Chapter 26 — Anorectal Malignancy — Clinical Approach
Anorectal malignancy is cancer arising in the anal canal or the rectum. It must always be considered in anorectal practice, since its early symptoms can closely resemble those of benign conditions described elsewhere in this manual. This chapter covers presentation, examination, differential diagnosis, investigations, and the general clinical approach, including how to recognise an obstructive complication. This is the same pattern used for every disease chapter in this manual, though the focus here is clinical recognition and referral, not detailed oncological treatment.
26.1 What Is Anorectal Malignancy
Anorectal malignancy is not one single disease. It falls into two main categories, with different biology and different treatment pathways.
Anal canal cancer is most often a squamous cell carcinoma. It is strongly linked to human papillomavirus infection, described in Chapter 24.
Rectal cancer is most often an adenocarcinoma. It arises from the glandular lining of the rectum, often through a polyp that has become malignant over time.
This distinction matters greatly. The two cancers look different, behave differently, and are treated differently.
26.2 Why It Happens
Anal cancer is strongly associated with high-risk human papillomavirus infection, described in Chapter 24. Anal intraepithelial neoplasia, also described in Chapter 24, is a recognised precursor. Immunosuppression, including HIV infection, and smoking, both increase the risk.
Rectal cancer risk increases with age. A family history of colorectal cancer increases risk. Inflammatory bowel disease, described in Chapter 28, increases risk with long-standing disease. Diet, and the presence of adenomatous polyps, also contribute. A smaller number of cases arise from inherited genetic syndromes, which carry a much higher lifetime risk and need specific genetic counselling and surveillance.
26.3 Classification
Anal canal cancer arises within the anal canal itself.
Anal margin cancer arises on the skin just outside the anal canal. It behaves more like a skin cancer than a true anal canal cancer.
Rectal cancer arises within the rectum, at varying distances from the anal verge. This distance affects both symptoms and treatment options.
Staging describes how far a cancer has spread, using the size of the primary tumour, the involvement of lymph nodes, and the presence of distant spread. Staging is confirmed through the investigations described in Section 26.8, and guides the treatment plan directly.
26.4 Presentation
Follow the history-taking method described in Chapter 2.
Bleeding is the most common symptom. It is also the symptom most likely to be dismissed as haemorrhoids, described in Chapter 10, without proper examination. This is a genuinely dangerous assumption, particularly in an older patient, or when any other alarm feature is present.
A change in bowel habit, particularly a new and persistent change, is a significant symptom.
Tenesmus, a persistent feeling of needing to defecate, can occur with a rectal tumour.
Pain is a notable feature of anal cancer, unlike rectal cancer, which is often painless until the disease is advanced.
A palpable lump, noticed by the patient themselves, sometimes prompts the initial visit.
Unexplained weight loss, and symptoms of anaemia such as fatigue or breathlessness, are systemic features that should raise concern.
An anal cancer can present as a non-healing ulcer. This is easily mistaken for a chronic fissure, described in Chapter 11. A fissure that fails to heal with standard treatment, or that looks atypical, should prompt biopsy rather than continued conservative management.
26.5 Examination
Follow the sequence described in Chapter 3.
Inspection, described in Chapter 4, may show a visible mass, an irregular ulcer, or an atypical lesion.
Digital rectal examination, described in Chapter 5, is essential. Feel specifically for a mass, for irregularity, and for fixation to the surrounding tissue, which suggests more advanced local disease.
Proctoscopy or sigmoidoscopy, described in Chapter 6, allows direct visualisation, and biopsy of any suspicious lesion.
Examine the groin for enlarged lymph nodes. This is particularly important for anal cancer, which characteristically spreads to the inguinal nodes.
A general examination should check for hepatomegaly, an enlarged liver that can indicate metastatic spread, and for signs of significant weight loss.
26.6 Assessment for Obstructive Complications
A growing rectal cancer can progressively narrow the bowel lumen. This can lead to bowel obstruction, which may present as a surgical emergency.
Ask specifically about worsening constipation, abdominal distension, colicky abdominal pain, and vomiting. Any of these, especially in combination, should raise concern for obstruction.
Examine the abdomen for distension and tenderness. A digital rectal examination may reveal a tumour that is nearly or completely occluding the lumen.
Urgent imaging, typically a CT scan, is needed if obstruction is suspected. Do not delay this assessment while arranging routine outpatient investigations.
A patient with acute obstruction often needs decompression first, sometimes with a defunctioning stoma, before any definitive oncological treatment can proceed safely. Recognising this need early prevents a patient reaching a crisis point before the underlying cancer has even been diagnosed.
26.7 Differential Diagnosis
Haemorrhoids, described in Chapter 10, and chronic fissure, described in Chapter 11, are the most common benign conditions mistaken for malignancy. The reverse mistake is just as serious. Dismissing a malignancy as one of these benign conditions is a preventable error.
Inflammatory bowel disease, described in Chapter 28, can cause similar symptoms of bleeding and altered bowel habit.
A benign polyp can mimic an early cancer on examination alone, and needs biopsy to distinguish the two with certainty.
Anal warts and anal intraepithelial neoplasia, described in Chapter 24, sit on a spectrum with early anal cancer, and need careful assessment to place a lesion correctly along that spectrum.
26.8 Investigations
Biopsy is essential for a tissue diagnosis. This is usually taken during proctoscopy or sigmoidoscopy, described in Chapter 6.
MRI, described in Chapter 8, is the standard investigation for local staging of rectal cancer, showing the depth of invasion and nearby lymph nodes clearly.
A CT scan of the chest, abdomen, and pelvis checks for distant spread, most commonly to the liver or lungs.
Endoanal ultrasound, described in Chapter 20, is useful for assessing an early, superficial rectal or anal lesion.
Blood tests typically include a full blood count, to check for anaemia, and liver function tests. A specific blood marker, carcinoembryonic antigen, is sometimes used to help monitor the disease over time, though it is not used alone to make the diagnosis.
26.9 Principles of Management
Anorectal malignancy is managed by a multidisciplinary team, bringing together surgery, oncology, and radiotherapy expertise for every patient.
Anal cancer is most often treated with combined chemotherapy and radiotherapy as the primary treatment, reserving surgery for cases that do not respond, or that recur afterward.
Rectal cancer is most often treated with surgery, sometimes combined with chemotherapy or radiotherapy given before or after the operation, depending on the stage of the disease at diagnosis.
The specific surgical and oncological techniques for treating anorectal malignancy are beyond the scope of this manual. The essential responsibility described in this chapter is timely recognition, and prompt referral to a specialist multidisciplinary team.
26.10 Special Situations
A patient with HIV, described in Chapter 24, carries a higher risk of anal cancer, and may need closer surveillance for anal intraepithelial neoplasia.
An obstructive presentation, described in Section 26.6, needs urgent assessment and management, often ahead of full oncological staging.
An elderly or frail patient needs a careful, individualised discussion about the aims of treatment. This may mean pursuing a cure, or it may mean prioritising symptom control and quality of life, depending on the patient's overall health and wishes.
26.11 Complications
Bowel obstruction, described in Section 26.6.
Perforation of the bowel, a life-threatening emergency.
Fistula formation between the tumour and adjacent structures, such as the bladder or vagina.
Metastatic spread, most commonly to the liver, lungs, or lymph nodes.
Complications of treatment itself, including those of surgery, chemotherapy, and radiotherapy, which should be discussed by the treating specialist team.
26.12 Common Mistakes to Avoid
Assuming rectal bleeding is due to haemorrhoids, without adequate examination, particularly in an older patient or when any alarm feature is present.
Treating a non-healing or atypical anal ulcer as a simple chronic fissure, without biopsy.
Delaying biopsy of a suspicious lesion, hoping it will resolve with conservative treatment.
Missing early signs of obstruction, described in Section 26.6, until the patient presents as an emergency.
Omitting a groin examination in a patient with suspected anal cancer.
26.13 Recording and Follow-Up
Record the examination findings, the biopsy result if available, and the referral made. For example: "Irregular ulcerated lesion, anal canal, biopsy taken, urgent referral to colorectal multidisciplinary team made. No signs of obstruction." Arrange urgent follow-up to review the biopsy result, and ensure the referral pathway is actively followed up, not simply initiated and left.
26.14 Why This Chapter Matters
Anorectal malignancy can mimic the benign conditions that make up most of this manual, and the cost of missing it is severe. A careful examination, a low threshold for biopsy, and prompt recognition of an obstructive complication together protect the patient from a delayed diagnosis. The next chapter turns to a different kind of anorectal emergency: trauma and foreign bodies.