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

Abstract

18 F-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 18 F-FDG imaging. The present study defines the incidence, patterns, and intensity of thymic 18 F-FDG uptake in relationship to age and time after treatment in a large cohort of patients. Methods: A total of 559 consecutive 18 F-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 (T1 or T2, respectively), pattern, and intensity (SUVmax) of increased 18 F-FDG uptake in the anterior mediastinum, localized by the CT component to the thymus. The overall incidence of 18 F-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 T1 studies (25%), with equal sex distribution. T1 studies were found in significantly younger patients (20.6 6 9.3 y vs. 27.4 6 8.4 y, P , 0.001). Most T1 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. T1 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% T1 studies at baseline, 6% during treatment, 8% at the end of treatment, and 27%-40% during follow-up. The average SUVmax of thymic 18 F-FDG uptake was 3.73 6 1.22. Conclusion: Thymic 18 F-FDG uptake was found in 28% of the present study population, more frequently after treatment. T1 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 18 F-FDG avidity can reduce the number of potential pitfalls in reporting PET/CT studies in cancer patients.

Rebound Thymic Hyperplasia Detected by18F-FDG PET/CT After Radioactive Iodine Ablation Therapy for Thyroid Cancer

“…However, incidence, age and sex distributions, and affecting factors of RHT after RIAT have not been clearly defined. According to our results, the overall incidence of RTHP was approximately 1.4%, which is significantly lower than that of a previous study carried out by Jerushalmi et al (25); this resulted from differences in age distribution and the cause of RTHP. The upper age limit in the previous study was 40 years, and total fatty involution of the thymus occurs by this time.…”

Evaluation of thymic tumors with 18F-FDG PET-CT: A pictorial review

“…In cases of thymic hyperplasia the thymus usually losses its bilobed appearance and appears oval in CT. In one series the classic triangle or inverted V shape of thymic 18F-FDG uptake was observed in 60% of patients in that series, unilateral and focal midline uptake represented additional patterns of thymic uptake in 40% of patients (12). It has been suggested that standardized uptake value (SUVmax) may be useful in differentiating thymic rebound from mediastinal lymphoma, with a thymic SUVmax !3.4 being a strong predictor of lymphoma (15) (Fig.…”

PET/CT scanning with a high HIV/AIDS prevalence

“…This must also be interpreted in the light of evidence that thymic uptake is often also seen well into adulthood on PET in HIV negative patients [27]. Without this knowledge these studies may be falsely reported as being positive for disease in the anterior mediastinum.…”