Anal carcinoma - exploring the epidemiology, risk factors, pathophysiology, diagnosis, and treatment

Abstract

Anal carcinoma is a relatively rare tumor that accounts for approximately 2% of gastrointestinal malignancies and less than 7% of anorectal cancers. Most anal tumors originate between the anorectal junction and the anal verge. Risk factors for the disease include human papillomavirus infection, human immunodeficiency virus, tobacco use, immunosuppression, female sex, and older age. The pathogenesis of anal carcinoma is believed to be linked to human papillomavirus-related inflammation, leading to dysplasia and progression to cancer. Squamous cell carcinoma is the most common type of anal tumor, with an annual incidence of approximately 1 to 2 per 100000 persons. Treatment regarding anal cancer has emerged over time. However, chemoradiation therapy remains the mainstay approach for early localized disease. Patients with metastatic disease are treated with systemic therapy, and salvage surgery is reserved for disease recurrence following chemoradiation. This article aims to provide background information on the epidemiology, risk factors, pathology, diagnosis, and current trends in the management of anal cancer. Future directions are briefly discussed.

Key Words: Anal cancer; Squamous cell carcinoma; Chemoradiation therapy; Human papillomavirus; Immunosuppression

Core Tip: Anal cancer is a rare malignancy, comprising 1%-6% of anorectal tumors. The incidence of anal carcinoma has been steadily increasing worldwide, and diagnosis is often challenging due to the similarities in clinical presentation to benign rectal diseases such as hemorrhoids. Roughly 85% of anal cancers are squamous cell carcinoma, and human papillomavirus infection and immunosuppression are major risk factors for the disease. Chemoradiation is the treatment of choice for early-stage cancer, while systemic therapy is used for metastatic disease. Novel cytotoxic agents in combination with immunotherapy have produced favorable outcomes in patients with advanced disease.

INTRODUCTION

Anal carcinoma is a rare disease entity that accounts for less than 5% of cancers of the intestine[1]. In the general population, the incidence of anal cancer has steadily increased from 1.2 in 1992 up to 1.9 per 100000 persons in the United States alone, according to data from the National Cancer Institute’s Surveillance, Epidemiology, and End Results Program in 2024[2]. Among anal cancers, squamous cell carcinoma (SCC) is the predominant histological subtype, followed by adenocarcinoma and rarer tumors such as basal cell carcinoma (BCC), melanoma, and small cell carcinoma[3,4].

Treatment of anal tumors has evolved over several decades, with concurrent mechanism identification. The primary choice of management has undergone a radical shift from abdominoperineal resection (APR) to organ-preserving chemoradiation therapy (CRT) and surgery[5]. Furthermore, recent developments using immunotherapy for advanced disease and metastasectomy in selected cases highlight the continuous changes and evolution of disease management[6,7]. In this article, we provide a scoping review of anal carcinoma and its epidemiology, risk factors, pathophysiology, diagnosis, and management.

EPIDEMIOLOGY

Anal cancer is an uncommon tumor that comprises less than 2% of large bowel malignancies and approximately 1%-6% of anorectal tumors[1,3,4]. This cancer has become more prevalent in Western nations such as Australia and the United States, with little to no change in Asian countries and Spain[8]. In the United States, between the period of 1994 to 2000, the incidence of anal cancer almost doubled, with a rate of 2.04 per 100000 in males and 2.06 per 100000 in females, in comparison with the period between 1973 and 1979, where the incidence was 1.06 per 100000 in males and 1.39 per 100000 in females[2,5]. Incidence rates in the United Kingdom and across Europe range from 0.7 to 1.7 per 100000 persons per year[9,10].

In 2020, the global anal cancer incidence was estimated at 0.65 per 100000 persons, with 50865 cases[11]. The worldwide incidence of anal SCC was 8.2, with a lower incidence observed in the male population (0.28 vs 0.53 in females) and a higher proportion observed in Northern and Western Europe, North America, and Oceania[11,12]. The global incidence of anal adenocarcinoma was 0.54 in the same year, with a higher incidence observed in the male population (0.25 vs 0.19 in females) and a higher proportion observed in eastern Asia and northern and sub-Saharan Africa[11,13]. The growing trend of anal cancer is believed to be linked to the rising rates of human papillomavirus (HPV) infection.

RISK FACTORS

The development of anal cancer is linked to multiple risk factors. Social and cultural dynamics over the last few decades have played a significant role in patient exposure and increased rates of anal tumors. The most notable risk factors are as follows.

HPV infection

HPVs are a group of double-stranded circular DNA oncovirus from the Papillomavirdae family, commonly transmitted by mucosa-mucosa or skin-to-skin contact and entering the body by mucosal or cutaneous trauma in the form of oral sex, vaginal, or anal intercourse[14]. The infection is the most common sexually transmitted disease worldwide, with prevalence variation regionally[15]. The lifetime risk of infection at least once among men and women is 50%, and HPV serves as the underlying cause of cervical cancer[14,16,17]. In fact, female patients with cervical cancers are at increased risk for anal cancer[18].

Anal malignancies are often associated with HPV infection, although it is relatively uncommon for most patients with this infection to acquire anal cancer[12,19]. Anal carcinoma is commonly linked to HPV infection, most notably subtype 16[20]. Another high-risk subtype, HPV-18, is found in some anal tumors, but is less common than HPV-16[19,21]. Subtypes 6 and 11 are less likely to become malignant and are commonly found in anogenital warts[22]. Other subtypes, including HPV-31, 33, and 45, have been implicated in a small number of anal cancer cases and have been linked to vulvar, cervical, and vaginal cancers in females, and penile cancer in males[20,23,24]. Verruca vulgaris (common warts) are often associated with the low-risk subtypes (1, 2, 4, and 7), with occasional high-risk subtypes (16 and 18) as a cause[25]. Anal SCC is the most common histopathological subtype, and HPV is implicated in more than 80% of tumor samples[26].

Male circumcision is associated with lower rates of HPV infection and transmission[27,28]. A meta-analysis across 32 studies that analyzed the relationship between male circumcision and HPV infection rates found decreased odds of prevalence of HPV infections [odds ratio = 0.45; 95% confidence interval (CI): 0.34-0.61], reduced incidence rate of HPV infection (incidence rate ratio = 0.69; 95%CI: 0.57-0.83), and increased risk of clearing HPV infections (risk ratio = 1.44; 95%CI: 1.28-1.61) at the glans penis in male subjects[29]. The HPV vaccine and condoms are also implicated in protecting and reducing virus transmission rates[30,31]. However, condoms do not offer complete protection because they do not mask all areas of the body that may be susceptible to infection, such as the anus[30,31].

Human immunodeficiency virus infection

Patients with human immunodeficiency virus (HIV) are considered high risk for anal cancer[32]. Incidence rates of anal carcinoma among HIV-infected individuals are 30 times higher compared to the general population[33]. A cohort study by Silverberg et al[34] that compared the incidence rates among 34189 HIV-infected [55% men who have sex with men (MSM), 19% other men, 26% women] and 114260 HIV-noninfected (90% men) individuals found the unadjusted incidence rate per 100000 person-years to be 131 for HIV-positive MSM, 46 for other HIV-infected men, and 2 for HIV-negative men. HIV-infected women had an anal carcinoma rate of 30 per 100000 person-years, with no cases observed for HIV-noninfected women[34]. The use of highly active retroviral therapy has not reduced the incidence of anal cancer, and a systematic review revealed a continued increase in the incidence of anal carcinoma among HIV-infected women, despite the use of highly active retroviral therapy[35,36].

Tobacco use

Tobacco use is an independent risk factor for anal carcinoma[37]. It is associated with anal cancer disease recurrence and higher mortality rates[37,38]. A study conducted by Phillips et al[39], which measured smoking-related DNA adducts in the anal epithelium among smokers (n = 20) and age-matched life-long non-smokers (n = 16), showed high adduct levels among smokers compared to controls. The authors concluded that tobacco smoke inflicts genotoxic damage in the anal epithelium of smokers as a plausible mechanism for its association with anal cancer.

Chronic immunosuppression not due to HIV

Compromised immune function is associated with increased risk for anal cancer[40]. According to a population-based cohort study conducted in Denmark by Sunesen et al[41], immunosuppression in the form of autoimmune diseases, solid organ transplantation, and hematologic malignancy are all strongly associated with the development of anal carcinoma.

Men who have sex with men

MSM are 20 times more likely to be diagnosed with anal cancer than heterosexual men, with a rate of approximately 40 cases per 100000 people[42]. HIV-infected MSM are up to 40 times more likely to develop anal cancer, resulting in a rate of 80 cases per 100000 people[42,43]. The high incidence is related to significant HPV infection rates[43,44]. Modifiable behavioral factors such as condom and drug use can lower infection rates and cancer incidence[43].

Female sex

Anal cancer has a predilection for females compared to males, most notably due to high HPV infection rates among the sex[45,46]. Several factors, including multiple sexual partners, sex at an early age, and having an uncircumcised sexual partner, all contribute to higher rates of HPV infection and, therefore, high rates of anal cancer compared to their male counterparts[45-47].

Older age

Anal cancer is rare in people younger than 35 and is mainly found in older adults[48,49]. The development of anal carcinoma in older patients is primarily reflected by the accumulation of cell DNA damage over time[50].

ANATOMY

The anal canal (Figure 1) is the terminal portion of the alimentary tract that extends from the anorectal junction to the anal margin[51]. The upper portion of the canal derives from the dorsal section of the cloaca (endoderm), and the lower part is derived from the proctodeum (ectoderm)[51,52]. The canal measures approximately 4 cm and is continual with the rectum at the anorectal junction that forms an angle (anorectal angle) at the levator ani[51-53]. The muscular layer of the canal shapes the internal and external anal sphincters, and the tube contains longitudinal folds of mucosa that join inferiorly to make semicircular anal valves[53,54]. These valves form the pectinate (dentate) line, which demarcates the superior two-thirds of the anal canal from the inferior one-third[55]. The pectinate line is a watershed area that varies in neurovascular supply[55,56]. The epithelium above this line is columnar, while below is a transition zone lined by nonkeratinized stratified squamous epithelium known as the anal pecten[57]. Further below, the anal pecten ends at the anocutaneous (white) line, where the epithelium resembles true skin (i.e. keratinized stratified squamous with sebaceous glands and hair)[57,58].

Figure 1 Image depicting parts of the large bowel. The anal canal is labeled as the terminal portion of the rectum.

The anal canal above the pectinate line receives blood supply from the superior rectal artery (branch of the inferior mesenteric artery), median sacral arteries, and anastomosing branches from the middle rectal artery[55,56,59]. The venous drainage anastomoses with the rectal venous plexus and, above the line, drains from the superior rectal vein to the inferior mesenteric vein (portal venous system)[57-60]. Below the dentate line, the middle and inferior rectal veins drain to the internal pudendal vein, a tributary of the internal iliac vein[58-60]. Lymph drains primarily to the internal iliac nodes above the line, while inferiorly, it drains to the superficial inguinal nodes[60,61]. Above the pectinate line, the anal canal receives visceral innervation via the inferior hypogastric plexus[62]. As a result, the superior portion of the canal is susceptible to stretching[62,63]. Below the line, the canal receives somatic innervation through the inferior rectal branches of the pudendal nerves and, consequently, is sensitive to pain, temperature, and touch[63,64].

PATHOPHYSIOLOGY

Integration of HPV double-stranded DNA into the host cell genome facilitates the expression of viral oncoproteins E6 and E7, promoting tumor oncogenesis of anal SCC[65]. These proteins express stimulatory properties, leading to a complex inflammatory progression to invasive cancer[65,66]. During integration, a destruction of a segment of the E2 domain of the viral genome occurs in the DNA of the infected cells, which leads to a loss of suppressor function of the E2 protein[65-67]. Due to this loss of activity, E6 and E7 proteins have increased expression with stimulatory properties, promoting invasion and keratinocyte immortalization[65-68].

HPV-related neoplasm requires E6 and E7 expressions to establish and maintain a transformed state[67,68]. Within this environment, the E7 complex interacts with the retinoblastoma protein, and E6 binds and inactivates tumor suppression protein p53[67-69]. This complex interplay serves as the primary mechanism for HPV-related anal SCC[65-69]. HIV-associated anal cancer is believed to be related to microsatellite instability, leading to the progression of invasive carcinoma[70].

IMMUNIHISTOCHEMISTRY

Regarding immunohistochemistry (Table 1), SCC express cytokeratin (CK) 5/6, CK 13/19, pan CK antibody (AE1/AE3), and p63 protein[66-71]. The presence of the p63 protein on immunohistochemistry is highly specific for anal SCC[68-71]. Adenocarcinoma of the anal gland, in contrast, commonly expresses CK 7/20 (+/-)[67-71]. Adenoid cystic tumors are usually CK 7+, and melanocytic markers (i.e. S-100, human melanoma black-45, and Melan-A) are positive in melanomas[72,73]. Neuroendocrine tumors are positive for neuroendocrine markers (i.e. chromogranin A and serum neuron-specific enolase), and lymphoid cancers express lymphoid markers[74,75]. Perianal Paget disease commonly expresses CK 7 and gross cystic disease fluid protein 15[76]. Paget disease with associated anorectal cancer is typically CK 20+[76].

Table 1 Immunohistochemistry markers for anal cancers.

Tumor markers	Squamous cell carcinoma	Anal adenocarcinoma	Melanoma	Basal cell carcinoma	Neuroendocrine	Paget primary	Paget secondary
AE1/AE2	+	+	-	+	+	+	+
Ber-EP4	-	+	-	+	-	+	+
CAM5.2	+	+	-	Nonspecific	+	+	+
CDX-2	-	Mostly-	-	-	-	-	+
CEA	-	-	-	-	-	+	+
Chromogranin	-	-	-	-	+	-	-
CK5/6	+	-		+	-	-	-
CK7/20	-/-	+/Mostly-	-	-/-	-/-	+/-	Mostly-/+
GCDFP-15	-	-	-	-	-	+	-
HMB45	-	-	+	-	-	-	-
MELAN-A	-	-	+	-	-	-	-
Mucin	-	+	-	-	-	+	+
p63	+	-	-	+	-	-	-
S100	-	-	+	-	-	-	-
Synaptophysin	-	-	-	-	+	-	-
Vimentin	-	-	+	-	-	-	-

AE1: Anti-pan cytokeratin AE1; Ber-EP4: Anti-epithelial cell adhesion antibody; CAM5.2: Anticytokeratin CAM5.2; CDX-2: Caudal-related homeobox transcription factor 2; CEA: Carcinoembryonic antigen; CK5/6: Cytokeratin 5/6; GCDFP-15: Gross cystic disease fluid protein 15; MELAN-A: Melanoma antigen recognized by T cells.

PRECURSOR LESIONS

Anal cancer commonly originates at the squamocolumnar junction and arises from precancerous lesions termed anal intraepithelial neoplasia (AIN)[77]. These lesions are frequently called squamous intraepithelial lesions, and are grouped according to their grade and degree of dysplasia[77,78]. AIN 2/3 are commonly classified as high-grade squamous intraepithelial neoplasia and are found in most patients with anal cancer[77,78]. Consequently, patients who are at increased risk for AIN 2/3, which includes MSM, solid organ transplant recipients, immunocompromised patients, and women with any history of vulvar, vaginal, and cervical cancer should undergo regular screening with anal cytology[77-79].

AIN has been found to be biologically similar to cervical intraepithelial neoplasia (CIN) due to its common association with HPV, which is found in most cancers of the anus and cervix[80]. Current modes of treatment include topical medications, laser therapy, fulguration, and ablative techniques in the form of infrared coagulation, surgical excision, and thermal ablation. Topical treatment options include imiquimod, 5-fluorouracil (5-FU), and trichloroacetic acid[81].

PATHOLOGY

The World Health Organization has classified tumors of the anal canal into intraepithelial and invasive neoplasms that are further divided into epithelial and nonepithelial tumors (Table 2). The most notable ones are as follows.

Table 2 World Health Organization histological classification of anal tumors.

Tumor type	Characteristics
Epithelial tumors
Premalignant lesions	Intraepithelial neoplasia (dysplasia), low-grade
	Intraepithelial neoplasia (dysplasia), high-grade
	Bowden disease
	Perianal squamous intraepithelial neoplasia
	Paget disease
Carcinoma	Squamous cell carcinoma
	Adenocarcinoma
	Mucinous adenocarcinoma
	Small cell carcinoma
	Undifferentiated carcinoma
	Others
Nonepithelial tumors	Leiomyoma
	Gastrointestinal stromal tumor
	Myofibroma
Carcinoid tumors	NA
Melanoma	NA
Secondary tumors	Direct extension from adjacent organs: Rectal, cervical, and prostate carcinoma

NA: Not applicable.

Paget disease

Anal Paget disease commonly involves the perianal skin and is found in apocrine-rich areas[76,82]. Malignant cells produce mucus, and perianal spread is characterized by the proliferation of pagetoid cells within the epithelium[76,82,83]. Tumor cells on histological appearance often show large cells with giant nuclei and pale cytoplasm[82-84]. Pagetoid cells also occasionally acquire a signet ring appearance[82-85].

SCC

More than 80% of anal cancers are SCC[3,4]. These tumors arise primarily at the transformation zone between the columnar and squamous epithelium of the canal[3,4,33-36]. The cells that constitute these tumors have varying features, with some having giant pale eosinophilic characteristics and others having no areas of keratinization[86]. Other cells facilitate tumor nests (tumor-cell islands) that may appear with central keratinization and intercellular bridges or peripheral palisading on histology[86,87]. SCC tumors lack a myoepithelial layer within and around the basement membrane, and may resemble adnexal, skin, and salivary gland neoplasms[86,87].

Adenocarcinoma

Adenocarcinoma accounts for approximately 5% to 10% of cancers of the anal canal[4,88,89]. This histological tumor type primarily originates from the anal epithelium and includes primary adenocarcinomas that arise in the mucosa, anal glands, and fistula[89,90]. These tumors may also appear as small, ulcerated lesions near the anal duct, and possess some association with ulcerative colitis, Crohn’s, and Paget’s disease[89-92]. Tumors are commonly mucin-positive and are treated similarly to rectal cancer[4,89-92].

BCC

BCC, the rarest type of anal cancer, accounts for less than 0.2% of anal neoplasms and predominantly affects the skin around the anal region[93,94]. It arises from perianal lesions and tends to remain local[93,94]. Anal BCC shares similar histological features with a known variant of SCC[94]. This variant, termed SCC with basaloid features, tends to show nests of oval cells with varying ratios of eosinophilic to basophilic cytoplasm, assorted mitotic activity, and peripheral nuclear palisading[94-96]. Perianal BCC has a better prognosis than SCC with basaloid features, and the nodular subtype is the predominant histologic type found in most cases[94-97].

Carcinoid tumors

Carcinoid tumors of the anal canal (rectal carcinoid) are rare and account for 1% to 1.3% of all anal cancers[98]. These tumors are commonly asymptomatic and are found incidentally on routine colonoscopy[98,99]. Rectal carcinoids are of neuroendocrine origin and are well differentiated[99]. Due to this fact, they are associated with a favorable prognosis, although they can metastasize[98,99]. This ability to spread is more likely with a tumor size greater than 10 mm and atypical features[99,100]. On histological appearance, these tumors show a trabecular growth pattern with nest- or rose-like structures[100,101]. Immunohistochemistry markers for neuroendocrine tumors include chromogranin A, synaptophysin, and neuro-specific enolase. However, the most sensitive marker for rectal carcinoids is SABT2, which is positive in more than 85% of rectal carcinoids[101,102].

Melanoma

Anal melanomas are rare and account for only 1% of all anorectal malignant tumors[103]. These are often acro-lentiginous and are usually found as pigmented ulcerated lesions[103,104]. Melanomas of the anal canal are typically aggressive and exhibit poor prognosis due to the lack of symptoms and late disease presentation[105]. The histological types of anal melanoma include lymphoma-like, spindle-cell, epithelioid, and pleomorphic[103-105]. The spindle-cell anal melanoma can be easily misdiagnosed as a gastrointestinal stromal tumor[105,106]. For this reason, analysis of immunohistochemistry is vital for the correct diagnosis[105]. Anal melanoma most commonly expresses S-100 on immunohistochemistry and c-Kit positive in over 70% of cases[103-106].

DIAGNOSTIC WORK-UP

Most patients with anal cancer typically present with symptoms resembling benign diseases such as fissures and hemorrhoids[4,107]. Patients may complain of abdominal/rectal pain and bleeding on toilet paper after wiping[4,107]. In other cases, patients may be asymptomatic[4,107,108]. A complete history and physical examination are crucial to prognosis as patients with early diagnosis have better clinical outcomes[108-110]. Physicians should have a high index of clinical suspicion for patients presenting with symptoms resembling benign disease, and possess a low threshold for imaging studies and biopsy of suspected lesions[107-111]. Imaging modalities used in the diagnosis of anal cancer are as follows.

Three-dimensional endoanal ultrasound

Three-dimensional endoanal ultrasound (3D-EAUS) is a valuable tool that allows a detailed evaluation of the anatomy and diseases of the anal canal[112]. It can accurately assess the depth of invasion of cancer into the anal sphincter complex and gauge tumor response to CRT[112,113]. 3D-EAUS is easy to perform and reproduce with high diagnostic accuracy[112-114]. A large study by Reginelli et al[114], which compared the diagnostic performance of 3D-EAUS and magnetic resonance imaging (MRI) in the detection of anal cancer, found a detection rate of 100% using ultrasound compared to 93.1% for MRI.

Computed tomography

Computed tomography (CT) scan is a valuable tool in the diagnosis of anal cancer[115]. Contrast-enhanced CT scan may visualize an anal tumor as a hypo-attenuated necrotic mass[5,115]. CT scan is also helpful in evaluating lymph node metastases and distant metastatic disease[115,116]. Although useful in the diagnostic work-up of anal cancer, recent guidelines have geared toward better imaging modalities such as MRI, positron emission tomography (PET), and PET/CT scan[115-117]. A systematic review and meta-analysis by Mahmud et al[117] comparing PET/CT or PET with standard imaging studies showed a 67% sensitivity to detect primary tumors with CT scan alone compared to 99% for PET or PET/CT scan.

Phased-array MRI

Phased-array MRI is a useful tool for the diagnosis and staging of anal cancer[118]. In 2014, the European Society of Medical Oncology guidelines proposed that pelvic MRI should be mandatory in the diagnostic work-up of anal cancer, citing superior soft-tissue resolution compared to conventional (standard) imaging modalities (i.e. 3D-EAUS and CT scan)[119]. MRI detects neoplastic nodes, the extent of tumor infiltration into nearby organs, and the circumferential tumor extent[119,120]. It is greater than 90% sensitive in identifying anal carcinoma, and provides specific detail regarding the position of the tumor[120].

PET/CT scan

PET/CT scan with 18F-fluorodeoxyglucose (FDG-PET/CT) is an accurate and effective imaging modality for the detection and staging of anal cancer[121]. PET detects tumors based on molecular alterations and plays a vital role in post-treatment restaging of anal cancer patients[121,122]. FDG-PET/CT provides details regarding crucial markers for staging, such as the size of the tumor, the lymph nodes involved, and the detection of metastasis[121-123]. Of note, FDG-PET/CT is particularly sensitive for localized tumors less than 2 cm compared to standard imaging modalities[5,121-123].

STAGING

The tumor, node, and metastasis (TNM) classification system (Table 3) published by the American Joint Committee on Cancer is used to stage anal cancer[124]. This system is based on three essential factors.

Table 3 Tumor, node, and metastasis classification system for anal cancer.

Primary tumor	Characteristics
TX	Primary tumor cannot be assessed
T0	No evidence of primary tumor
Tis	HSIL (previously termed carcinoma in situ, Bowden disease, anal intraepithelial neoplasia II-III, high-grade anal intraepithelial neoplasia)
T1	Tumor ≤ 2 cm in greatest dimension
T2	Tumor > 2 cm but ≤ 5 cm in greatest dimension
T3	Tumor > 5 cm in greatest dimension
T4	Tumor of any size invades adjacent organ(s) (e.g., vagina, urethra, bladder)
Lymph nodes
NX	Regional lymph nodes cannot be assessed
N0	No regional lymph node metastasis
N1	Metastasis in inguinal, mesorectal, internal iliac, or external iliac nodes
	N1a: Metastasis in inguinal, mesorectal, or internal iliac lymph nodes
	N1b: Metastasis in external iliac lymph nodes
	N1c: Metastasis in external iliac with any N1a nodes
Metastasis
M0	No distant metastasis
M1	Distant metastasis

HSIL: High-grade squamous intraepithelial lesion.

Tumor

Tumor (T) describes the extent of the primary tumor and its size in relation to growth into nearby structures or organs[124,125]. A larger value after the T reflects a bigger tumor size and growth into nearby structures[125]. TX indicates that the primary tumor cannot be assessed due to lack of information, and T0 means there is no evidence of a primary tumor[120,125]. T1 indicates tumor sizes less than 2 cm in the greatest dimension, T2 reflects tumor size between 2 and 5 cm, and T3 indicates tumor sizes more than 5 cm[124]. Lastly, T4 represents a tumor of any size that invades adjacent organs[120,124,125].

Regional lymph node

Lymph node (N) describes the spread of cancer to nearby lymph nodes[120,124]. NX indicates that regional lymph nodes cannot be assessed due to insufficient information[120,124]. N0 means no regional lymph node metastasis, and N1 indicates metastasis to nearby lymph nodes, such as the internal or external iliac nodes[120,124,125].

Distant metastasis

Metastasis (M) describes the spread of cancer to distant lymph nodes or organs[124]. M1 represents distant metastasis[120,124]. Anal cancer is staged based on TNM status (Table 4)[124]. Stage I and II are localized diseases[120,124]. Stage III indicates locally advanced cancer and stage IV represents metastatic disease[120,124,125].

Table 4 Anatomic stage of anal cancer.

Stage	T	N	M
0	Tis	N0	M0
I	T1	N0	M0
IIA	T2	N0	M0
IIB	T3	N0	M0
IIIA	T1-T2	N1	M0
IIIB	T4	N0	M0
IIIC	T3-T4	N1	M0
IV	Any T	Any N	M1

M: Metastasis; N: Node; T: Tumor.

TREATMENT OF LOCOREGIONAL ANAL CANCER

Anal SCC was predominantly treated with radical resection before the 1970s[121-125]. This procedure involved the removal of the distal colon, rectum, anus, and anal sphincter complex through perineal and anterior abdominal wall techniques, which formed a permanent colostomy[126]. This procedure resulted in 5-year overall survival (OS) rates between 40% and 70% and a surgical mortality rate of 3%[127].

A shift in the approach to the treatment of anal SCC came when researchers at Wayne State University in 1974 investigated a therapeutic regimen for SCC consisting of chemotherapy and radiation[128]. This protocol (the Nigro regimen) consisted of 5-FU, fluoropyrimidine, and mitomycin-C (MMC) with a concurrent dose of gray[128]. The results were promising, as the first three patients treated with the regimen experienced complete tumor regression[128-130]. A subsequent follow-up study was conducted in which patients treated with the protocol only underwent a salvage APR if there was clinical evidence of disease residue[130]. The 5-year OS rate was 67%, and the 5-year colostomy-free survival rate was 59%. A total of 84% of patients in the follow-up study showed complete response to CRT.

These results sparked further investigation into the role of CRT in the management of anal cancer, and several randomized controlled trials (RCTs) have since been done to cement CRT as the definitive treatment for anal cancer[131-135]. These trials have investigated the benefits of CRT vs radiotherapy (RT) alone, sequencing, and optimal chemotherapy protocols with simultaneous RT, and high-dose RT.

CRT vs RT

The United Kingdom Coordination Committee on Cancer Research phase 3 RCT, which involved 585 patients with anal SCC, compared RT to CRT[136]. The trial concluded that the use of 5-FU and MMC in combination with RT resulted in a 46% risk reduction in the local failure rate [relative risk (RR) = 0.54, 95%CI: 0.42-0.69], with a local failure rate of 36% with CRT vs 59% with RT. CRT was also associated with a significant reduction in cancer mortality, citing a 3-year anal cancer mortality of 39% with RT compared to 28% with CRT (RR = 0.71, 95%CI: 0.53-0.95). A RCT by the European Organization for Research and Treatment compared the use of RT or CRT (5-FU and MMC) in patients with anal cancer[135]. CRT was found to be associated with more improvement in local and regional control rates. A 32% difference in colostomy-free time was observed between the two groups, and an 18% difference in local and regional failure rates.

Chemotherapy protocol and sequence

MMC, although associated with toxicity, is an essential drug in chemotherapy regimens[137]. A phase 3 intergroup RCT that compared combined 5-FU and MMC to 5-FU alone showed a favorable colostomy-free rate (71% vs 59%, P = 0.014) and disease-free survival (DFS) (73% vs 51%, P = 0.0003) with combination chemotherapy at 4 years[134]. The addition of MMC to 5-FU, however, was associated with a greater risk of high-grade toxicity (23% vs 7%, P ≤ 0.001). Two RCTs then compared MMC to combined cisplatin and 5-FU, which revealed conflicting results[132,133]. The United States gastrointestinal (GI) intergroup radiation therapy oncology group (RTOG) 98-11 phase 3 trial randomized anal cancer patients to CRT including 5-FU and MMC vs two induction cycles of 5-FU and cisplatin followed by CRT including 5-FU and cisplatin[133]. The 5-FU and MMC combinations were associated with better 5-year DFS and OS than the 5-FU and cisplatin combinations. DFS was 68% for 5-FU and MMC compared to 58% for 5-FU and cisplatin at 5 years; OS was 78% and 71%, respectively. The overall severity of toxicity was similar between the treatment arms. The results of RTOG 98-11 continued to support the use of combined MMC and 5-FU with RT for patients with anal SCC.

A 2 × 2 factorial trial, ACT II, conducted by the United Kingdom Coordination Committee on Cancer Research, also investigated the substitution of MMC with cisplatin[132]. Patients with anal cancer were first randomized to receive CRT with either MMC or cisplatin, followed by randomization to receive adjuvant therapy with 5-FU and cisplatin or observation. Both arms had similar complete response rates at approximately 90%. Toxicity rates were about 70%, with comparable OS and DFS. The progression-free survival was 74% for maintenance chemotherapy vs 73% for no maintenance at 3 years. The results of the ACT II and RTOG 98-11 trials have together solidified 5-FU and MMC as the standard of care for managing locoregional anal SCC[132,133]. Several studies have been published recently, which highlight that capecitabine and MMC appear to have equal efficacy to infusional 5-FU and MMC in the treatment of locally advanced anal SCC[138-140].

Intensity modulated RT

Intensity modulated RT (IMRT) is the preferred method for RT in the treatment of locoregional anal SCC[141]. This modality utilized variations in radiation intensities that facilitate more precise mapping of tumor targets and spares nearby structures[141,142]. The goals of treatment are to optimize radiation dose and duration to limit toxicity and locoregional recurrence[141,143]. IMRT is preferred over conventional RT based on the results of a phase two multicenter prospective RCT (RTOG 0529) that compared 3D-conformal RT to dose-painted IMRT[144]. The primary endpoint was a decrease in the combined rate of grade 2 + GI and genitourinary adverse events by at least 15% compared to previous results in the RTOG 98-11 trial[133,144]. Significant reductions in grade 2 hematologic, grade 3 dermatologic, and GI toxicity were observed in the IMRT group. However, despite the improved toxicity profile, retrospective studies have failed to show a difference in survival outcomes between cancer patients who received standard RT vs IMRT[145,146].

Surgery

In locoregional anal cancer, CRT should be exhausted as the number one option for treatment due to better outcomes historically vs surgery[147,148]. APR and the formation of end colostomy should be reserved for salvage therapy in patients with tumor progression, disease recurrence following CRT, or patients who are ineligible for definitive CRT[148,149]. The exception to CRT as a first-line treatment for locoregional cancer is early-stage (stage I disease) perianal disease excluding the anal sphincter and superficially invasive anal SCC, for which a wide local excision is a treatment option[126,141].

TREATMENT OF RECURRENT AND METASTATIC DISEASE

Locoregional disease recurrence

Recurrent disease is defined as tumor discovery after 6 months post-CRT and a residual tumor if present within 6 months of treatment[150,151]. The average time for tumor recurrence of anal cancer is less than 12 months after CRT, and suspicion should prompt a detailed work-up with imaging and biopsy of all lesions to exclude metastasis[150]. The goal of management is to attain negative tumor margins, and patients are treated with APR with a permanent end colostomy and the creation of a sizable pelvic floor defect[151,152]. Salvage APR results in the control of disease locally in approximately 60% of all cases[151-153]. Salvage surgery also has a 5-year OS rate between 30 and 60%[154,155]. RT or brachytherapy following CRT has limited use in the management of local disease recurrence[141,156]. Similarly, the use of immunotherapy or systemic chemotherapy has limited data, and further research needs to be done to support use, although some articles have justified the use of MMC in the definitive management of local disease recurrence despite higher toxicity levels[141,157,158].

Metastatic disease recurrence

Systemic therapy is the current standard of treatment for patients with metastatic cancer or metastatic disease recurrence[159]. More than 10% of patients commonly experience distant relapse following CRT, and less than 10% of patients with de novo metastatic disease[159,160]. This first-line therapy is a platinum-based doublet regimen with either cisplatin or carboplatin and 5-FU or paclitaxel[161,162]. The phase 2 InterAACT trial randomized cancer patients with advanced anal SCC to treatment with either carboplatin and weekly paclitaxel or infusional 5-FU and cisplatin. Results suggested improvements in OS (20 months vs 12 months, P = 0.01) with the carboplatin/paclitaxel regimen and a favorable toxicity profile (36% vs 62% adverse event rate, P = 0.02) in comparison to the cisplatin and 5-FU regimen[163]. This trial established carboplatin/paclitaxel as the primary therapy for patients with metastatic disease and disease recurrence, and remains the standard of treatment for patients in the United States. Other studies have been done on regimens such as docetaxel, cisplatin, and 5-FU (DCF) and 5-FU, leucovorin, and cisplatin (FOLFCIS)[164,165]. These studies have shown promising results, making DCF and FOLFCIS good alternative options for patients with metastatic disease.

SURVEILLANCE FOLLOWING PRIMARY TREATMENT

According to guidelines from the National Comprehensive Cancer Network, patients should be re-evaluated with a digital rectal examination and anoscopy 8 weeks to 12 weeks after completion of CRT[166]. Following evaluation, patients should be classified according to whether they have complete remission of the disease, persistent disease, or progressive disease.

Patients with complete remission should have digital rectal examinations and lymph node exams every 3 months to 6 months for 5 years after treatment. Anoscopy is also recommended every 6 months to 12 months for 3 years post-treatment. Imaging is suggested for stage II and III anal cancers in complete remission every year for 3 years and takes the form of a CT of the chest, abdomen, and pelvis with contrast or a CT of the chest without contrast, plus an MRI of the abdomen and pelvis with contrast.

Patients who are found to have remaining cancer that has not grown, or spread are classified as having persistent disease. Watchful waiting for a maximum of 6 months can be considered, as some anal cancers take longer to respond to treatment. Following this period, the patient should be reassessed, and then be categorized as progressive disease if there is persistent cancer or worsening following this period. If the cancer goes into remission, surveillance follows the complete remission protocol.

Lastly, if a follow-up examination 8 weeks to 12 weeks after CRT finds that the cancer has grown or spread, the patient should be classified as having progressive disease, and a biopsy should be performed. If biopsy results confirm progression, imaging (CT scan with contrast or non-contrast CT and MRI) should be done to assess the location and spread of the disease, and to determine treatment.

PREVENTION AND SCREENING

Anal cancer is a rare disease that accounts for approximately 0.5% of all new cancers in the United States and less than 3% of cancers of the intestine[4,167,168]. While the incidence of anal cancer is too low for general population screening, there is a specific subset of groups with increased risk for AIN 2/3, including HIV-positive patients, MSM, solid organ transplant recipients, women with any history of cervical, vulvar, or vaginal cancer, and immunocompromised patients not due to HIV[167-169]. The high burden of anal cancer in these populations is partly attributed to the higher prevalence of HPV infection[169,170]. As a result, these patients should be regularly screened with anal cytology (also termed anal pap testing, using a Dacron swab inserted into the anal canal)[171,172]. Patients with an abnormal pap test should then undergo frequent surveillance with high-resolution anoscopy[173,174].

A single-center prospective study that evaluated the diagnostic accuracy of early anal cancer detection in high-risk patients (AIN-2+) using combined anal pap testing and high-resolution anoscopy showed that the diagnostic accuracy of anal cancer improves when the two tests are combined in comparison to anal pap testing alone[175].

HPV vaccination is a proven and cost-effective approach to the prevention of anal cancer in the younger population[176-178]. The administering of the 9-valent HPV vaccine to children could effectively prevent almost all anal tumors (primary prevention of anal SCC)[177]. Furthermore, the vaccine may help prevent AIN 2/3 and possible progression to SCC in high-risk groups[176-178]. This is true for populations that include MSM, HIV-positive men and women, solid organ transplant recipients, and women with a history of cervical cancer or CIN 3[177,179].

FUTURE DIRECTIONS

Although significant progress has been made with respect to the diagnosis and management of anal cancer, knowledge gaps still exist. More research is needed to address the prevention of anal carcinoma, especially in patients who are deemed high risk. Additionally, research focusing on minimizing the adverse effects of CRT in patients with early disease is crucial to improving the quality of life and survivorship care. Immunotherapy is an active area of research in the treatment of anal SCC. Therapeutic agents targeting programmed cell death receptor 1 have been shown to be effective against other HPV-related cancers, such as cutaneous SCC of the skin[6,180]. Due to this fact, several clinical trials are currently ongoing to investigate the efficacy of anti-programmed cell death receptor 1 agents in both metastatic and localized anal cancer (Table 5). Investigators are also currently assessing the role of monoclonal antibodies for use in anal cancer after the success of anti-epidermal growth factor receptor (anti-EGFR) therapies for cancers of the head and neck[181-183]. Cetuximab, an EGFR inhibitor combined with an interleukin-15 receptor superagonist, is currently being studied in the treatment of advanced SCC[184].

Table 5 Ongoing clinical trials investigating the use of anti-programmed cell death-1 agents in anal squamous cell carcinoma.

NCT number	Official title	Interventions	Study type	Phase	Status	Location
NCT04719988	Ezabenlimab (BI 754091) and mDCF (Docetaxel, Cisplatin, and 5-fluorouracil) Followed by Chemoradiotherapy in Patients With Stage III Squamous Cell Anal Carcinoma. A Phase II Study	Biological: Blood sample collection	Interventional	Phase 2	Active	France
		Procedure: Biopsy
NCT04894370	Spartalizumab, mDCF (Docetaxel, Cisplatin, and 5-fluorouracil) and Radiotherapy in Patients With Metastatic Squamous Cell Anal Carcinoma. A Phase IIA Study	Biological: Sample collection	Interventional	Phase 2	Recruiting	France
NCT04708470	A Phase I/II Study of Combination Immunotherapy for Advanced Cancers Including HPV-Associated Malignancies, Small Bowel, and Colon Cancers	Drug: Bintrafusp Alfa	Interventional	Phases 1 and 2	Recruiting	United States
		Drug: NHS-IL12
		Drug: Entinostat
NCT04432597	Phase I/II Trial of HPV Vaccine PRGN-2009 Alone or in Combination With Anti-PD-L1/TGF-Beta Trap (M7824) in Subjects With HPV-Positive Cancers	Biological: PRGN-2009 (Phase I)	Interventional	Phases 1 and 2	Active	United States
		Biological: PRGN-2009 (Phase II)
		Biological: M7824
		Diagnostic test: MRI
		Diagnostic test: Bone scan
		Diagnostic test: CT scan
		Diagnostic test: Brain CT
		Diagnostic test: Brain MRI
		Procedure: Biopsy (Phase I)
		Procedure: Biopsy (Phase II)
NCT05544929	A Phase I, Open-label, Multicenter Study of KFA115 as a Single Agent and in Combination With Pembrolizumab in Patients With Select Advanced Cancers	Drug: KFA115	Interventional	Phase 1	Recruiting	United States
		Drug: Pembrolizumab
NCT04357873	Phase II Basket Trial Evaluating the Efficacy of a Combination of Pembrolizumab and Vorinostat in Patients With Recurrent and/or Metastatic Squamous Cell Carcinoma	Drug: Pembrolizumab; vorinostat	Interventional	Phase 2	Active	France
NCT04802876	Efficacy of Tislelizumab and Spartalizumab Across Multiple Cancer-types in Patients With PD1-high mRNA Expressing Tumors Defined by a Single and Pre-specified Cutoff	Drug: Spartalizumab	Interventional	Phase 2	Recruiting	Spain
		Drug: Tislelizumab

CT: Computed tomography; HPV: Human papillomavirus; mDCF: Modified DCF regimen; MRI: Magnetic resonance imaging; PD1: Programmed cell death 1; PD-L1: Programmed cell death ligand 1; TGF: Transforming growth factor.

Additional novel therapies, including triplet immunotherapy and HPV-targeted vaccines, are presently being studied in clinical trials. Pembrolizumab, in combination with a bifunctional EGFR/transforming growth factor β fusion protein, is actively being investigated in the phase I KEYNOTE-E28 clinical trial in patients with recurrent or metastatic SCC[185]. UCPVax, derived from telomerase, is a CD4 helper T-inducer vaccine currently being studied in combination with immune checkpoint inhibitors for HPV-positive malignancies, including SCC, in the VolATIL trial[186]. Lastly, circulating tumor DNA is an emerging tool being investigated in the current NOAC9 phase 2 clinical trial[187]. This trial compares standard surveillance vs HPV-positive circulating tumor DNA-guided image follow-up to assess for improved detection of early treatment failure or recurrence.

CONCLUSION

Although anal cancer is rare in the general population, its incidence has been steadily increasing over the past decade. SCC is the predominant subtype of anal cancer and is HPV-mediated in most cases. The overall management of anal cancer has evolved over the past few decades. Currently, CRT, with the use of 5-FU and MMC, is the standard treatment for patients with early and locoregional disease. Wide local excision is a treatment option for a small subset of patients with stage I disease. Recurrence occurs in more than 20% of patients and requires salvage APR. A platinum-based doublet regimen is the mainstay of treatment for patients with metastatic disease. Immunotherapy is a treatment option for disease progression, and HPV vaccination is an effective long-term approach to limiting HPV-related anal cancer, especially in younger patients. Physical examination, in conjunction with routine screening, may increase detection of anal cancer in high-risk patients, which ultimately may reduce patient morbidity and mortality.