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

¹ Department of Nuclear Medicine, Western Galilee Hospital, Nahariya, Israel; ² Rambam Health Care Campus, Haifa, Israel; and ³ B. and R. Rappaport School of Medicine, The Technion, Haifa, Israel

Correspondence: For correspondence or reprints contact: Jacqueline Jerushalmi, Department of Nuclear Medicine, Western Galilee Hospital, Box 21, Nahariya, Israel 22100. E-mail: Jacqueline.Jerushalmi{at}naharia.health.gov.il

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 ± 9.3 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 treatment-related incidence of physiologic thymic ¹⁸F-FDG avidity can reduce the number of potential pitfalls in reporting PET/CT studies in cancer patients.

Key Words: thymus • PET/CT • FDG avidity

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