Anal squamous-cell carcinoma

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Anal squamous-cell carcinoma can resemble haemorrhoids, fissure, fistula or inflammatory ulcer, so persistence and induration matter. The assessment must inspect the anal margin, examine the canal when tolerable and palpate both groins. Oncogenic HPV is the main causal association; HIV, other immunosuppression, smoking and previous HPV-related dysplasia increase risk without being diagnostic.

Biopsy establishes squamous histology, pelvic MRI maps the primary and pelvic nodes, and diagnostic CT with pathway-selected PET-CT assesses regional and distant disease. Inguinal nodes are a regional basin and suspicious nodes need tissue confirmation where it changes fields. Most non-metastatic anal-canal SCC is treated with definitive combined chemoradiotherapy. ACPGBI describes an early clinical assessment around 6–8 weeks after treatment and continuing interval review because a steadily regressing lesion can reach complete response by six months after completion of chemoradiotherapy; a lesion that is enlarging or shows no response needs earlier biopsy and restaging rather than automatic waiting.

Key points

Persistent anal pain, bleeding, mass, ulceration, discharge or change in continence warrants examination rather than repeated empirical haemorrhoid treatment.
Human papillomavirus is the major causal association; HIV, immunosuppression, smoking and prior HPV-related disease increase risk.
Assessment includes inspection, digital examination when tolerable, proctoscopy or examination under anaesthesia and bilateral inguinal-node palpation.
MRI maps the primary and pelvis while CT or PET-CT assesses nodes and distant spread according to the specialist pathway.
Most anal-canal squamous cancers are treated with combined chemoradiotherapy, reserving salvage abdominoperineal excision for persistent or recurrent disease.
Assess clinically about 6–8 weeks after chemoradiotherapy and continue interval review toward the six-month response point after completion of chemoradiotherapy when the lesion is steadily regressing; biopsy and restage earlier if it enlarges or does not respond.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Persistent oncogenic HPV infection

High-risk HPV oncoproteins disrupt p53 and retinoblastoma cell-cycle control, allowing anal intraepithelial neoplasia to progress to invasive squamous carcinoma in susceptible epithelium.

Immunosuppression and smoking

HIV, transplantation, immunosuppressive treatment and tobacco exposure impair viral clearance or promote carcinogenesis, increasing risk and sometimes complicating treatment tolerance.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Invasion through anal epithelium
Dysplastic squamous cells breach basement membrane and infiltrate the canal, sphincter or adjacent organs, causing induration, pain, bleeding and altered continence.
2
Regional lymphatic spread
Drainage varies with tumour position and includes mesorectal, internal iliac and inguinal basins; groin disease is regional and changes staging and radiotherapy fields.
3
Delayed chemoradiotherapy regression
Radiation and concurrent chemotherapy continue to cause tumour-cell death after treatment ends, so residual thickening at an early review may later resolve clinically.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Persistent or indurated anal lesion

Bleeding, pain, discharge or a presumed fissure that is lateral, ulcerated, indurated, enlarging or non-healing should prompt specialist inspection and biopsy rather than repeated topical treatment.

Anal-canal symptoms

Tenesmus, altered stool calibre, painful defaecation, seepage and new continence change may reflect a canal tumour even when no external mass is initially visible.

HPV and immune risk

Prior high-grade anogenital intraepithelial neoplasia, HIV, transplantation or other immunosuppression and smoking increase SCC risk and influence toxicity support and follow-up.

Bilateral groin examination

Anal lymphatic drainage includes inguinal basins; record each node’s side, size, mobility and tenderness because nodal involvement changes stage and radiotherapy planning.

Local complication signs

Abscess, fistulation, uncontrolled bleeding, severe pain, stenosis or obstructive symptoms require prompt treatment alongside the cancer pathway and may limit tolerability of examination.

Red flags requiring action

Fever with systemic illness during chemoradiotherapy, a fluctuant spreading perianal infection, obstructive vomiting or uncontrolled haemorrhage requires acute care rather than routine cancer-clinic review.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Specialist examination with biopsyFirst step
Why
Define the primary site, size and relationship to sphincters and establish histological type.
Interpretation and limitations
Use examination under anaesthesia when pain or stenosis prevents adequate assessment; avoid wide unplanned excision of an anal-canal tumour that may compromise definitive treatment.
02
Pelvic MRI
Why
Map primary-tumour size, adjacent-organ extension and mesorectal or pelvic nodes.
Interpretation and limitations
Include the anal canal, sphincter complex and regional nodal basins in the report; MRI supplies anatomy but does not replace tissue.
03
CT chest, abdomen and pelvis or pathway-selected PET-CT
Why
Assess distant disease and clarify regional nodal burden before radiotherapy planning.
Interpretation and limitations
PET-CT can reveal metabolically active inguinal or pelvic nodes, but inflammatory nodes can also be avid and suspicious accessible nodes may need sampling.
04
Ultrasound-guided inguinal-node sampling
Why
Confirm whether a clinically or radiologically suspicious groin node contains metastasis.
Interpretation and limitations
A palpable node is not automatically malignant because infection and inflammation also enlarge groin nodes; cytology or core histology informs treatment fields.
05
HIV, renal, marrow and fitness assessment
Why
Identify immune status and organ reserve relevant to chemotherapy and acute toxicity.
Interpretation and limitations
Consent for HIV testing and coordinate specialist care; optimise infection risk, renal function, nutrition and baseline continence without delaying tissue diagnosis.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Chronic anal fissure

A typical fissure is usually midline with defaecatory pain; lateral position, rolled or indurated edges and failure to heal require biopsy for malignancy or inflammatory disease.

Perianal Crohn disease or fistula

Fistula openings, abscesses and inflammatory ulcers can mimic cancer, and longstanding fistula disease can itself harbour carcinoma; MRI and tissue resolve the distinction.

Haemorrhoids or thrombosis

Haemorrhoids can bleed or prolapse and a thrombosed external pile is acutely painful, but neither explains a fixed ulcerated canal mass.

Anal adenocarcinoma or melanoma

Different epithelial or melanocytic primaries require different staging and treatment; morphology and immunohistochemistry establish lineage before the MDT plan.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Worked case: ulcer with an inguinal nodeConfirm both the primary and regional spread before chemoradiotherapyFirst stepA 57-year-old has six months of worsening anal pain and bleeding despite fissure treatment. Examination shows a 3.5 cm indurated canal ulcer and a firm 2 cm left inguinal node.

1Take an HPV, smoking, HIV, immune-suppression and continence history, then inspect the perineum and palpate both groins with consent; provide analgesia and avoid forcing an intolerable office examination.
2At examination under anaesthesia, biopsy the ulcer without performing wide excision. Histology confirms p16-positive squamous-cell carcinoma.
3Pelvic MRI maps the canal tumour and pelvic nodes; CT and PET-CT show no distant disease but confirm uptake in the left groin, and ultrasound-guided core biopsy proves regional SCC metastasis.
4DefinitiveThe anal-cancer MDT plans definitive mitomycin and fluoropyrimidine-based chemoradiotherapy including the involved groin, with renal, marrow, HIV, nutrition and skin-toxicity support.
5At the 6–8-week clinical response assessment the lesion is clearly smaller but still palpable, so interval examinations continue; it regresses completely by the six-month assessment after completion of chemoradiotherapy. Verify ongoing anal and groin surveillance rather than labelling an early, steadily improving residual as failure.

02Atypical presumed fissureBiopsy a non-healing lateral lesion rather than repeating ointmentA 46-year-old who smokes has a lateral anal ulcer with rolled indurated edges, minor bleeding and no palpable groin nodes after eight weeks of fissure treatment.

1Reassess the exact site and appearance and ask about immune suppression, HPV-related disease, discharge, weight loss and continence; a lateral indurated ulcer is not a typical acute midline fissure.
2Refer to the specialist anal pathway and obtain biopsy under adequate analgesia. Histology shows invasive SCC rather than benign fissuring.
3Complete pelvic MRI, systemic imaging and bilateral groin assessment before the MDT assigns stage and treatment intent.
4DefinitiveVerify definitive chemoradiotherapy, toxicity support and the scheduled clinical-response plan; counsel against interpreting early post-radiotherapy induration without specialist review.

03Biopsy-proven local recurrenceUse salvage surgery only after restaging persistent or recurrent diseaseEighteen months after complete clinical response to chemoradiotherapy, a patient develops renewed anal pain and a growing ulcer. Biopsy confirms recurrent SCC and CT shows no distant metastases.

1Restage the pelvis and groins with examination and MRI, complete systemic imaging and review the original radiotherapy fields and current urinary, sexual and continence function.
2Discuss resectability at a specialist salvage and plastic-reconstruction MDT rather than repeating chemoradiotherapy or assuming symptoms are radiation injury.
3The patient undergoes salvage abdominoperineal excision with planned perineal reconstruction; pathology confirms recurrent SCC removed with clear margins.
4Verify wound healing, stoma training and final pathology and continue groin, pelvic and systemic follow-up, with rapid investigation of new pain, ulceration or nodal enlargement.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Local stenosis, fistula or sepsis

Progressive tumour or treatment injury can narrow the canal, fistulate into adjacent structures or cause abscess, requiring symptom control alongside oncological treatment.

Regional or distant recurrence

Pelvic and inguinal nodal relapse may precede liver, lung or other metastases and can remain amenable to specialist salvage when detected early.

Acute chemoradiotherapy toxicity

Perineal skin breakdown, diarrhoea, dehydration, cystitis, marrow suppression and infection can interrupt treatment and require coordinated oncology support.

Late pelvic morbidity

Fibrosis may cause chronic proctitis, stenosis, incontinence, sexual dysfunction, urinary symptoms, pelvic fracture risk or lower-limb lymphoedema.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

During chemoradiotherapy, review blood count, renal function, hydration, diarrhoea, urinary symptoms, perineal and groin skin, pain, nutrition and treatment interruptions.
Perform an early clinical response assessment around 6–8 weeks after chemoradiotherapy, documenting the primary by inspection and palpation when tolerable and examining both groins; compare at interval reviews because regression may continue to the explicit six-month assessment point after completion of chemoradiotherapy.
Biopsy and restage an enlarging or non-responding lesion earlier through the specialist team; reserve continued observation for a residual lesion that is objectively regressing, rather than waiting six months despite progression.
After response, ask about stenosis, bleeding, faecal urgency or incontinence, sexual function, urinary symptoms, lymphoedema and chronic skin injury and refer to the relevant supportive service.
After salvage surgery, verify margin and nodal pathology, perineal wound healing, stoma independence and the next oncological imaging and clinical-review dates.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Benign labels can delay diagnosis

A fissure or haemorrhoid diagnosis should be reconsidered when an ulcer is indurated, atypical or non-healing.

Groins are part of staging

Inguinal nodes may be the first clinically apparent regional spread.

Treatment preserves sphincter

Definitive chemoradiotherapy avoids immediate radical excision for most anal-canal squamous cancers.

Late effects need ownership

Skin, bowel, sexual, urinary and lymphatic toxicities can persist after cancer control.

11Common pitfallsFrequent interpretation and management errors.

01
Repeating topical therapy for an indurated non-healing ulcer delays biopsy and staging.
02
Omitting inguinal-node examination misses a regional drainage basin central to staging.
03
Performing wide local excision of an anal-canal tumour can compromise definitive planning.
04
Declaring failure at the first 6–8-week review despite objective regression can trigger premature salvage surgery, but waiting to six months despite enlargement or no response delays biopsy and restaging.

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