Rebound adenotonsillar and thymic uptake of 18F-FDG in paediatric patients with lymphoma.

To evaluate the treatment-dependent changes in FDG uptake in the adenoid, palatine tonsils, and thymus in paediatric patients with lymphoma. Eight hundred PET/CT scans of 212 paediatric patients between 2007 and 2019 (mean age, 11.9 years; median follow-up, 26.2 months) were retrospectively reviewed for discernible FDG uptake in the adenoid (A+), palatine tonsils (P+), and thymus (T+). The distribution of metabolic activity in the interested lymphoid organs was examined. Statistical analysis was performed using SPSS packages. There were 513 (64 %) A + scans, 548 (69 %) P + scans, 270 (48 %) T + scans identified. The percentage of A + was 88 % at baseline, decreased to 48 % at the end of treatment, and then rebounded to 73 % during follow up; P + went from 79 % to 45 % then to 82 %; and for T + was 75 %, 21 %, 72 %. SUVmax was significantly higher (P < 0.001) in scans performed during follow-up than that of the baseline (A + 7.0 ± 3.5 vs. 5.8 ± 2.5; P + 9.4 ± 3.5 vs. 8.2 ± 2.8; T + 4.0 ± 1.4 vs. 3.4 ± 1.1). A + and P + peaked between 6-12 months of follow-up with a SUVmax of 7.6 ± 3.2, 10.6 ± 3.2, accordingly; T + peaked within 3-12 months with a SUVmax of 4.4 ± 1.4. Despite that A + and T + were more commonly seen in younger patients at any given study time, evident uptake rebound persisted in patients aged ≥16. In paediatric patients with lymphoma, evident and benign rebound adenotonsillar and thymic 18F-FDG uptake commonly occur during post-treatment surveillance.