Does a Standardized Distance Cut-off Accurately Predict the Length of the Rectum? Using MRI to Analyze the Height of the Peritoneal Reflection

Aim: Standardized distance cut-offs are frequently utilized as a surrogate in determining whether neoadjuvant therapy is needed in treating upper rectal cancer. With patient-to-patient variation in rectal length this method can prove inaccurate. This article establishes the range of natural variation in the height of this structure in patients and if standardized measurement cut-offs are inappropriate in characterizing its location. Method: Retrospective chart review, from 2015 to 2019, of patients in whom pre-operative rectal cancer staging magnetic resonance imaging was undertaken. Measurement from the anal verge to the anterior peritoneal reflection (APR) and sigmoid take-off (ST) was performed. Differences between genders were compared and distance measurement correlations with height, weight, age, and body mass index were investigated. Results: Mean overall height of the APR was 11.9±2.0 cm from the anal verge. When genders were compared this measurement was 12.3±2.1 cm in males and 11.3±1.5 cm in females (p=0.003). Overall, the 75th, 90th, and 95th percentile of the height of the APR was 13.2 cm, 14.5 cm, and 15.5 cm, respectively. Average height of the rectum at the ST from the anal verge was 19.3±2.4 cm and 14.3±2.1 cm, for men and women, respectively. No anthropometric measurements had a strong correlation with APR height. Conclusion: Males possess a higher APR and ST over females. This difference resembles the difference between genders in anal canal length. Currently utilized standardized rectal length cut-offs may inappropriately categorize patients as rectal cancer whose tumor may lie above the peritoneal reflection.