FDG uptake in the morphologically normal thymus: comparison of FDG positron emission tomography andCT

Abstract

The purpose of this study was to evaluate the correlation between fluorine-18 fluorodeoxyglucose (FDG) thymic uptake and a normal appearing thymus on CT. Non-attenuation corrected FDG positron emission tomography (PET) data from 94 young persons (mean age 25.4 years, range 18-29 years) with a normal thymus diagnosed on CT were retrospectively evaluated. No subject had clinical symptoms suggestive of thymus-related disease or mediastinal tumour (follow-up period 6-69 months). PET images were visually assessed and the count ratio between the thymus and the lung (T/L ratio) was calculated. Increased FDG uptake occurred in 32 (34%) subjects. In these 32 cases, the T/L ratio was 2.86+/-0.49 (range 2.02-3.99). In 86 subjects whose CT images were available to calculate the CT attenuation of the thymus (CAT), the CAT value was -17.5+/-45.7 HU (range -103.6 HU to 79.9 HU). The T/L ratio correlated with the CAT value (r=0.58). CAT values in subjects with positive PET findings were significantly higher than CAT values in subjects with negative PET findings (p<0.001, unpaired t-test). These results suggest that even in young adults, if the thymus has a relatively high CT attenuation value, the presence of physiological thymic uptake in FDG-PET is a normal variant. In this study, the diagnosis of normal thymus was based on CT appearance and clinical course. Further studies are needed to clarify the relationship between histopathology and FDG uptake in the thymus.

Physiologic Thymic Uptake of 18F-FDG in Children and Young Adults: A PET/CT Evaluation of Incidence, Patterns, and Relationship to Treatment

“…This percentage was slightly higher than the previously reported range of 11%-25% thymic enlargement demonstrated on CT (14,16) and similar to the 11%-36% reported for 67 Ga uptake (17)(18)(19). This incidence is, however, lower than the previously reported 31%-50% for 18 F-FDG PET, most probably because previous studies included mainly children and adolescents, with none or few young adult patients (2,10,15,20). 18 F-FDG uptake in the thymus has been previously associated mainly with treatment-related hyperplasia in children with cancer and has been only sporadically described in adult patients after chemotherapy (2,21).…”

“…Although as a group the T1 population was significantly younger, the incidence reached 12% in patients in the 20-to 40-y age group; furthermore, 18 F-FDG uptake in the thymus was found in a previously unknown 8% of patients in the fourth decade of life. A slow process of fatty infiltration of the thymus can be a possible explanation for the presence of 18 F-FDG uptake in older patients (10). An additional reason for the higher incidence of T1 studies in adult patients may be related to the use of PET/CT, compared with PET only, in previously published studies.…”

“…Patients were instructed to fast, except for noncaloric fluid intake, for at least 5 h before the injection of 18 F-FDG (370-555 MBq [10][11][12][13][14][15]). Children were administered a weight-adjusted dose of 18 F-FDG (5.328 MBq/kg [0.144 mCi/kg]).…”

Evaluation of normal fatty replacement of the thymus with chemical‐shift MR imaging for identification of the normal thymus

“…However, normal physiologic uptake in the thymus in PET with FDG was identified in all subjects prior to puberty and in over 30% of subjects in early adulthood (17)(18)(19).…”

A pilot study of the use of 2‐[18F]‐fluoro‐2‐deoxy‐D‐glucose–positron emission tomography to assess the distribution of activated lymphocytes in patients with systemic lupus erythematosus

“…Although age might be a confounding factor, thymic uptake was seen in patients with active disease even in their fourth decade of life, when the thymus is usually involuted (10). In fact, 3 of 5 patients with thymic uptake were age 29 years or older.…”