Overview
Anal cancer is much less common than rectal cancer, but it is important because it is the classic cancer cured by radiotherapy rather than surgery. Almost all anal cancers are squamous cell carcinomas (SCC), and most are linked to human papilloma virus (HPV) infection. Treatment is usually managed by a multidisciplinary team (MDT), and the aim is cure with preservation of the anus and sphincter function.
Anal cancer differs from rectal cancer in two big ways: the histology is squamous rather than adenocarcinoma, and the primary treatment is chemoradiotherapy, with surgery kept only as a salvage option. This is the reverse of rectal cancer, where surgery is the main curative treatment.
Anatomy
The anal canal is about 3–5 cm long, running from the anorectal ring—the palpable upper border of the anal sphincter and puborectalis muscles—down to the anal verge, the lowest edge of the sphincter muscles at the anal opening. Two muscle rings, the internal and external anal sphincters, surround the canal and control continence. Preserving them is the whole point of treating with chemoradiotherapy rather than surgery.
Two regions are distinguished. The anal canal is the tube described above. The anal margin is the pigmented perianal skin within about a 5–6 cm radius of the anal verge. This distinction matters for treatment: small anal margin tumours behave more like skin cancers and can sometimes be removed by local excision, whereas anal canal cancers are treated with chemoradiotherapy.
A landmark called the dentate line lies in the middle of the canal, and it matters for two reasons:
- Tissue type: below the dentate line the lining is squamous, like skin, which is why most anal cancers are squamous cell carcinomas; above it the lining is glandular.
- Drainage: tumours below the dentate line drain to the inguinal (groin) nodes, while those above drain to the pelvic nodes.
Lymphatic drainage
Because a tumour can drain either way, radiotherapy treats both the groin and pelvic node areas:
- Inguinal (groin) nodes
- External iliac nodes
- Internal iliac nodes
- Obturator nodes
- Presacral nodes
- Mesorectal (perirectal) nodes
Diagnosis and Staging Work-up
Before treatment is planned, every patient needs a standard set of investigations so the MDT knows the size of the tumour and the state of the nodes. The basics you should know are:
- Digital rectal examination (DRE) and clinical examination — often an examination under anaesthetic; the groins are examined for enlarged nodes.
- Anoscopy, colonoscopy and biopsy — confirms the diagnosis of squamous cell carcinoma.
- Inguinal node assessment — fine-needle aspiration of suspicious groin nodes if it will change the plan.
- MRI of the pelvis — the key local staging test for tumour size and extent.
- CT of the chest, abdomen and pelvis — looks for distant spread.
- PET-CT — recommended for T2 and larger tumours; helps decide which nodes to treat.
- Additional assessment — HIV testing and, in women, a pelvic examination with up-to-date cervical screening.
Staging: TNM Basics
Staging uses the TNM system. The key point in anal cancer is that the T category is based on tumour size, not on depth of invasion—the opposite of rectal cancer.
| Category | What it means |
|---|---|
| T1 | Tumour 2 cm or less. |
| T2 | Tumour more than 2 cm, up to 5 cm. |
| T3 | Tumour more than 5 cm. |
| T4 | Tumour invades an adjacent organ such as the vagina, urethra or bladder. Invading the sphincter, rectal wall or skin alone is not T4. |
| N0 / N1 | No regional nodes / regional nodes involved, including inguinal, mesorectal or iliac nodes. |
| M0 / M1 | No distant spread / distant spread present. |
Treatment Overview
Treatment is decided by the MDT, which includes surgeons, oncologists, radiologists and pathologists. For almost all patients with localised disease, the answer is definitive chemoradiotherapy.
Chemoradiotherapy
Radiotherapy given together with chemotherapy is the curative treatment for anal cancer. It cures most patients while keeping the anus and sphincter, so a permanent colostomy is avoided.
Role of surgery
Surgery is not the first treatment. An abdominoperineal resection—removing the anus and rectum and forming a permanent colostomy—is reserved as salvage for tumours that persist or return after chemoradiotherapy. Small, early anal margin tumours are the exception: a well or moderately differentiated tumour ≤2 cm (T1) that is node-negative can be treated by local excision, aiming for a ≥1 cm margin without compromising the sphincter. Selected small T2 lesions away from the sphincter may also be suitable. All other tumours are treated with chemoradiotherapy.
Chemotherapy
The standard concurrent chemotherapy is mitomycin C with 5-fluorouracil (5-FU), or mitomycin C with capecitabine tablets. Cisplatin can replace mitomycin if there is a contraindication.
Assessing the response
Anal tumours regress slowly and can take months to disappear, so response is assessed at a delayed point. It is important not to biopsy too early, as a tumour still present at 6–8 weeks may still resolve completely. Patients with genuinely persistent or recurrent disease are then considered for salvage surgery.
Radiotherapy in Anal Cancer
Radiotherapy is delivered to the grossly visible primary anal tumour (GTVp), enlarged nodes (GTVn), and the risk regions likely to harbour microscopic disease, known as the clinical target volume (CTV).
The elective draining lymph-node clinical target volumes receive 45 Gy in 30 fractions. The elective volume includes the inguinal, external iliac, internal iliac, obturator, presacral and mesorectal nodes. The primary tumour and enlarged nodes receive 54 Gy in 30 fractions. Lymph nodes smaller than 3 cm can be treated to a reduced dose of 50.4 Gy in 30 fractions. All target volumes are treated simultaneously using a simultaneous integrated boost (SIB) technique.
How a course is planned
- Simulation: a planning CT scan is taken supine with a comfortably full bladder (>250 ml) and an empty rectum. An anal marker may be placed at the anal verge, and a wire marker on the perineal scar if the patient has had surgery.
- A simulation MRI and, where available, a diagnostic pelvic MRI and PET scan are fused with the planning CT to help delineate the tumour.
- Contouring—target volumes: the primary tumour (GTVp) is grown by 15 mm to make its CTVp, which also covers the whole anal canal and sphincters; involved nodes (GTVn) are grown by 5 mm to make their CTVn. The PTV adds a further 5 mm.
- Elective nodal volume: inguinal, external iliac, internal iliac, obturator, presacral and mesorectal nodes.
- Organs at risk: bowel bag or small bowel, bladder, femoral heads, external genitalia and penile bulb.
- Technique: intensity-modulated radiotherapy (IMRT) using RapidArc (VMAT).