The prevalence and clinical significance of ¹⁸F–2‐fluoro‐2‐deoxy‐D‐glucose (FDG) uptake in the thyroid gland on PET or PET‐CT in patients with lymphoma

Abstract

F(18) -2-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) has become a well established tool in staging and assessing therapy response in lymphoma. Incidental thyroid uptake on PET is not uncommon and can pose a diagnostic and management challenge. We retrospectively evaluate the prevalence and clinical significance of incidental FDG uptake in the thyroid gland in patients with lymphoma. 1868 lymphoma patients underwent PET and PET-CT between August 2002 and August 2008. 52 patients (2.8%) demonstrated FDG thyroid uptake (M = 17, F = 35; mean age 63 yr). Thyroid uptake was determined as focal or diffuse, maximum standardized uptake values (SUVmax) recorded as well as SUV max ratio compared to background mediastinum activity (SUVR). Corresponding CT findings on PET-CT were evaluated independently. Results were correlated with clinical, histopathological and imaging follow-up. 30 (1.6%) patients had focal thyroid uptake. 16 (53%) had histological confirmation either by surgery (n = 7) or FNA under USS (n = 9). The final diagnosis was benign in 12/30 patients and malignant in 9/30. The malignancy risk for focal thyroid uptake was 30%. Five patients had intercurrent thyroid cancer (four papillary, one microinvasive follicular) and four had lymphomatous involvement. There was no significant difference between the mean sizes of benign (23.7 mm, range 12-34) and malignant nodules (23.6 mm, range 8-48). The mean SUVmax of malignant and benign nodules was 4.4 (range 1.8-10.1) and 3.2 (range 1.8-6.9) respectively with no statistically significant difference. 22 (1.2%) patients had diffuse FDG uptake in thyroid and benign aetiology was found in all with adequate follow-up (15/22). Focal FDG thyroid uptake on PET or PET-CT in lymphoma patients warrants further investigations. The malignancy risk is 30% either due to intercurrent thyroid cancer or lymphomatous involvement. SUVmax, SUVR and CT attenuation characteristics are not helpful in distinguishing between benign and malignant aetiologies. Diffuse thyroid uptake has a benign aetiology.

Positron emission tomography in non‐medullary thyroid cancer

“…They also reported that the maximum SUV was significantly greater for malignant than benign lesions 84 . In our own experience of 1868 patients with lymphoma, abnormal focal FDG uptake within the thyroid was seen in 1.6% of patients, 30% of which was subsequently found to be malignant, comprising a mix of intercurrent thyroid carcinomas as well as lymphomatous involvement of the thyroid, and we observed no difference in SUV in benign compared with malignant causes of focal thyroid uptake 85 …”

Lymphomas and thyroid: Bridging the gap

“…The purpose of this review is to enter into an initial exploratory phase in order to understand the real nature and impact of the problems related to lymphoma and thyroid. This is done through a careful examination of existing evidence regarding a range of issues that are central to the topic: thyroid diseases at risk of lymphoma development, primary thyroid lymphoma (PTL), and thyroid involvement in primitive nodal or extranodal lymphomas . Also, another learning objective is to understand the risk of thyroid dysfunction during therapy with new agents for lymphomas, how they will influence clinical practice, and the significance of incidental imaging–related findings …”

The management impact of clinically significant incidental lesions detected on staging FDG PET-CT in patients with non-small cell lung cancer (NSCLC): An analysis of 649 cases

“…Incidental focal thyroid uptake carries an approximately 30% risk of malignancy [14,19] while 50-80% cases of focal colorectal FDG uptake are due to pre-malignant polyps or carcinoma [15]. A marked difference in metabolic activity between the known site of malignancy and the incidentally detected lesion, or a discordant metabolic response to therapy, are also scan features suggestive of a second pathology [9].…”

“…Although the FDG avidity of incidental malignant or pre-malignant lesions tended to have a higher SUV max (mean = 8.8 vs. 4.8) in our cohort, the number of patients for each specific organ location was too small for us to make general recommendation and warrants further studies with a larger number of patients. From our previous experience of incidental focal thyroid or colorectal lesions found on PET, there is no reliable cut-off or threshold based on metabolic activity where a malignant lesion is more likely [14,15,19].…”

“…thyroiditis) whereas approximately 30% of focal uptake can be malignant [14]. Moreover, the degree of FDG avidity in terms of standardised uptake values (SUV), does not appear to be sufficiently adequate in distinguishing between benign and malignant incidental thyroid [14] and colorectal lesions [15].…”