HYPHYCA: a prospective study in 613 patients conducting a comprehensive analysis for predictive factors of physiological 18F-FDG anal uptake

Abstract

Background: Anal cancer is a relatively rare tumor of which incidence increases in developed countries. 18F-FDG PET has been increasingly used for its post radio-chemotherapy evaluation. However, several authors have reported the risk of local false-positive findings leading to low specificity and positive predictive values. These false-positive results could be due to post-radiotherapy inflammation or infection but certainly also to physiological anal canal uptake that is observed on a regular basis in clinical practice. The purpose of this prospective study (NCT03506529; HYPHYCA) was therefore to seek predictive factors of physiological anal canal hypermetabolism.

Materials and methods: Over a 2-month period, patients aged 18 years old and more, referred for 18F-FDG PET-CT at two EARL-accredited PET centers were included, after obtaining their informed and written consent. They were asked to fill in a questionnaire including seven closed questions about usual intestinal transit, ongoing medications relative to intestinal transit, history of digestive, and anal and/or pelvic diseases. Age, gender, and body mass index (BMI) were recorded. A single nuclear medicine physician visually and quantitatively analyzed anal canal uptake (SUVmax_EARL) and assessed visual rectal content (air, feces, or both) and the largest rectal diameter (mm).

Results: Six hundred and thirteen patients were included (sex ratio F/M = 0.99) and 545 (89%) questionnaires were entirely completed. Significantly more males presented anal canal hypermetabolism (sex ratio (M/F) = 1.18 versus 0.85, p = 0.048). Moreover, patients with anal canal hypermetabolism had higher BMI (27.6 (5.7) kg/m2 versus 23.9 (4.5) kg/m2, p < 0.0001), higher rate of hemorrhoid history (43% versus 27%, p = 0.016), and higher rate of rectum filled with only feces (21% versus 12%, p = 0.019) as compared to patients with no anal canal uptake. On logistic regression, all these variables were found to be independent predictors of the occurrence of an anal canal hypermetabolism. Odds ratio were 1.16 (1.12-1.20) per unit of BMI (kg/m2) (p < 0.0001), 1.48 (1.04-2.11) for males (p = 0.030), 1.64 (1.10-2.45) for hemorrhoids history (p = 0.016), and 1.94 (1.147-3.22) for the rectum filled with only feces (p = 0.010).

Conclusion: According to our study, the predictive factors of physiological anal canal hypermetabolism are high BMI, male gender, hemorrhoid history, and rectum filled with only feces. This may pave the way to a more specific interpretation of post radio-chemotherapy PET evaluations of anal canal cancer, provided that other studies are conducted in this specific population.

Trial registration: This prospective study was registered at Clinicaltrial.gov: NCT03506529; HYPHYCA on April 24, 2018.

Keywords: 18F-FDG; Anal cancer; PET; Physiological uptake.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Additional details on PET examinations indications

Fig. 2

Visual versus quantitative analysis of anal canal uptake. Data are presented as Tukey’s boxplots and o represents outliers

Fig. 3

Linear regression of SUVmax_EARL and body mass index (kg/m2)

Fig. 4

Illustration of rectal content classification and its association with anal canal uptake. a Empty rectum. b Air rectal content classified 0. c Fecal rectal content classified 1. d Mixed rectal content classified 2

Fig. 5

Axial EARL-accredited images centered on the anal canal (left panel) and the liver (right panel) of a patient classified with significant anal canal hypermetabolism on visual analysis who would be classified “negative” using liver uptake as reference (a) and a patient classified with basal anal canal hypermetabolism who would be classified as “positive” using liver uptake as reference (b)