Physiologic thymic uptake of ¹⁸F-FDG in children and young adults: A PET/CT evaluation of incidence, patterns, and relationship to treatment

¹⁸F-FDG uptake in the thymus, mainly related to hyperplasia after chemotherapy, has been described. Thymic uptake can challenge the accurate assessment of cancer patients by ¹⁸F-FDG imaging. The present study defines the incidence, patterns, and intensity of thymic ¹⁸F-FDG uptake in relationship to age and time after treatment in a large cohort of patients. Methods: A total of 559 consecutive ¹⁸F-FDG PET/CT studies in 160 patients (86 men, 74 women; age, 3-40 y) performed at baseline, during treatment, at the end of treatment, and during follow-up were retrospectively reviewed. PET/CT studies were assessed for the presence or absence (T+ or T-, respectively), pattern, and intensity (SUVmax) of increased ¹⁸F-FDG uptake in the anterior mediastinum, localized by the CT component to the thymus. The overall incidence of ¹⁸F-FDG avidity in the thymus in relationship to the patient's age and time after treatment administration were statistically evaluated. Results: There were 137 of 559 T+ studies (25%), with equal sex distribution. T+ studies were found in significantly younger patients (20.6±3 y vs. 27.4±8.4 y, P< 0.001). Most T+ patients (60%) showed an inverted V pattern of thymic uptake, with additional unilateral mediastinal extension in 24% and focal midline uptake in 16% of studies. T+ studies were encountered in 80% of patients younger than 10 y, compared with 8% of patients in the 31- to 40-y age group. There were 17% T+ studies at baseline, 6% during treatment, 8% at the end of treatment, and 27%-40% during follow-up. The average SUVmax of thymic ¹⁸F-FDG uptake was 3.73 ± 1.22. Conclusion: Thymic ¹⁸F-FDG uptake was found in 28% of the present study population, more frequently after treatment. T+ patients were significantly younger. Thymic uptake was found in 73% of untreated patients up to the age of 13 y and in 8% of patients in the fourth decade of life. Knowledge of this age- and treatmentrelated incidence of physiologic thymic ¹⁸F-FDG avidity can reduce the number of potential pitfalls in reporting PET/CT studies in cancer patients.