Quantifying 18F-Fluorodeoxyglucose Uptake in Perianal Fistulas on PET/CT: A Retrospective Analysis

Rationale and ObjectivesThe use of 18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) in assessing inflammatory diseases has shown significant promise. Uptake patterns in perianal fistulas, which may be an incidental finding on PET/CT, have not been purposefully studied. Our aim was to compare FDG uptake of perianal fistulas to that of the liver and anal canal in patients who underwent PET/CT for hematologic/oncologic diagnosis or staging. Materials and MethodsWe retrospectively identified patients who underwent FDG- PET/CT imaging between January 2011 and May 2023, where the report described a perianal fistula or abscess. PET/CTs of patients included in the study were retrospectively analyzed to record the maximum standardized uptake value (SUVmax) of the fistula, abscess, anal canal, rectum, and liver. Fistula-to-liver and Fistula-to-anus SUVmax ratios were calculated. We statistically compared FDG activity among the fistula, liver, and anal canal. We also assessed FDG activity in patients with vs. without anorectal cancer, as well as across different St. James fistula grades. ResultsThe study included 24 patients with identifiable fistulas. Fistula SUVmax (mean=10.8{+/-}5.28) was significantly higher than both the liver (mean=3.09{+/-}0.584, p<0.0001) and the anal canal (mean=5.98{+/-} 2.63, p=0.0005). Abscess fistula SUVmax was 15.8 {+/-} 4.91. St. James grade 1 fistulas had significantly lower SUVmax compared to grades 2 and 4 (p= 0.0224 and p=0.0295 respectively). No significant differences existed in SUVmax ratios between anorectal and non-anorectal cancer groups. ConclusionPerianal fistulas have increased FDG avidity with fistula SUVmax values that are significantly higher than the anal canal.