Thyroid Research and Practice

CASE REPORT
Year
: 2020  |  Volume : 17  |  Issue : 2  |  Page : 82--85

Fluorodeoxyglucose-avid thyroid incidentaloma


Deepanjan Mitra1, Sujoy Ghosh2,  
1 Department of Nuclear Medicine, AMRI Hospitals, Kolkata, West Bengal, India
2 Department of Endocrinology, Institute of Post Graduate Medical Education and Research, Kolkata, West Bengal, India

Correspondence Address:
Prof. Sujoy Ghosh
Department of Endocrinology, Institute of Post Graduate Medical Education and Research, Kolkata, West Bengal
India

Abstract

Positron emission tomography–computed tomography (PET-CT) has become a standard investigation oncology workup.18F-fluorodeoxyglucose (18F-FDG) is the main radiopharmaceuticals for most of the studies. Since malignant cells overexpress glucose transporter receptors, they internalize FDG, and FDG undergoes the first rate-limiting step of glycolysis to form FDG-6-phosphate. In a whole-body PET-CT study, a lot of incidental findings are noticed and often bear significant clinical relevance. A normal thyroid gland does not show any demonstrable FDG uptake. However, sometimes, a normal-looking thyroid gland shows up increased FDG uptake in a diffuse manner in both lobes without any obvious focal FDG-avid lesion. This is attributed to autoimmune lymphocytic thyroid diseases and hypothyroidism. A hyperthyroid gland may appear diffusely enlarged with intense FDG uptake throughout the gland suggesting the hyperfunctioning of its follicular cells. However, any focal FDG uptake in either lobe of the thyroid needs an evaluation, particularly if there is any hypodense nodule on corresponding CT study. It is observed that FDG-avid thyroid nodules can harbor malignancy in almost 33% of cases. Hence, any FDG-avid incidentaloma needs an image-guided fine-needle aspiration cytology to rule out occult malignancy.



How to cite this article:
Mitra D, Ghosh S. Fluorodeoxyglucose-avid thyroid incidentaloma.Thyroid Res Pract 2020;17:82-85


How to cite this URL:
Mitra D, Ghosh S. Fluorodeoxyglucose-avid thyroid incidentaloma. Thyroid Res Pract [serial online] 2020 [cited 2020 Nov 26 ];17:82-85
Available from: https://www.thetrp.net/text.asp?2020/17/2/82/290002


Full Text



 Introduction



Positron emission tomography–computed tomography (PET-CT) has become a standard investigative modality in recent oncology workup. The major workhorse for PET-CT study is18 F-fluorodeoxyglucose (18 F-FDG). Malignant cells overexpress glucose transporter (GLUT) receptors on their surface and hence internalize18 F-FDG. The FDG undergoes the rate-limiting step of glycolysis and forms FDG-6-phosphate. However, due to the absence of hydroxyl group in the second carbon of native glucose molecule, FDG does not undergo the further steps of glycolysis and gets trapped within the cell. The trapping of FDG is directly proportional to the number of GLUT receptors expressed and rate of glycolysis, which is increased in malignant cells (Warburg effect). This forms the basis of increased amount of FDG uptake by malignant cells compared to normals.[1]

However, FDG is not malignant specific. FDG is often taken up by infective inflammatory cells due to increased glycolytic activity by inflammatory macrophages as well.[2] Hence, careful assessment of FDG uptake is necessary in order to increase the specificity of PET-CT findings.

Many organs normally take up glucose for their metabolism such as brain, lymphoid tissue in Waldeyer's ring, myocardium (often show variable uptake), liver, kidney, and bladder (due to urinary excretion of FDG) and hence show increased uptake of FDG in a PET-CT study.

A normal thyroid gland does not show any demonstrable FDG uptake in a PET-CT study. However, sometimes, a normal-looking thyroid gland shows up increased FDG uptake in a diffuse manner in both lobes without any obvious focal FDG-avid lesion.[3] This is attributed to autoimmune lymphocytic thyroid diseases and hypothyroidism. A hyperthyroid gland may appear diffusely enlarged with intense FDG uptake throughout the gland suggesting the hyperfunctioning of its follicular cells.[3] However, any focal FDG uptake in either lobe of the thyroid needs an evaluation, particularly if there is any hypodense nodule on corresponding CT slides. It is observed that FDG-avid thyroid nodule can harbor malignancy in almost 33% of cases.[3],[4] Hence, any FDG-avid incidentaloma needs an image-guided fine-needle aspiration cytology (FNAC) to rule out occult malignancy.

We present two cases in this context to give an impression.

 Care Reports



Case 1

A 67-year-old gentleman presented with right hypochondrial pain for 15 days. He had no other symptoms and been otherwise fit. On a screening Ultrasonogram abdomen, a contracted and thickened gallbladder was detected along with a space-occupying lesion in the liver. FNAC of the liver lesion showed features of metastatic disease. A three-phase CT scan of the liver showed thick-walled contracted gallbladder with a lesion in liver parenchyma that raised a possibility of gallbladder malignancy with liver metastasis.

The patient came to our side for an18 F-FDG PET-CT scan. PET-CT was performed as per standard protocol, and a contrast-enhanced CT study was performed along with. The scan demonstrated a thick-walled contracted gallbladder on CT with no evidence of FDG uptake [Figure 1]. There was a hypodense lesion in segment IV of the liver with intense FDG uptake [Figure 2]. Bilateral lung fields showed multiple small nodules. Most of the nodules in the lung did not show any definite FDG uptake (might be due to limited resolution of PET system – small metastatic nodules in the lung often do not show any FDG uptake) except one nodule in apical segment of the lower lobe of the right lung [Figure 3]. Hypodense nodules were seen in both lobes of the thyroid, the larger one measuring 15 mm × 12 mm in size in the right lobe. The right lobe nodule showed intense FDG uptake, whereas the left lobe nodule was FDG nonavid [Figure 4].{Figure 1}{Figure 2}{Figure 3}{Figure 4}

The impression of an occult thyroid carcinoma was made due to intensely FDG-avid nodule in the right lobe. Due to the absence of any FDG uptake in the nodule in the left lobe, the possibility of malignancy appeared lower. Multiple lung nodules with variable FDG uptake appeared metastatic in nature. The FDG-avid solitary liver lesion was also reported as metastatic lesion, probably from thyroid origin. Due to no definite FDG uptake in thickened gallbladder, there was a suggestion of chronic cholecystitis.

The role of FDG PET in gallbladder carcinoma and cholangiocarcinoma is doubtful.[5] It is observed that both the malignancies may be of less metabolic potential and hence may appear less FDG avid. The liver lesion appeared intensely FDG avid on PET-CT study and thus reduced the possibility of it being a cholangiocarcinoma. If it was metastatic lesion from gall bladder then Gall bladder should have same intense FDG uptake also. Discordant finding between gallbladder and liver lesions was contradictory for it being a metastatic disease from the gallbladder. With an FDG-avid incidentaloma in the right lobe of the thyroid along with multiple lung nodules and an FDG-avid liver lesion, a possibility of occult thyroid malignancy with metastatic lung–liver disease was raised. A USG-guided FNAC of the nodule from the right lobe of the thyroid revealed follicular thyroid carcinoma. A repeat biopsy from the liver lesion revealed this to be a metastatic disease from thyroid carcinoma with follicular thyroid cells.

Case 2

A 30-year-old female presented with chronic nonproductive cough for the past 2 years. Her initial diagnosis was made as dilated cardiomyopathy which was changed to tuberculosis after seeing miliary mottling on her chest CT study. She was started with antitubercular drugs (ATDs) 1.5 years back. She did not respond to the treatment even after 4 months of her ATD therapy. The diagnosis was changed to sarcoidosis and given steroid therapy with no effect.

Recently, she had undergone a repeat chest CT scan that showed miliary mottling in bilateral lung fields with a nodule of 1.5 cm diameter in the lower lobe of the left lung. There were associated minimal bilateral pleural and pericardial effusions as well. The FNAC of the nodule could not be successful due to the small size of the lesion. Bronchoscopic lavage showed evidence of adenocarcinoma, and hence, she was diagnosed as a case of bronchogenic carcinoma.

She was referred for a PET-CT scan for staging of the disease. The FDG PET-CT scan revealed diffusely increased FDG uptake in bilateral lung fields corresponding to miliary mottling in both parenchyma [Figure 5] and [Figure 6]. No obvious FDG uptake was noted in pleuropericardial effusion. The nodule in the lower lobe of the left lung was also FDG avid [Figure 7]. However, in the whole-body PET-CT image, an FDG-avid hypodense nodule was detected in the left lobe of the thyroid [Figure 8]. In the given clinical context, we raised a possibility of an occult thyroid carcinoma with metastatic lung disease. FNAC of the thyroid nodule showed papillary thyroid carcinoma. The review of the bronchial lavage showed a papillary variety of adenocarcinoma.{Figure 5}{Figure 6}{Figure 7}{Figure 8}

 Discussion



Thyroid nodular disease is a worldwide clinical problem with prevalence ranging from 20% to 50% of the general population with higher prevalence in currently and previously iodine-deficient areas.[6] However, thyroid cancers are rare, accounting for about 2%–5% of all thyroid nodules.[7],[8],[9] The diagnostic evaluation of thyroid nodules includes laboratory tests and thyroid ultrasonography (US). US features are also employed to stratify the risk of malignancy and to select those nodules requiring US-guided FNAC.[10],[11]18 F-FDG PET is widely used to detect malignancy in many other tumor types, and FDG-avid thyroid nodules found incidentally on FDG-PET scan performed for nonthyroidal disease present a risk of malignancy ranging between 14% and 50% in most studies.[4],[12] The American Thyroid Association recommends performing a “prompt evaluation” by FNAC of all thyroid nodules showing FDG uptake even if in the absence of suspicious US findings.[13] On the other hand, the systematic use of this technique in the presurgical evaluation of thyroid nodules is currently not recommended. This is due to discordant results among studies reporting a sensitivity and specificity of FDG-PET in detecting malignancy ranging from 60% to 100% and 40% to 90%, respectively.[14],[15] These discrepancies seem to be related to the small samples studied and/or to the differences in the inclusion criteria. Most studies reported a low positive predictive value (around 30%) essentially due to a high number of false-positive results represented by benign nodules with FDG uptake. The reported negative predictive value is usually high (80%–100%) essentially due to the small proportion of thyroid cancers among nodules, suggesting that FDG-PET could be useful to avoid unnecessary surgery for benign nodules.[16]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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