Physiological 18F-FDG uptake in the normal adult anal canal: evaluation by PET/CT

Despite their benefit for detecting primary tumors, data for normal ¹⁸F-fluoro-2-deoxy-d-glucose (FDG) uptake in the anal canal are insufficient. Here we used positron emission tomography–computed tomography (PET/CT) to determine the uptake of FDG in the normal adult anal canal (AC) and to evaluate its clinical significance compared with that of anal cancer.

We conducted a retrospective study of–PET/CT images in the anal region, of 201 consecutive patients without symptoms or pathology taken from January 2015 to August 2019, after excluding two patients (one each with Crohn’s disease and hemorrhoid). These patients were included in the normal group, and data of eight patients with anal cancer were collected from January 2011 to August 2019 for comparison. FDG uptake was quantitatively evaluated (compared with the maximum standardized uptake value [SUV_max] to the SUV_max values of liver and distal rectum) and qualitatively (compared with background) in early and delayed phases. Normal grade 3 uptake was qualitatively defined as FDG uptake higher than the surrounding muscles.

In the normal group, mean anal canal SUV_max of early phase was: 2.26 (range 1.00–6.30), and delayed phase: 2.52 (range 1.00–8.80). Their ratios to liver SUV_max were early: 0.74 (range 0.24–2.25), and delayed: 0.81 (range 0.23–2.32); ratios to rectal SUV_max were early: 0.87 (range 0.30–1.89), and delayed: 0.90 (range 0.30–1.27). Qualitatively, 25 patients (15.4%) had normal grade 3 uptake during the early and delayed phases. In contrast, qualitative data showed that all patients with anal cancer exhibited high FDG uptake in the anal canal. The mean early- and delayed-phase values of SUV_max of the anal canal and anal cancer group were 11.09 (range 5.40–17.73) and 14.23 (range 6.70–22.85), respectively. There was a significant difference between the mean-early and -delayed anal SUV_max values of the normal grade 3 and anal cancer groups. Furthermore, the ratios to liver SUV_max were significantly different between the two groups.

PET/CT scans occasionally showed high FDG uptake in the anal canal of healthy adults. Comparing the SUV_max values of liver FDG uptake may help differentiate between normal tissue and anal cancer.