹⁸F-FDG PET/CT in evaluating treatment response and as a prognostic marker in oncological patients treated with immunotherapy. However, further studies are needed.

CONCLUSIONS

Diffuse ¹⁸F-FDG thyroid uptake is a relatively rare event commonly due to benign diseases, among which thyroiditis is the most common. The rate of neoplastic disease with diffuse ¹⁸F-FDG thyroid uptake is very low. Diffuse ¹⁸F-FDG thyroid uptake requires further investigation and clinical evaluation for the correct diagnosis. Currently, cases of diffuse thyroid uptake with non-¹⁸F-FDG radiotracer are only anecdotal.

Diffusely Increased 18F-FDG Uptake in the Thyroid Gland and Risk of Thyroid Dysfunction: A Cohort Study

The impact of incidentally identified diffuse thyroid FDG uptake on ¹⁸F-FDG PET/CT scan on the incidence of thyroid dysfunction remains unclear. We examined the association of diffuse thyroid FDG uptake with the development of thyroid dysfunction. This cohort study involved 39,098 Korean adults who were free of malignancy and thyroid disease at baseline and underwent regular health checkup examinations including an ¹⁸F-FDG whole body PET/CT scan, thyroid-stimulating hormone and free thyroxine. The participants were annually or biennially followed for up to 5 years. A parametric proportional hazard model was used to estimate the adjusted hazard ratio (HR) and 95% confidence interval (CI). Diffuse thyroid uptake was positively associated with increased risk of thyroid dysfunction in both the cross-sectional and cohort studies. During 104,261.4 person-years of follow-up, 102 incident hypothyroidism cases and 172 hyperthyroidism cases were identified. Multivariable-adjusted HR (95% CI) for incident hypothyroidism or hyperthyroidism comparing diffuse thyroid uptake to no uptake were 15.72 (9.23–26.77) and 7.38 (4.23–12.87), respectively. In this large cohort, incidentally, identified diffuse thyroid uptake on ¹⁸F-FDG PET/CT was associated with increased risk of both prevalent and incident thyroid dysfunction. Therefore, baseline and follow-up evaluations in individuals with diffuse thyroid uptake may help identify individuals with thyroid dysfunction.

Clinical significance of thyroid incidentalomas detected on fluorodeoxyglucose positron emission tomography scan (PETomas): An Indian experience

Thyroid incidentalomas (TIs) are being frequently detected on positron emission tomography (PET) scan. The risk of malignancy in these focal hot spots is substantially high as compared to incidentalomas detected on ultrasonography (USG)/magnetic resonance imaging/computed tomography (CT). Majority of the studies on the prevalence of TIs in PET and the risk of malignancy in them are retrospective and have had varied results. Very few prospective studies are available and very few Indian studies have been done on the subject. Hence, this study was undertaken to evaluate the clinical significance of TIs detected on fluorodeoxyglucose (FDG)-PET scan. The study included all patients undergoing FDG-PET scan for nonthyroid illness from October 2015 to October 2016. Twenty-three consecutive patients detected to have focal TI (FTI) were prospectively evaluated with detailed history and clinical examination, serum thyroid-stimulating hormone, total T4 and total T3 levels, USG neck, fine-needle aspiration cytology (FNAC), and surgery when indicated. The prevalence of FTI was 2.26%. Out of the 23 FTI cases, 19 patients agreed to undergo further evaluation and malignancy was detected in 5 patients (all papillary carcinomas) making a risk of malignancy of 26.3%. There was no significant correlation between CT attenuation characteristics and size of benign and malignant PETomas or between the maximum standardized uptake value (SUV_max) of benign and malignant PETomas. Hence, the risk of malignancy in thyroid PETomas is substantially high and warrants USG-guided FNAC and further work-up. Their SUV_maxvalues, size, and CT attenuation characteristics do not contribute in differentiating benign from malignant lesions.

The Role of 18F-FDG PET/CT in the Management of the Autoimmune Thyroid Diseases

Objective: It is a well-known fact that positron emission tomography (PET) is an effective tool in the assessment of thyroid focal lesions, however only few studies so far have investigated its role in monitoring of autoimmune thyroid diseases (AITDs). The aim of this study is to assess if PET scan may be useful for the assessment of the thyroid gland in patients with an AITD-Hashimoto's thyroiditis. Methods: We evaluated twenty subjects with diagnosed Hashimoto's thyroiditis (proven by presence of elevated thyroid antibodies level and by thyroid imaging). The maximum standardized uptake value (SUV-max) of the thyroid parenchyma was measured using ¹⁸F-FDG-PET/CT. Control group consisted of patients who have been in carcinoma remission for other reasons than thyroid cancer and who had been investigated by PET scan to exclude carcinoma recurrence. All control group subjects had their thyroid glands intact, none of them had a medical history of thyroid disease including thyroid nodules. AITDs had been excluded in all control group subjects. STATISTICA 13.1 software was used for statistical analysis. Results: Results: The SUV-max was significantly higher in patients with an AITD than in healthy subjects (median SUV-max 3.94 vs. 1.95; p = 0.005). Conclusions: ¹⁸F-FDG-PET/CT scan may differentiate normal thyroid parenchyma from the diffused inflammatory changes of the thyroid gland in patients with AITDs. However, the researchers must be continued.

Correlation between anti-thyroid peroxidase antibody levels and diffuse thyroid uptake of 18F-fluorodeoxyglucose in Hashimoto's thyroiditis: a retrospective study

Background

On ¹⁸F-fluorodeoxyglucose (FDG)-positron emission tomography (PET), diffuse uptake in the thyroid gland is often observed in patients with Hashimoto’s thyroiditis. In this study, we evaluated the factors associated with diffuse uptake by comparing Hashimoto’s thyroiditis patients with or without diffuse uptake in the thyroid.

Methods

A retrospective study was conducted of 18 patients with Hashimoto’s thyroiditis who underwent blood tests, thyroid ultrasonography, and FDG-PET during the period from 2014 to 2015. The patients were divided into two groups: one with diffuse thyroid uptake (group 1, n = 13) and one without diffuse thyroid uptake (group 2, n = 5). Clinical and laboratory parameters, including maximum standardized uptake in the thyroid (SUVmax), which was defined as the higher value obtained in either the right or left thyroid lobe, were compared in the two groups.

Results

The frequency of abnormal findings, such as a rough or heterogeneous pattern, was significantly higher in group 1 (p <  0.01), as were anti-thyroid peroxidase (TPO) antibody titers, anti-thyroglobulin (Tg) antibody titers, and SUVmax (p <  0.01). The frequency of hypothyroidism did not differ significantly in the two groups. Anti-TPO and anti-Tg titers were positively correlated with SUVmax (r = 0.856, p <  0.01 and r = 0.821, p <  0.01, respectively); in univariate analysis, anti-TPO titer was predictive of SUVmax (p <  0.01).

Conclusions

The results of the current study suggest that Hashimoto’s thyroiditis patients with high titers of anti-thyroid antibodies are likely to exhibit intense diffuse FDG uptake in the thyroid, and that thyroid function may be clearly impaired, even in the presence of mild FDG uptake in the thyroid.

Primary Thyroid Lymphoma: External Beam Radiation Therapy Induced Thyroiditis Mimics Residual Disease on Serial 18F-FDG PET/CT Imaging

A 67-year-old female patient with no prior history of benign thyroid disease was diagnosed with primary thyroid lymphoma and was staged with¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT). She was treated with chemotherapy and external beam radiation therapy, and a follow-up PET/CT showed significant reduction in the size of the thyroid lymphoma with persistent intense ¹⁸F-FDG uptake, which was interpreted as partial response to therapy. However, two subsequent PET/CT studies showed no change in the persistent intense ¹⁸F-FDG uptake in the thyroid and a biopsy confirmed the presence of thyroiditis with no evidence of residual lymphoma. Follow-up PET/CTs performed over the subsequent three years showed stable intensely ¹⁸F-FDG avid thyroiditis with no evidence of lymphoma recurrence. We present the imaging characteristics of a long term radiation treatment induced thyroiditis mimicking ¹⁸F-FDG avid residual disease on PET/CT.

Clinical significance of patterns of increased [18F]-FDG uptake in the thyroid gland: a pictorial review

In the diagnosis and staging of oncologic patients, [¹⁸F]-fluorodeoxyglucose positron emission tomography (FDG-PET) is well recognized as an important functional imaging modality. FDG-PET also has been used for cancer screening in healthy individuals. In general, the normal thyroid gland shows absent or low uptake on FDG-PET, which is often identified as an incidental finding on PET. Today, thyroid FDG uptake can be seen in three patterns: diffuse; focal; and diffuse-plus-focal. Diffuse thyroid uptake is mainly considered an indicator of chronic thyroiditis. Focal thyroid uptake has been associated with malignancy (range 25-50%). Diffuse-plus-focal uptake is not well recognized and might also indicate a risk of malignancy. Understanding the patterns of thyroid FDG uptake is thus important for nuclear medicine physicians or radiologists when giving recommendations to the referring physician. In this pictorial review, we show the clinical significance of different patterns of thyroid uptake on FDG-PET [PET/computed tomography (CT)], including ultrasonography (US) findings.

Malignancy rate of focal thyroid incidentaloma detected by FDG PET-CT: results of a prospective cohort study

OBJECTIVE

To evaluate the malignancy rate of focal thyroid incidentaloma (fTI) in a population of patients undergoing a 18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG PET-CT) for a non-thyroid purpose.

DESIGN

We conducted a prospective cohort study from January 2013 to November 2014. All consecutive patients referred for a FDG PET-CT were prospectively screened. Patients with known neoplastic thyroid disease were excluded from the analysis. All patients presenting one or more fTI and who accepted to benefit from a complementary thyroid ultrasonography (US) were included and managed according to the French endocrine society consensus. Prevalence of fTI in our population and malignancy rate was assessed.

RESULTS

During the inclusion period, 10,171 patients were referred for a FDG PET-CT in our center. Fifty-three patients presenting a known thyroid disease were excluded. Among the remaining 10,118 patients, 127 (1.3%) with 131 fTI were individualized. US could not be performed in 37 patients. The remaining 90 patients (92 fTI) were explored by US ± fine-needle aspiration biopsy (FNAB). US results demonstrated a nodule aspect in 80 cases of which 60 benefited from FNAB. Nineteen of 92 fTI underwent surgery with 10 malignant lesions among the 60 patients performing both US and FNAB.

CONCLUSION

The prevalence of fTI discovered on FDG PET-CT in our population was 1.3% with 10 malignant lesions among the 60 patients performing both US and FNAB.

Risk of malignancy in thyroid incidentalomas identified by fluorodeoxyglucose-positron emission tomography

BACKGROUND

Thyroid incidentalomas detected by 2-deoxy-2-18F-fluoro-D-glucose positron emission tomography/computed tomography (18F-FDG PET/CT) have been reported in 1% to 4% of the population, with a risk of malignancy of 27.8% to 74%. We performed a retrospective review of FDG-avid thyroid incidentalomas in cancer screening subjects and patients with nonthyroid cancer. The risk of malignancy in thyroid incidentaloma and its association with the maximal standardized uptake value (SUVmax) in 18F-FDG PET/CT were evaluated to define the predictor variables in assessing risk of malignancy.

METHODS

A total of 2,584 subjects underwent 18F-FDG PET/CT for metastatic evaluation or cancer screening from January 2005 to January 2010. Among them, 36 subjects with FDG-avid thyroid incidentalomas underwent further diagnostic evaluation (thyroid ultrasonography-guided fine needle aspiration cytology [FNAC] or surgical resection). We retrospectively reviewed the database of these subjects.

RESULTS

Of the 2,584 subjects who underwent 18F-FDG PET/CT (319 for cancer screening and 2,265 for metastatic evaluation), 52 (2.0%) were identified as having FDG-avid thyroid incidentaloma and cytologic diagnosis was obtained by FNAC in 36 subjects. Of the subjects, 15 were proven to have malignant disease: 13 by FNAC and two by surgical resection. The positive predictive value of malignancy in FDG-avid thyroid incidentaloma was 41.7%. Median SUVmax was higher in malignancy than in benign lesions (4.7 [interquartile range (IQR), 3.4 to 6.0] vs. 2.8 [IQR, 2.6 to 4.0], P=0.001).

CONCLUSION

Thyroid incidentalomas found on 18F-FDG PET/CT have a high risk of malignancy, with a positive predictive value of 41.7%. FDG-avid thyroid incidentalomas with higher SUVmax tended to be malignant.

Thyroid lesions incidentally detected by (18)F-FDG PET-CT - a two centre retrospective study

Background.

Incidental ¹⁸F-FDG uptake in the thyroid on PET-CT examinations represents a diagnostic challenge. The maximal standardized uptake value (SUV_max) is one possible parameter that can help in distinguishing between benign and malignant thyroid PET lesions.

Patients and methods.

We retrospectively evaluated ¹⁸F-FDG PET-CT examinations of 5,911 patients performed at two different medical centres from 2010 to 2011. If pathologically increased activity was accidentally detected in the thyroid, the SUV_max of the thyroid lesion was calculated. Patients with incidental ¹⁸F-FDG uptake in the thyroid were instructed to visit a thyroidologist, who performed further investigation including fine needle aspiration cytology (FNAC) if needed. Lesions deemed suspicious after FNAC were referred for surgery.

Results.

Incidental ¹⁸F-FDG uptake in the thyroid was found in 3.89% — in 230 out of 5,911 patients investigated on PET-CT. Malignant thyroid lesions (represented with focal thyroid uptake) were detected in 10 of 66 patients (in 15.2%). In the first medical centre the SUV_max of 36 benign lesions was 5.6 ± 2.8 compared to 15.8 ± 9.2 of 5 malignant lesions (p < 0.001). In the second centre the SUV_max of 20 benign lesions was 3.7 ± 2.2 compared to 5.1 ± 2.3 of 5 malignant lesions (p = 0.217). All 29 further investigated diffuse thyroid lesions were benign.

Conclusions.

Incidental ¹⁸F-FDG uptake in the thyroid was found in 3.89% of patients who had a PET-CT examination. Only focal thyroid uptake represented a malignant lesion in our study — in 15.2% of all focal thyroid lesions. SUV_max should only serve as one of several parameters that alert the clinician on the possibility of thyroid malignancy.

Clinical significance of patterns of incidental thyroid uptake at (18)F-FDG PET/CT

Incidental uptake of 2-[(18)F]-fluoro-2-deoxy-d-glucose ((18)F-FDG) in the thyroid gland is not uncommonly encountered in day-to-day practice of oncological (18)F-FDG positron-emission tomography/computed tomography (PET/CT). These are often felt to be "nuisance lesions" by referring clinicians and radiologists alike. However, recognition of the importance of different patterns of FDG uptake in the thyroid gland and knowledge of the possible underlying aetiologies are crucial in ensuring that patients are managed appropriately in the clinical context of their primary diagnosis, as the underlying pathological condition may be clinically important in a significant minority of such cases. This review describes the various patterns of (18)F-FDG uptake within the thyroid and discusses the clinical significance and possible impact on patient management. Incidental low-grade homogeneous diffuse increased thyroid (18)F-FDG uptake is usually seen in the patients with chronic thyroiditis, Grave's disease, and hypothyroidism. Thyroid function tests and antibody profiling are advised in these patients. Incidental focal (18)F-FDG thyroid uptake should raise the possibility of underlying malignancy. Ultrasound with or without fine-needle aspiration cytology is usually recommended for the evaluation of these lesions. Heterogeneous uptake with prominent focal uptake in the thyroid should be further evaluated to exclude malignancy.

Does the intensity of diffuse thyroid gland uptake on F-18 fluorodeoxyglucose positron emission tomography/computed tomography scan predict the severity of hypothyroidism? Correlation between maximal standardized uptake value and serum thyroid stimulating hormone levels

Context:

F-18 fluorodeoxyglucose (F-18 FDG) positron emission tomography/computed tomography (PET/CT) scan and hypothyroidism.

Aims:

The aim was to determine whether the intensity of diffuse thyroid gland uptake on F-18 FDG PET/CT scans predicts the severity of hypothyroidism.

Materials and Methods:

A retrospective analysis of 3868 patients who underwent F-18 FDG PET/CT scans, between October 2012 and June 2013 in our institution for various oncological indications was done. Out of them, 106 (2.7%) patients (79 females, 27 males) presented with bilateral diffuse thyroid gland uptake as an incidental finding. These patients were investigated retrospectively and various parameters such as age, sex, primary cancer site, maximal standardized uptake value (SUVmax), results of thyroid function tests (TFTs) and fine-needle aspiration cytology results were noted. The SUVmax values were correlated with serum thyroid stimulating hormone (S. TSH) levels using Pearson's correlation analysis.

Statistical Analysis Used:

Pearson's correlation analysis.

Results:

Clinical information and TFT (serum FT3, FT4 and TSH levels) results were available for 31 of the 106 patients (27 females, 4 males; mean age 51.5 years). Twenty-six out of 31 patients (84%) were having abnormal TFTs with abnormal TSH levels in 24/31 patients (mean S. TSH: 22.35 μIU/ml, median: 7.37 μIU/ml, range: 0.074-211 μIU/ml). Among 7 patients with normal TSH levels, 2 patients demonstrated low FT3 and FT4 levels. No significant correlation was found between maximum standardized uptake value and TSH levels (r = 0.115, P > 0.05).

Conclusions:

Incidentally detected diffuse thyroid gland uptake on F-18 FDG PET/CT scan was usually associated with hypothyroidism probably caused by autoimmune thyroiditis. Patients should be investigated promptly irrespective of the intensity of FDG uptake with TFTs to initiate replacement therapy and a USG examination to look for any suspicious nodules.

PET/CT in the management of thyroid cancers

OBJECTIVE

Thyroid cancer is the most common endocrine cancer. This review evaluates the established use of (18)F-FDG PET/CT in papillary, follicular, Hürthle cell, anaplastic, and medullary thyroid cancers. The significance of incidental diffuse and focal thyroid FDG uptake is discussed. The evolving value of non-FDG radiotracers, including (124)I, (18)F-dihydroxyphenylalanine, and (68)Ga somatostatin analogs, is summarized.

CONCLUSION

PET/CT is a valuable imaging test, in the appropriate clinical context, for the management of thyroid cancers.

Unfolding the role of PET FDG scan in the management of thyroid incidentaloma in cancer patients

Thyroid incidentaloma detected on FDG-PET scan has been reported repeatedly in the last several years, though conflicting data are reported. Our aim is to identify the incidence and outcome of incidental FDG-PET detected thyroid lesions in patients undergoing FDG-PET scan for other primary malignancies and to suggest a management algorithm. This is a retrospective review of all patients who had a FDG-PET detected incidental thyroid lesion between January 2002 and December 2009 at Peter MacCallum Cancer Center. Demographics, data relating to PET scan findings, FNA diagnoses, operative details, and histopathology were reviewed. Of the 1,034 subjects who underwent the FDG-PET study, 51 (4.9%) were identified as having thyroid incidentaloma, 31 females and 20 males with a mean age of 60 years. Thyroid malignancy was noted in 39.5% (19/48 patients) who underwent FNAB. Sixteen underwent thyroidectomy. The histopathology revealed 12 patients with papillary carcinoma, 5 with follicular carcinoma and 2 with medullary carcinoma. The high rates are in concordance with analysis of the rates published in the literature. In patients with thyroid PET incidentaloma, the incidence of primary thyroid malignancy is very high as reported in our study and based on analysis of published data, necessitating further investigation. If assessment of these incidentalomas suggests malignancy, then appropriate surgical management may be warranted according to the patient's medical condition.

Thyroid incidentalomas discovered on positron emission tomography CT scanning - Malignancy rate and significance of standardised uptake values

AIMS

Increasing use of 18F-Fluorodeoxy glucose (FDG) Positron Emission Tomography Computed Tomography (PET CT) has resulted in an increased frequency of incidentally discovered areas of focally increased FDG uptake within the thyroid gland - thyroid incidentalomas. We aimed to compare radiological characteristics of thyroid incidentalomas with cytology, histology and ultrasound findings.

MATERIALS AND METHODS

We examined all FDG PET CT scan reports for all patients undergoing this investigation over a 6 year period in a single tertiary cancer centre. All PET CT scans followed an agreed proforma allowing reports mentioning "thyroid" to be identified. Reports commenting on a positive finding within the thyroid gland were investigated further manually. Incidental mentions of thyroid with no underlying abnormality were discounted from analysis.

RESULTS

In the study period, 7221 patients underwent FDG PET CT scanning in our unit. Within this group 75 (1%) showed diffuse FDG uptake and 81 (1.1%) showed focal uptake (thyroid incidentalomas). Only 30 patients (37%) with incidentalomas had further investigation and malignancy rate was 23% (7/30). Median Standardised Uptake Values (SUV) in malignant lesions was 9.9 (range 3.5-17.8) whilst in benign lesions and diffuse lesions it was 5.4 (2.8-32) and 4.2 (2.1-25.6) respectively (p = 0.0013, Kruskal Wallis).

CONCLUSION

There remains a need to develop a standardised approach to the investigation and management of thyroid incidentalomas discovered on FDG PET CT scanning. Up to 1 in 4 of these patients will harbour thyroid malignancy.

Risk of malignancy in focal thyroid lesions identified by (18)F-fluorodeoxyglucose positron emission tomography or positron emission tomography/computed tomography: evidence from a large series of studies

Focal thyroid incidentaloma identified on (18)F-fluorodeoxyglucose positron emission tomography or positron emission tomography/computed tomography ((18)F-FDG PET or PET/CT) indicates a high risk of thyroid malignancy. A meta-analysis was performed to investigate whether the maximum standardized uptake value (SUVmax) could discriminate between benign and malignant tissues in focal lesions and to explore the cutoff value of SUVmax for the diagnosis of malignancy. A total of 29 studies were involved in this meta-analysis. The results indicated that there was no statistically significant difference in the size of the two benign and malignant groups when measured by ultrasonography (95 % confidence interval (CI), -0.79 to 0.03 min; p=0.07), while a significantly higher focal SUVmax was observed in the malignant group (95 % CI, 0.34 to 1.05; p=0.0001). In conclusion, the findings of this meta-analysis suggest that a higher focal (18)F-FDG SUVmax was associated with a higher risk of thyroid malignancy, especially at a threshold of 3.3 or more.

[Interpretation of thyroid incidentalomas in (18)F-FDG PET/CT studies]

OBJECTIVE

Thyroid findings or incidentalomas in (18)F-FDG PET/CT studies are relatively frequent, being its clinical significance subject of controversy. The aim of this study was to show our experience in the detection of thyroid incidentalomas by PET/CT studies as well as its follow up.

MATERIAL AND METHODS

A retrospective and descriptive review was conducted on patients who had thyroid incidentalomas detected in (18)F-FDG PET/CT studies between June 2010 and March 2013. Patient's medical records were reviewed for age, genre, maximum standardized uptake value (SUVmax), thyroid diseases, TSH and antithyroid antibodies levels, ultrasound, fine-needle aspiration (FNA) and cytology.

RESULTS

4085 PET/CT studies for several purposes were performed. Eighty-three of these studies (2.03%) showed thyroid incidentalomas. Thirty-seven patients showed a diffuse increase of glucose metabolism in the thyroid gland and 46 showed a focal increase of glucose metabolism. Five out of 46 patients with focal uptake were diagnosed of a neoplastic disease by cytology (11%). The SUVmax of malignant pathology did not differ from that of benign thyroid diseases (Mean: 10,26 and 5,92 respectively).

CONCLUSION

In our experience, focal thyroid incidentalomas detected in (18)F-FDG PET/CT studies are related to a significant risk of malignancy (11%). Therefore, in these situations, an ultrasound study with fine needle biopsy should be recommended. Moreover, a diffuse increase of glucose metabolism in the thyroid gland is often associated with benign thyroid pathology.

The Clinical Role of Dual-Time-Point (18)F-FDG PET/CT in Differential Diagnosis of the Thyroid Incidentaloma

Thyroid incidentalomas are common findings during imaging studies including ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) for cancer evaluation. Although the overall incidence of incidental thyroid uptake detected on PET imaging is low, clinical attention should be warranted owing to the high incidence of harboring primary thyroid malignancy. We retrospectively reviewed 2,368 dual-time-point ¹⁸F-FDG PET/CT cases that were undertaken for cancer evaluation from November 2007 to February 2009, to determine the clinical impact of dual-time-point imaging in the differential diagnosis of thyroid incidentalomas. Focal thyroid uptake was identified in 64 PET cases and final diagnosis was clarified with cytology/histology in a total of 27 patients with ¹⁸F-FDG-avid incidental thyroid lesion. The maximum standardized uptake value (SUVmax) of the initial image (SUV1) and SUVmax of the delayed image (SUV2) were determined, and the retention index (RI) was calculated by dividing the difference between SUV2 and SUV1 by SUV1 (i.e., RI = [SUV2 - SUV1]/SUV1 × 100). These indices were compared between patient groups that were proven to have pathologically benign or malignant thyroid lesions. There was no statistically significant difference in SUV1 between benign and malignant lesions. SUV2 and RI of the malignant lesions were significantly higher than the benign lesions. The areas under the ROC curves showed that SUV2 and RI have the ability to discriminate between benign and malignant thyroid lesions. The predictability of dual-time-point PET parameters for thyroid malignancy was assessed by ROC curve analyses. When SUV2 of 3.9 was used as cut-off threshold, malignancy on the pathology could be predicted with a sensitivity of 87.5 % and specificity of 75 %. A thyroid lesion that shows RI greater than 12.5 % could be expected to be malignant (sensitivity 88.9 %, specificity 66.3 %). All malignant lesions showed an increase in SUVmax on the delayed images compared with the initial images. But in the group of benign lesions, 37.5 % (6/16) showed a decrease or no change in SUVmax. Dual-time-point ¹⁸F-FDG PET/CT, obtaining additional images 2 h after injection, seems to be a complementary method for the differentiation between malignancy and benignity of incidental thyroid lesions.

Diffuse thyroid uptake incidentally found on 18F-fluorodeoxyglucose positron emission tomography in subjects without cancer history

Objective

We investigated the clinical significance of incidental diffuse thyroid uptake (DTU) on ¹⁸F-FDG PET in subjects without a history of cancer.

Materials and Methods

This study included 2062 studies from adults who underwent ¹⁸F-FDG PET as a cancer screening program. Subjects were divided into the following two groups: with (group I) or without (group II) DTU. The presence of DTU and the thyroid visual grading score were compared with thyroid function tests, serum anti-microsomal antibody (AMA) levels, and the presence of diffuse parenchymal change (DPC) on ultrasonography (USG).

Results

DTU was found in 6.6% of the scans (137/2062). Serum thyroid stimulating hormone (TSH) and AMA levels were significantly higher in group I than in group II. Increased AMA level (55.1%) and DPC (48.7%) were more frequently found in group I (p < 0.001). The proportion of subjects with any abnormal results in serum free thyroxine, triiodothyronine, TSH, or AMA levels or DPC on USG was significantly higher in group I than in group II (71.5% vs. 10.6%, p < 0.001), and was significantly and gradually increased according to the visual grading score group (0 vs. 1-2 vs. 3-4 = 10.6% vs. 58.5% vs. 90.9%, p < 0.001). TSH and is AMA levels were significantly increased according to the visual grading score.

Conclusion

The presence or degree of incidental DTU on ¹⁸F-FDG PET is closely correlated with increased serum AMA and TSH levels, and the presence of DPC on USG. Therefore, the most plausible pathological cause of DTU may be cell damage by an autoimmune mechanism.

Clinical Significance of Diffuse (18)F-FDG Uptake in Residual Thyroid Gland after Unilateral Thyroid Lobectomy

PURPOSE

We investigated the clinical significance of diffuse uptake in remaining thyroid after unilateral lobectomy for thyroid cancer.

METHODS

A total of 144 thyroid cancer patients who underwent (18)F-FDG PET/CT after lobectomy were enrolled in the present study. The PET/CT images were evaluated for the presence of diffuse (18)F-FDG uptake with maximum SUV (SUVmax) >2.0 in the residual thyroid and placed into one of two groups: with diffuse uptake and without diffuse uptake group. Clinical, laboratory, and PET/CT parameters in both groups were compared. Correlations between SUVmax of thyroid and available parameters were analyzed.

RESULTS

Forty-two of 144 patients (29.2%) had diffuse thyroid uptake (mean SUVmax: 3.2 ± 1.1). All patients with diffuse uptake and 96 (94.1%) without diffuse uptake were receiving thyroxine therapy (P = 0.09). Thyroid function tests showed that most patients were euthyroid status (78.6 vs. 85.3%, P = 0.36). TgAb levels were significantly higher in patients with diffuse uptake (338.0 ± 664.6 vs. 57.3 ± 46.4, P < 0.0001). Mean attenuation values in the diffuse uptake group were significantly lower (72.2 ± 15. vs. 97.0 ± 16.0, P < 0.0001). An inverse correlation was found between SUVmax and mean attenuation values of residual thyroid in all patients (r = -0.57, P < 0.0001) and subgroup with diffuse uptake (r = -0.31, P < 0.05).

CONCLUSION

In this study, diffuse (18)F-FDG uptake in the residual thyroid after unilateral lobectomy was a relatively frequent finding and may be associated with chronic thyroiditis. This uptake is not influenced by thyroid status or thyroxine therapy. The (18)F-FDG uptake is inversely correlated with mean attenuation value of thyroid.

The role of 18F-fluorodeoxyglucose positron emission tomography in thyroid neoplasms

(18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) has established itself as an important imaging modality in many oncological and nononcological specialties and, as a consequence, it is increasingly being used in clinical practice. Since the first report of FDG being taken up by metastatic differentiated thyroid carcinoma (DTC) cells >20 years ago, various groups of investigators have explored the potential role of FDG-PET scanning in patients with benign and malignant thyroid neoplasms. With the increasing demand for FDG-PET scanning, clinicians are faced with the challenge of managing an increasing number of FDG-PET-detected thyroid incidentalomas because their significance remains unclear. The aims of this review are to address some of these issues, specifically, the clinical significance of FDG-PET-detected thyroid incidentalomas, the ability of FDG-PET to characterize thyroid nodules, especially those with indeterminate fine needle aspiration cytology results, and the role of FDG-PET in patients with confirmed primary DTC and with suspected recurrent DTC, by reviewing the current literature.

Incidence of diffuse FDG uptake in the thyroid of patients with hypothyroidism

Positron emission tomography (PET) positive lesions are common in the thyroid. The uptake can be focal or diffuse. Diffuse thyroid uptake is thought to be indicative of autoimmune thyroiditis and not for lesions of malignant potential. Hashimoto's thyroiditis as a cause for diffusely positive thyroid glands has been demonstrated. We determine the incidence of diffuse thyroid PET positivity in hypothyroid patients, presumed to have Hashimoto's thyroiditis. The study design was retrospective database and electronic medical record review. The study setting includes tertiary care and academic health sciences center. The subjects were patients at our medical center who underwent positron emission tomography. Hypothyroid patients were identified who had total body PET imaging performed for any reason. Patients were excluded if they were not taking levothyroxine, had a history of neck surgery, neck irradiation, Graves' disease, taking lithium, thalidomide, amiodarone or interleukin. Patients remaining after the application of these exclusion criteria were presumed to be hypothyroid from Hashimoto's thyroiditis. Only 9.5% of PET scans of hypothyroid patients display diffuse thyroid activity. Only a small minority of presumed Hashimoto's thyroiditis patients will display diffuse thyroid activity after PET imaging. The etiology of this effect is unknown. Diffuse thyroid activity rarely requires surgical intervention.

PET and PET/CT in the management of thyroid cancer

The introduction of PET(-CT) has brought about a major paradigm shift in the management of thyroid carcinoma, especially from the diagnostic standpoint. From the viewpoint of patient management, the areas where it has made significant impact include the following: (1) the detection of disease focus in patients with differentiated thyroid carcinoma with elevated Tg levels and negative radioiodine scan. When localized disease is identified with F-18 FDG-PET-CT, surgery or focused radiotherapy could be utilized to eradicate the tumor; (2) the localization of disease in patients of MTC with elevated serum calcitonin levels; (3) the detection of unsuspected focal F-18 FDG uptake in the thyroid in patients undergoing whole body F-18 FDG PET for a different indication. This would prompt a workup to rule out thyroid carcinoma. The use of I-124 is evolving at this time and has been of great promise with regard to (a) its better efficacy of lesion detection and (b) the ability to provide lesion-specific dosimetry. In addition, F-18 FDG PET appears to be of potential value in patients with thyroid lymphoma in making the initial diagnosis, monitoring therapeutic response, and assessing for residual disease and/or recurrence.

Physiology and pathophysiology of incidental findings detected on FDG-PET scintigraphy

A routine feature of positron emission tomography/computed tomography (PET/CT) imaging is whole-body acquisition that results in many unexpected findings identified outside of the primary region of abnormality. Furthermore, (18)F-fluorodeoxyglucose (FDG) is a marker of glycolysis and does not specifically accumulate in malignancy. Understanding the physiology and pathophysiology of normal FDG distribution and common incidental findings is therefore essential to the physician interpreting whole-body FDG-PET/CT studies. Whereas many incidental findings are benign and of limited clinical significance, others represent uncommon manifestations of the primary malignancy, second malignancies, or various clinically significant pathologic processes. Patients with a single malignancy are at greater risk of developing synchronous or metachronous second malignancies, possibly related to exposure to shared carcinogenic agents or presence of prooncogenic mutations. The decision of how to pursue an intervention on the basis of an incidental finding is generally left to clinical judgment.

Incidental head and neck (18)F-FDG uptake on PET/CT without corresponding morphological lesion: early predictor of cancer development?

PURPOSE

To retrospectively determine whether increased/asymmetric FDG uptake on PET without a correlating morphological lesion on fully diagnostic CT indicates the development of a head and neck malignancy.

METHODS

In 590 patients (mean age 55.4 +/- 13.3 years) without a head and neck malignancy/inflammation FDG uptake was measured at (a) Waldeyer's ring, (b) the oral floor, (c) the larynx, and (d) the thyroid gland, and rated as absent (group A), present (group B), symmetric (group B1) or asymmetric (group B2). Differences between groups A and B and between B1 and B2 were tested for significance with the U-test (p < 0.05). An average follow-up of about 2.5 years (mean 29.5 +/- 13.9 months) served as the reference period to determine whether patients developed a head and neck malignancy.

RESULTS

Of the 590 patients, 235 (40%) showed no evidence of enhanced FDG uptake in any investigated site, and 355 (60%) showed qualitatively elevated FDG uptake in at least one site. FDG uptake values (SUV(max), mean+/-SD) for Waldeyer's ring were 3.0 +/- 0.89 in group A (n = 326), 4.5 +/- 2.18 in group B (n = 264; p < 0.01), 5.4 +/- 3.35 in group B1 (n = 177), and 4.1 +/- 1.7 in group B2 (n = 87; p < 0.01). Values for the oral floor were 2.8 +/- 0.74 in group A (n = 362), 4.7 +/- 2.55 in group B (n = 228; p < 0.01), 4.4 +/- 3.39 in group B1 (n = 130), and 5.1 +/- 2.69 in group B2 (n = 98, p = 0.01). Values for the larynx were 2.8 +/- 0.76 in group A (n = 353), 4.2 +/- 2.05 in group B (n = 237; p < 0.01), 4.0 +/- 2.02 in group B1 (n = 165), and 4.6 +/- 2.8 in group B2 (n = 72; p = 0.027). Values for the thyroid were 2.4 +/- 0.63 in group A (n = 404), 3.0 +/- 1.01 in group B (n = 186; p < 0.01), 2.6 +/- 0.39 in group B1 (n = 130), and 4.0 +/- 1.24 in group B2 (n = 56; p < 0.01). One patient developed a palatine tonsil carcinoma (group B1, SUV(max) 3.2), and one patient developed an oral floor carcinoma (group B1, SUV(max) 3.7).

CONCLUSION

Elevated/asymmetric head and neck FDG accumulation without a correlating morphological lesion can frequently be found and does not predict cancer development. In populations in which goitre is endemic, FDG uptake by the thyroid is common and not associated with thyroid cancer.