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Health condition

Anal intraepithelial neoplasia, HPV, high-resolution anoscopy and why high-grade lesions get treated.

AIN is a precancer, not a cancer - but high-grade lesions are the direct precursor to anal squamous cell carcinoma. UK practice is shifting to treat, not just watch.

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Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against ANCHOR, IANS, BASHH and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects ANCHOR-era practice, HRA-led surveillance and the case for treating high-grade lesions.

Key facts

AIN at a glance.

The essentials, in plain English - what it is, who is affected, how it is found and why high-grade lesions get treated.

  • What it is

    HPV-driven precancer of the anal canal and perianal skin - graded low (LSIL / AIN1) or high (HSIL / AIN2 or AIN3), with high-grade lesions the direct precursor to anal squamous cell carcinoma.

  • Who it affects

    Prevalence is highest in HIV-positive men who have sex with men (30 to 50 per cent), HIV-positive women (10 to 20 per cent) and HIV-negative MSM (20 to 30 per cent). Solid-organ transplant recipients and women with prior HPV-related cervical or vulval disease are also at raised risk.

  • Cause

    Persistent infection with high-risk HPV types - HPV 16 dominates, with HPV 18 a distant second.

  • How it is detected

    High-resolution anoscopy (HRA) is the gold standard - a colposcope-style magnified view with acetic acid and Lugol's iodine plus targeted biopsy.

  • Screening

    No NHS screening programme exists yet, but UK working groups are building pathways for the highest-risk groups.

  • Why treatment matters

    The 2022 ANCHOR trial showed that treating HSIL in people with HIV cuts the risk of progression to anal cancer - now the basis for active treatment of high-grade disease.

Why this guide matters

A shifting picture, and a real chance to prevent cancer.

AIN sits in the same family as cervical intraepithelial neoplasia (CIN) - the same HPV, the same logic of screening and treatment. The three points below shape everything else on this page.

  • HRA is the reference test

    High-resolution anoscopy is the anal equivalent of colposcopy - it is what any serious surveillance pathway is built around.

  • Treat high-grade lesions

    The 2022 ANCHOR trial changed practice - treating HSIL in people with HIV reduces the risk of anal cancer. UK practice now leans in.

  • Risk factors are modifiable

    Stopping smoking, optimising HIV care and completing HPV vaccination lower recurrence and progression risk over time.

How the diagnosis is made

From risk review to an HRA-led plan.

The steps a UK HIV, GUM or colorectal team will normally follow, in order - so you know what to expect and why.

  1. 01

    Assessing

    Risk review and symptom check

    A structured history covering HIV status, prior HPV disease (cervix, vulva, penis), transplant or immunosuppression, smoking and sexual health.

  2. 02

    Assessing

    Digital rectal examination

    A careful DRE for palpable lesions, masses or induration - AIN itself is usually not felt, but a mass raises concern for invasive cancer.

  3. 03

    Assessing

    Anal cytology (Anal Pap)

    A soft brush sample from the anal canal, sent for cytology with reflex high-risk HPV testing - used as a screening triage in higher-risk groups.

  4. 04

    Confirming

    High-resolution anoscopy (HRA)

    The gold-standard examination - acetic acid and Lugol's iodine highlight abnormal areas that are then biopsied under magnified view.

  5. 05

    Confirming

    Targeted biopsy and grading

    Histology confirms LSIL (AIN1) or HSIL (AIN2 / AIN3) and rules out early invasion.

  6. 06

    Planning

    STI and HIV screen

    A full sexual-health screen with an HIV test - because status changes the surveillance interval and treatment threshold.

  7. 07

    Planning

    Multidisciplinary planning

    Colorectal, HIV or genitourinary medicine and HRA teams agree the treatment or surveillance plan and vaccination status.

Typical timeline: an initial visit to a settled surveillance or treatment plan in weeks, not months.

Symptoms

What AIN can look and feel like.

AIN is often silent - the features below are the ones that should prompt an HRA rather than a repeat course of haemorrhoid cream.

  • Often silent

    Most AIN causes no symptoms and is found on examination - which is why targeted surveillance in high-risk groups matters.

  • Perianal warts

    Visible condylomas often coexist with AIN - any wart in a high-risk patient deserves an HRA referral.

  • Itch, discharge or bleeding

    Non-specific anal symptoms - worth investigating rather than assuming haemorrhoids or fissure.

  • Palpable lump or ulcer

    A firm mass, ulcer or non-healing lesion is a red flag for invasive anal cancer, not simple AIN.

  • Pigmented or leukoplakic patch

    A persistent white or pigmented plaque in the perianal skin - worth a biopsy rather than watchful waiting.

  • History of HPV disease

    Previous CIN, VIN, VaIN or penile intraepithelial neoplasia raises the pre-test probability - flag it at every review.

  • Immunosuppression

    HIV, transplant medication or biologic therapy raises both the risk of AIN and the risk of progression.

  • Red flag - suspected invasion

    Pain, weight loss, bleeding or a palpable mass - refer urgently on a suspected cancer pathway.

Treatment

How AIN is managed in the UK.

Low-grade lesions are usually watched. High-grade lesions are treated - typically by HRA-directed ablation or a topical, with excision for anything larger or suspicious.

  • Observation for LSIL (AIN1)

    Low-grade lesions are usually watched, not treated - with HRA surveillance and modification of risk factors including smoking.

  • Electrocautery ablation

    Targeted destruction of HSIL under HRA - the most commonly used ablation method for accessible lesions, per the ANCHOR trial.

  • Infrared coagulation (IRC)

    A handheld probe delivers infrared energy to ablate HSIL - well tolerated in the outpatient setting.

  • Laser or cryotherapy

    Selective options for larger or awkward lesions, or where cautery is not tolerated.

  • Trichloroacetic acid (TCA)

    A chemical ablation applied under HRA - useful for small, discrete HSIL foci.

  • Topical imiquimod 5%

    A self-applied immune modulator - an option for multifocal or perianal HSIL, often over several weeks.

  • Topical 5-fluorouracil (5-FU)

    A cytotoxic cream applied under supervision - reserved for selected multifocal disease.

  • Surgical excision

    Reserved for larger, focal or suspicious lesions where diagnosis of invasion needs to be excluded.

Surveillance

A predictable rhythm of HRA reviews.

Six-monthly HRA is typical after treatment or in high-risk groups. Stable, treated lesions move to annual HRA. Smoking cessation, HPV vaccination and HIV optimisation sit alongside every review.

What this guide is based on

The sources behind every claim on this page.

Landmark trial data, international consensus guidelines and UK specialist society standards, current at the time of last review.

Key references

Trials, guidelines and society standards.

A quiet reminder

This guide is for information, not medical advice.

Your HIV, GUM or colorectal team knows your history and can tell you which parts apply to you. If in doubt, get seen.

  • ANCHOR Study Investigators. Treatment of anal high-grade squamous intraepithelial lesions to prevent anal cancer. NEJM, 2022.

  • International Anal Neoplasia Society (IANS). Consensus guidelines for anal cancer screening.

  • BASHH / BHIVA. UK guidance on anal cancer screening in people living with HIV.

  • British Society of Gastroenterology / Association of Coloproctology of Great Britain and Ireland. Anal cancer position statements.

  • UK Health Security Agency. HPV vaccination programme guidance.

Red flags

When AIN needs urgent attention.

Most AIN is managed on a scheduled HRA pathway. These are the situations that need a faster response - and where a specialist opinion is essential.

  • Suspected invasive anal cancer

    A hard mass, non-healing ulcer, persistent pain or bleeding needs urgent referral on a suspected cancer pathway - not routine HRA.

  • Rapid change in a known lesion

    A previously stable lesion that grows, ulcerates or becomes painful between surveillance visits deserves early re-review.

  • New HIV diagnosis with anal symptoms

    Start antiretroviral therapy and arrange HRA - do not delay assessment on the assumption that the immune reconstitution will settle things.

  • Immunosuppression flare

    A change in transplant or biologic therapy can drive rapid HSIL progression - bring the surveillance interval forward.

  • Multifocal HPV disease in women

    Vulval, vaginal or cervical HSIL alongside anal symptoms warrants combined gynaecology and HRA input.

  • Perianal Bowen's or extramammary Paget's

    Pigmented or eczematous perianal patches that will not heal - biopsy rather than treat as dermatitis.

  • Warts that do not clear

    Persistent or recurrent condylomas in a high-risk patient - refer for HRA rather than repeated topical courses.

  • Pregnancy

    Treatment options are narrower in pregnancy - imiquimod, 5-FU and podophyllotoxin are avoided. Coordinate with obstetrics.

  • Bleeding on anticoagulation

    Symptoms may be attributed to anticoagulants - do not skip examination and HRA when risk factors are present.

Living with it

A precancer, not a life sentence.

Four things that make the biggest difference over the long run - vaccination, stopping smoking, HIV optimisation and turning up to every HRA.

A quiet reminder

Surveillance beats surprises.

Small, steady habits and a scheduled HRA are what turn AIN into a manageable, long-term condition rather than an emergency.

  1. 01 Vaccinate

    HPV vaccination still helps

    Gardasil 9 is offered as secondary prevention after AIN - it lowers the risk of new HPV infections and recurrence. Partner vaccination is also worth discussing.

  2. 02 Smoking

    Stop smoking

    Smoking is one of the strongest modifiable risk factors for HSIL progression and recurrence. Free NHS support makes a real difference.

  3. 03 HIV care

    Optimise HIV treatment

    Consistent antiretroviral therapy with a suppressed viral load and a stable CD4 count reduces the risk of persistent HSIL.

  4. 04 Attend

    Do not skip HRA reviews

    HSIL can recur or progress silently - six-monthly HRA in high-risk patients and annual review in stable disease keeps you ahead of it.

Frequently asked

Everything we get asked about AIN.

Quick answers on HPV, HRA, screening, treatment and recurrence.

  • What is anal intraepithelial neoplasia (AIN)?

    AIN is a precancerous change in the lining of the anal canal or perianal skin caused by persistent infection with high-risk human papillomavirus (HPV). It is graded as low-grade (LSIL / AIN1) or high-grade (HSIL / AIN2 or AIN3). High-grade lesions are the direct precursor to anal squamous cell carcinoma - they are not cancer, but they can become cancer if left untreated.

  • Who is most at risk?

    The highest-risk groups are HIV-positive men who have sex with men (with prevalence estimates of 30 to 50 per cent), HIV-positive women (10 to 20 per cent) and HIV-negative MSM (20 to 30 per cent). Women with a history of HPV-related cervical, vulval or vaginal high-grade disease and people on long-term immunosuppression - including solid-organ transplant recipients - are also at raised risk. Smoking further increases risk in every group.

  • How is AIN diagnosed?

    The gold standard is high-resolution anoscopy (HRA) - a magnified view of the anal canal using acetic acid and Lugol's iodine to highlight abnormal areas, followed by targeted biopsy. Anal cytology (an "anal Pap") and reflex HPV testing are used as screening triage in higher-risk groups. Digital rectal examination and a full sexual-health screen usually go alongside.

  • Is there an NHS screening programme?

    Not yet. The UK does not have a national anal-cancer screening programme, but working groups in HIV medicine, coloproctology and genitourinary medicine are building pathways for the highest-risk groups. The 2022 ANCHOR trial showed that treating high-grade lesions in people with HIV lowers the risk of anal cancer, and this is shifting UK practice towards active surveillance and treatment for those groups.

  • How is high-grade AIN treated?

    Following the ANCHOR trial, HSIL is now actively treated in most high-risk patients. Options include electrocautery ablation, infrared coagulation (IRC), laser or cryotherapy, chemical ablation with trichloroacetic acid, and topical treatments such as imiquimod 5% or 5-fluorouracil. Surgical excision is reserved for larger or suspicious lesions where invasion needs to be excluded.

  • What can I do to reduce recurrence?

    Four things matter most - stop smoking, keep HIV suppressed with antiretroviral therapy if relevant, complete HPV vaccination (Gardasil 9 as secondary prevention) and attend every scheduled HRA. Six-monthly HRA in high-risk patients and annual review in stable disease catches recurrence early.

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