title: 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 (T+ or T-, 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 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 (18)F-FDG uptake was 3.73 +/- 1.22.
  
  
  CONCLUSIONS
  Thymic (18)F-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 (18)F-FDG avidity can reduce the number of potential pitfalls in reporting PET/CT studies in cancer patients.
authors.affiliation: Department of Nuclear Medicine, Western Galilee Hospital, Nahariya, Israel. Jacqueline.Jerushalmi@naharia.health.gov.il
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Jerushalmi
authors.forename: Alex
authors.initials: A
authors.lastname: Frenkel
authors.forename: Rachel
authors.initials: R
authors.lastname: Bar-Shalom
authors.forename: Jabour
authors.initials: J
authors.lastname: Khoury
authors.forename: Ora
authors.initials: O
authors.lastname: Israel
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D002648:Child; D005260:Female; D019788:Fluorodeoxyglucose F18 / Q000493:pharmacokinetics*; D006801:Humans; D008297:Male; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000378:metabolism*; D013950:Thymus Gland / Q000378:metabolism*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D055815:Young Adult
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18