Evaluating the Change in F-Fluorodeoxyglucose Uptake in Perianal Fistulas on PET/CT over Time: A Serial Retrospective Analysis.

Rationale And Objectives: Perianal fistulas onF-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) can be an incidental site of FDG uptake in patients undergoing PET for other indications. There are no longitudinal studies describing FDG uptake patterns in perianal fistulas. Therefore, we aimed to analyze changes in FDG uptake over time in patients with incidental perianal fistulas.

Patients And Methods: Patients who underwent at least two FDG-PET/CTs between January 2011 and May 2023, with incidental perianal fistula, were retrospectively identified. We analyzed all sequential PET/CTs to determine the presence of a perianal fistula and recorded the fistula's maximum standardized uptake value (SUV). Statistical analysis compared fistula FDG-avidity in the initial versus final PET/CT examinations and assessed the correlation between initial fistula SUV and percent change over time.

Results: The study included 15 fistulas in 14 patients, with an average of 5 PET/CT examinations per patient. The average interval between the first and last PET/CT was 24 months (range: 6-64). The average initial fistula SUV (11.28 ± 3.81) was significantly higher than the final fistula SUV (7.22 ± 3.99) (p = 0.0067). The fistula SUV declined by an average of 32.01 ± 35.33% with no significant correlation between initial fistula SUV and percent change over time (r = -0.213, p = 0.443, 95% CI -0.66-0.35).

Conclusion: FDG uptake in perianal fistulas shows temporal fluctuations but follows a decreasing SUV trend, possibly indicating a relationship with inflammatory activity. Further studies with larger cohorts paired with perianal fistula pelvic MR imaging are needed to validate these observations and their utility in guiding further management.