A standard colonoscopy can occasionally detect anal cancer, but it was not designed for that purpose and frequently misses the anal canal entirely. The colonoscope is optimized to examine the colon and rectum; as it is withdrawn, it typically passes through the short anal canal too quickly and at an angle that leaves much of the tissue unseen. Dedicated techniques like rectal retroflexion or anoscopy during the procedure improve visualization dramatically, but they are not always performed. The result is that anal cancers found during routine colonoscopy tend to be incidental discoveries rather than reliable screening outcomes.
Why the Colonoscope Has a Blind Spot
The anal canal is only about three to four centimeters long, and it sits at the very end of the gastrointestinal tract. When an endoscopist withdraws the colonoscope after examining the colon and rectum, the instrument moves through this short stretch of tissue in seconds. The optics of the scope, designed to look forward and slightly to the side inside the spacious colon, do not wrap around to inspect the narrow, angled walls of the anal canal the way a purpose-built tool would. Endoscopists themselves have raised concerns that the colonoscope does not give an adequate view of this area.1PubMed Central. Anal neoplasm in colonoscopy: What endoscopists need to know
This matters because anal cancer, while relatively uncommon compared to colorectal cancer, arises in tissue the colonoscope is poorly positioned to inspect. The two cancers are often conflated by patients, but they develop in different tissue types and in different locations. Colorectal cancer arises from glandular cells higher in the bowel, while most anal cancers are squamous cell carcinomas originating in the thin lining of the anal canal. A colonoscopy that finds nothing in the colon provides no reassurance about the anal canal unless the endoscopist specifically examined it.
How Retroflexion Changes the Picture
One technique that significantly improves anal canal visualization during colonoscopy is rectal retroflexion, where the endoscopist bends the tip of the scope back on itself while still inside the rectum to look downward at the anal canal from above. Combined with anoscopy, this approach can provide what researchers describe as “perfect visualization” of the anal canal.1PubMed Central. Anal neoplasm in colonoscopy: What endoscopists need to know
A prospective study across three centers looked at what retroflexion actually adds. Among patients where small polyps and hemorrhoids were excluded, distinct lesions in the anorectal area were found in about 3.4% of patients. Of those, roughly a third were detected only on the retroflex view and none were found only on the standard straight view, meaning that without bending the scope back, those lesions would have been missed entirely.2PubMed Central. Diagnostic Yield and Therapeutic Impact of Rectal Retroflexion: A Prospective, Single-Blind Study Conducted in Three Centers The lesions caught only by retroflexion included sessile polyps averaging about 7.5 mm, large enough to warrant removal and pathological examination.
The catch is that retroflexion during colonoscopy is not universally performed. Some endoscopists routinely do it; others skip it, particularly when they consider the procedure finished once the scope has been withdrawn past the rectum. There is no binding requirement in most settings that every colonoscopy include a careful inspection of the anal canal. If you are at elevated risk for anal cancer and are undergoing a colonoscopy, it is reasonable to ask your endoscopist beforehand whether they will perform retroflexion and inspect the anal canal during withdrawal.
Incidental Findings During Routine Colonoscopy
Despite the limitations, colonoscopy does catch anal neoplasia sometimes, purely by chance. A study evaluating over 22,000 colonoscopies that included anal examination found 88 lesions suspected of neoplasia in the anal canal, amounting to about 0.5% of procedures. Of those, 23 turned out to be confirmed neoplasias, including 9 squamous cell carcinomas and 5 adenocarcinomas.1PubMed Central. Anal neoplasm in colonoscopy: What endoscopists need to know Those numbers are small in absolute terms, but for the individual patients whose cancers were caught, the discovery was potentially life-saving.
A separate analysis of 121 patients found to have incidental anal squamous dysplasia during routine colonoscopy broke down the findings further. Endoscopists detected low-grade precancerous changes in about 53% of cases, high-grade changes in 43%, and actual squamous cell carcinoma in roughly 4%.3PubMed Central. Detection of anal squamous dysplasia at routine colonoscopy in 121 patients: implications for anal cancer screening The high proportion of high-grade lesions is striking because those are the precursors most likely to progress to invasive cancer if left untreated. Finding them incidentally during colonoscopy, even if the procedure was not designed for that purpose, gave those patients a head start on treatment they might not have gotten otherwise.
The Misdiagnosis Problem
One of the more frustrating aspects of anal cancer detection is how often it gets mistaken for something benign. Symptoms like bleeding, pain, and a lump near the anus overlap almost perfectly with hemorrhoids and anal fissures, which are far more common. A study examining diagnostic delays found that at first medical visits after symptom onset, a rectal examination was performed in only about 54% of patients. Roughly 27% were initially told they had hemorrhoids, and when that misdiagnosis was made, patients needed substantially more visits before the actual cancer was identified.4PubMed Central. Reasons for delays in diagnosis of anal cancer and the effect on patient satisfaction
Patients themselves sometimes contribute to the delay. In the same study, about 19% of patients waited more than six months before seeking medical attention after symptoms appeared, and 40% believed they were personally responsible for the delay in diagnosis.4PubMed Central. Reasons for delays in diagnosis of anal cancer and the effect on patient satisfaction Embarrassment about anal symptoms is a real barrier. The discomfort of discussing the area, combined with the assumption that bleeding is “just hemorrhoids,” creates a pattern where early, treatable cancers are allowed to grow.
Case reports reinforce this pattern. Small and early anal cancers are not uncommonly misdiagnosed as benign conditions like chronic anal fissures, sometimes for extended periods before the correct diagnosis is made.5GE – Portuguese Journal of Gastroenterology. A Case Series of Anal Carcinoma Misdiagnosed as Idiopathic Chronic Anal Fissure The practical lesson here is that any anal symptom that does not resolve with standard treatment for hemorrhoids or fissures within a few weeks deserves further investigation, including a physical examination and potentially imaging or biopsy.
Who Should Be Screened Specifically for Anal Cancer
For most people, routine anal cancer screening is not recommended. The cancer is uncommon enough in the general population that mass screening would produce more false alarms than useful catches. But certain groups face dramatically higher risk, and for them, targeted screening makes sense.
The International Anal Neoplasia Society published consensus guidelines recommending that men who have sex with men and transgender women living with HIV begin screening at age 35. For other people living with HIV, as well as men who have sex with men and transgender women who are not HIV-positive, screening was recommended starting at age 45. Solid organ transplant recipients were advised to begin screening ten years after transplant.6PubMed. International Anal Neoplasia Society’s consensus guidelines for anal cancer screening Australian guidelines have arrived at similar age thresholds, reinforcing the international alignment on who qualifies for proactive screening.7PubMed. Australian guidelines for anal cancer screening using anal human papillomavirus testing with cytology triage in people living with HIV
The common thread among high-risk groups is human papillomavirus. HPV, particularly the high-risk strains, drives the vast majority of anal squamous cell carcinomas through a slow progression from normal tissue to precancerous changes and eventually to invasive cancer. Conditions that suppress the immune system, whether HIV, immunosuppressive drugs after transplant, or other causes, reduce the body’s ability to clear HPV and control abnormal cell growth, raising the likelihood that precancerous lesions will progress.
Precancerous Changes and Why They Matter
Anal cancer does not typically appear out of nowhere. It usually develops through a recognizable precursor stage called anal intraepithelial neoplasia, a premalignant change in the mucosal lining. Progression from these precancerous changes to invasive cancer happens over several years in some individuals, which creates a window for intervention.8PubMed Central. Anal intraepithelial neoplasia: A review of diagnosis and management Screening programs for high-risk populations aim to catch disease at this precancerous stage, when treatment can prevent cancer from ever developing.
This is where colonoscopy’s limitations become most relevant. Precancerous changes are often flat, subtle, and not visible to the naked eye during a standard scope withdrawal. They require either magnification, special staining techniques, or high-resolution imaging to be identified. A colonoscope rushing past the anal canal on its way out simply cannot detect what it cannot see, and these early changes are precisely the ones worth finding.
Dedicated Tools for Anal Cancer Screening
The gold standard for examining the anal canal in high-risk patients is high-resolution anoscopy, a procedure specifically designed for this purpose. It uses a magnifying scope and acetic acid or Lugol’s iodine solution applied to the anal lining to highlight abnormal tissue that would otherwise be invisible. Without high-resolution anoscopy, only a small percentage of suspicious lesions are identified.9PubMed Central. High-resolution anoscopy: Unchartered territory for gastroenterologists?
Before high-resolution anoscopy, many screening programs use a triage step. Anal cytology, sometimes called an anal Pap test, collects cells from the anal canal to look for abnormalities under a microscope. HPV testing can be done on the same sample. An Italian study found that combining both tests raised the sensitivity for detecting significant precancerous changes to 100%, catching all cases that anal cytology alone would have missed. The trade-off was lower specificity, meaning more false positives that required follow-up anoscopy, but the researchers considered this worthwhile given the consequences of missed disease.10PubMed Central. PAP-HPV Co-Testing in Anal Cancer Screening: An Italian Experience
A systematic review and meta-analysis examining different biomarker strategies found that DNA-based high-risk HPV testing offers the best sensitivity as a primary screening tool, while mRNA-based HPV testing provides higher specificity. Importantly, both can be performed on the same sample collected in liquid-based cytology, eliminating the need for a second patient visit.11The Lancet Regional Health. Accuracy of DNA HRHPV, mRNA HPV, DNA HPV16 and p16 staining biomarkers in anal canal smears for identifying anal high-grade squamous intraepithelial lesions and cancer The emerging consensus favors a multi-step approach: HPV testing as the initial screen, followed by cytology or high-resolution anoscopy for those who test positive.
Why Early Detection Changes Outcomes
The argument for finding anal cancer early is not abstract. A large French nationwide cohort study compared outcomes between early-stage and locally advanced anal cancer. Three-year overall survival was about 92% in the early-stage group compared to roughly 78% in the locally advanced group. Disease-free survival showed an even starker gap, at about 84% versus 64%.12PubMed. Treatment, outcome, and prognostic factors in non-metastatic anal cancer: The French nationwide cohort study FFCD-ANABASE Male sex, advanced stage, and poor overall health at diagnosis were all associated with worse outcomes.
Anal cancer treatment typically centers on chemoradiation rather than surgery for most cases, which preserves the sphincter and avoids a permanent colostomy. But the intensity and side effects of treatment scale with how advanced the cancer is at diagnosis. Catching disease early means less aggressive treatment, fewer complications, and a better quality of life during and after therapy.
There is also an interesting clinical wrinkle in the relationship between anal cancer and colorectal cancer. The American Society of Colon and Rectal Surgeons notes that while anal cancer itself is not a risk factor for colon cancer, colorectal neoplasms have been found in up to 15% of patients with anal cancer. Their guidelines recommend that patients diagnosed with anal cancer undergo colonoscopy to rule out synchronous colorectal tumors.13Diseases of the Colon & Rectum. The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for Anal Squamous Cell Cancers (Revised 2018) So while colonoscopy is not the right tool for finding anal cancer, it becomes part of the workup once anal cancer is found.
Staging After a Diagnosis
Once anal cancer is confirmed, imaging becomes essential for determining how far it has spread. Two tools dominate staging: endoanal ultrasound and MRI. Both are accurate for assessing tumor size and whether nearby lymph nodes are involved. A prospective study comparing the two found high concordance between them, but endoanal ultrasound identified all cancer patients with 100% sensitivity compared to about 89% for MRI, and was more accurate for early-stage tumors.14PubMed. Staging anal cancer: prospective comparison of transanal endoscopic ultrasound and magnetic resonance imaging A separate analysis confirmed that three-dimensional endoanal ultrasound is particularly strong for staging small, early tumors, while MRI brings broader field-of-view advantages for assessing spread into surrounding structures.15PubMed Central. Diagnostic performance of magnetic resonance imaging and 3D endoanal ultrasound in detection, staging and assessment post treatment, in anal cancer
In practice, many centers use both modalities in combination, letting ultrasound characterize the local tumor and MRI assess the wider pelvic region. PET-CT also plays a role in checking for distant spread. The point for patients is that a confirmed diagnosis triggers a well-established staging pathway that does not depend on colonoscopy at all.
Artificial Intelligence and the Future of Screening
One of the bottlenecks in anal cancer screening is the shortage of clinicians trained in high-resolution anoscopy. The procedure requires experience to interpret the staining patterns and identify which areas to biopsy. Researchers have begun exploring whether artificial intelligence can help. A convolutional neural network trained on high-resolution anoscopy images achieved an overall accuracy of about 90% in identifying anal squamous cell carcinoma precursors, with a sensitivity of roughly 91% and specificity of about 90%.16Springer Link / Tech Coloproctol. Artificial intelligence and high-resolution anoscopy: automatic identification of anal squamous cell carcinoma precursors using a convolutional neural network
These are early results, and the technology is not yet deployed in routine clinical practice. But if AI-assisted anoscopy proves reliable in larger validation studies, it could expand access to effective anal cancer screening beyond the handful of specialized centers that currently offer it. For people in high-risk groups living far from a major medical center, that kind of expansion could be the difference between catching a precancerous change and presenting with advanced disease years later.