S1987 Go With Your Gut Feeling: A Case of Invasive Colonic Squamous Cell Carcinoma Not Detected by DNA-Based Stool Testing

Abstract

Introduction: Though less common, colonic squamous cell carcinoma is a type of colorectal cancer. Current colorectal cancer screening recommendations for average-risk patients include the use of stool DNA-based testing as an alternative to screening colonoscopy. This clinical vignette is an example of colonic squamous cell carcinoma that went undetected by a stool DNA-based colorectal cancer screening test. Case Description/Methods: 57 year-old female with no prior medical history presented to outpatient gastroenterology office for evaluation of tenesmus, mucus in stool with associated diarrhea and LLQ pain of 3 months duration. Physical exam was unremarkable. Recent CBC, BMP, and albumin were within normal limits. Patient had undergone average-risk colorectal cancer screening with a stool DNA-based test 11 months prior to presentation (8 months prior to symptom onset) with a negative result. Patient was asymptomatic at the time of the screening test. Due to her new symptoms, it was recommended she undergo infectious stool studies and colonoscopy. C difficile screen, bacterial stool culture and parasitology exam were negative. Colonoscopy revealed a 6-cm partially circumferential frond-like/villous, infiltrative and ulcerated partially obstructing large mass 4-11cm from the anal verge (Figure 1A, B). Biopsy was positive for invasive squamous cell carcinoma. The patient was referred to oncology and completed definitive chemo-radiation therapy with no evidence on residual disease on most recent imaging. Discussion: This case demonstrates an average-risk patient who underwent stool DNA-based testing for colorectal cancer screening that was negative with subsequent presentation and diagnosis of invasive squamous cell carcinoma of the colon in less than one year’s time. It highlights the importance of colorectal cancer screening and the ability of colonoscopy to detect and diagnosis colorectal cancer. DNA-based stool testing is an alternative non-invasive screening test to colonoscopy in average-risk patients. The current data supports a false negative rate of DNA-based stool testing to be less than 1 in 1500. However, the presence of a false negative test provides patients with a false sense of confidence and reassurance that they do not have cancer. It is imperative that for any new lower GI symptoms or other clinical suspicion for colorectal malignancy, the patient should still undergo evaluation with colonoscopy – endoscopic evaluation should not be deferred because of a recent negative DNA-based stool test.Figure 1.: A. Colon with mass, B. Detailed view of ulcerated mas.