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What is anal cancer?
Anal cancer develops in the anal canal, the short passage at the end of the digestive tract. Most cases are squamous cell carcinomas and are strongly linked to the human papillomavirus (HPV). It's an uncommon cancer, and one of the great success stories of cancer care: a combination of radiation and chemotherapy cures most cases without removing the anus, so patients keep normal bowel function. Surgery is now reserved mainly for the rare cancers that don't respond.
The main types
Doctors group anal cancer by where it starts and how it behaves:
| Type | What it means, simply |
|---|---|
| Squamous cell carcinoma | The most common type (~90%), usually HPV-related and very responsive to chemoradiation. |
| Anal canal vs. anal margin | Cancers inside the canal versus on the skin at the opening; location affects the treatment plan. |
| Adenocarcinoma | A less common type that behaves more like rectal cancer and may be treated differently. |
| Precancers (AIN) | Anal intraepithelial neoplasia — HPV-related precancerous changes that can be monitored or treated early. |
Staging, in plain terms
Anal cancer is staged with TNM (tumor size, lymph nodes, metastasis). Tumor size is especially important here, since most anal cancers are diagnosed while still local and highly curable with chemoradiation.
| TNM | What it generally means |
|---|---|
| Stage 0 | Pre-invasive changes confined to the surface lining. Treated early to prevent progression. |
| Stage I | A small tumor (up to ~2 cm) with no lymph nodes involved. Excellent cure rate. |
| Stage II | A larger tumor, still without lymph-node spread. |
| Stage III | Spread to nearby lymph nodes or invading neighboring structures, but not distant organs. |
| Stage IV | Spread to distant organs — uncommon, and increasingly treated with chemotherapy plus immunotherapy. |
The standard of care
Anal Cancer is almost always treated by a team that may include a surgeon, a medical oncologist, and a radiation oncologist, combining therapies for the best result. The usual building blocks are:
Chemoradiation (the cure)
The standard first treatment: radiation combined with chemotherapy (mitomycin plus 5-FU). This 'Nigro protocol' approach cures most anal cancers and preserves the anus.
Surgery (reserved)
Major surgery is now a backup, used mainly if chemoradiation doesn't fully clear the cancer or it returns.
Active surveillance after treatment
Because the tumor can keep shrinking for months, doctors watch closely rather than rushing to surgery.
Immunotherapy (advanced)
For metastatic disease, immunotherapy added to chemotherapy is a newer standard, following 2025 FDA approval of retifanlimab with chemotherapy.
How radiation treatment works
Radiation uses focused, high-energy x-rays to damage the DNA of cancer cells so they can no longer divide, while healthy tissue repairs and recovers. Given together with chemotherapy, radiation is especially effective against anal cancer — the chemo sensitizes the cancer cells so a moderate radiation dose can cure the disease. Modern intensity-modulated radiation shapes the dose tightly around the tumor and lymph nodes while protecting nearby skin and organs, which makes treatment far more tolerable than it once was. The main side effects — skin irritation and bowel changes in the treated area — are temporary and managed closely by your team.
The main ways radiation is delivered for anal cancer:
Intensity-modulated radiation therapy (IMRT)
The preferred technique: it delivers a high dose to the tumor and at-risk lymph nodes while sparing the skin, hips, bladder, and genitals — reducing the harsh side effects of older radiation methods.
Concurrent chemotherapy
Mitomycin and 5-fluorouracil given during radiation make the cancer cells far more sensitive to it, dramatically improving cure rates.
Image-guided delivery
Daily imaging keeps the beams precisely on target through a treatment course of about 5-6 weeks.
Latest studies shaping care
Care keeps improving — often toward getting the same excellent results with less burden on patients. A few developments:
ASTRO radiation guideline (2025): ASTRO published a clinical practice guideline standardizing IMRT-based chemoradiation for anal squamous cell cancer, reinforcing organ-preserving, non-surgical cure as the standard of care.[1]
ASTRO Clinical Practice Guideline (2025)
Retifanlimab approval (2025): The FDA approved the immunotherapy retifanlimab with carboplatin and paclitaxel for advanced or metastatic anal cancer — the first major systemic advance for late-stage disease in years.[2]
FDA approval; PODIUM-303 phase III trial
IMRT reduces toxicity: Switching from older radiation techniques to IMRT has cut serious skin and bowel side effects while maintaining the high cure rates of the Nigro chemoradiation approach.[3]
RTOG 0529 and IMRT experience
Common questions
Will I need a colostomy or surgery? Usually not. The whole point of modern chemoradiation is to cure anal cancer while preserving normal function. Surgery is reserved for the minority of cases that don't fully respond.
How long is treatment? Radiation is typically given daily over about 5-6 weeks, with chemotherapy during the first and last weeks. Your plan is tailored to the tumor's size and location.
What are the side effects? Skin soreness in the treated area, diarrhea, and fatigue are common during treatment and usually settle within weeks afterward. Your team will help you manage them.
References
Numbered sources for the studies cited above. Links open the primary publication on PubMed or the publisher’s site.