Anal Cancer Facts: Symptoms, Causes, HPV Link, and Types Explained
What is the Anus?
The anus is the small opening at the end of your digestive system where solid waste (stool) leaves your body. It sits at the very bottom of the tract, right after the rectum, which is the last part of the large intestine. Even though the anus is small, about the size of a coin, it has an important job. It works as the final checkpoint that controls when and how stool leaves the body.
Connected to the anus is a short tube called the anal canal. According to the National Cancer Institute (NCI), the anal canal is about 1 to 1.5 inches long. It links the rectum above to the anal opening below. This short tube is surrounded by two ring-shaped muscles called sphincters. The internal sphincter works automatically, without you thinking about it, while the external sphincter is one you can control on purpose. Together, these muscles squeeze shut to hold in stool and relax to let it pass.
The anal canal is unusual because it is made of two very different kinds of tissue. The upper part is lined with the same type of tissue found inside the rectum and intestines. The lower part, closer to the outside of the body, is covered with skin-like tissue similar to what covers the rest of the body. The place where these two tissue types meet is called the dentate line, or the anal transition zone. This detail matters because different types of anal cancer tend to start in different tissue types.
The anus also contains small glands and blood vessels. Some of these blood vessels can swell and form hemorrhoids, which are very common and usually harmless. The area also has many nerve endings, which is why anal problems, including anal cancer, can cause pain, itching, or a feeling of pressure.
Understanding the anus is different from understanding the rectum or colon, even though they are all part of the same digestive system. Many people confuse anal cancer with colorectal cancer, but according to the Cancer Society (ACS), anal cancer is a separate disease from colon and rectal cancer. It behaves differently, is caused by different things most of the time, and is treated differently.
Source: Cancer.Gov - National Cancer Institute
The skin around the outside of the anus is called the perianal skin. This skin has hair follicles, sweat glands, and oil glands, just like skin found elsewhere on the body. Cancer can also form in this outer skin area, not only inside the anal canal itself.
Blood and lymph fluid drain from the anus into nearby lymph nodes, which are small, bean-shaped structures that are part of the immune system. Lymph nodes near the anus are located in the groin area and around the pelvis. This drainage pattern is important because it can affect how cancer spreads if it forms in this area.
Because the anus is a private part of the body, many people feel embarrassed to talk about symptoms there. Doctors stress that no symptom in this area should be ignored out of embarrassment, since early detection makes anal cancer much easier to treat.
How does Anal Cancer Start?
Anal cancer starts when normal, healthy cells in the anus begin to change and grow out of control. Healthy cells in the body normally grow, divide, and die in an orderly way. Cancer happens when something damages the instructions inside a cell, called DNA, causing the cell to keep dividing instead of dying when it should.
According to the American Cancer Society, most anal cancers are linked to infection with human papillomavirus, commonly called HPV. HPV is a very common virus that spreads through skin-to-skin contact, most often during sexual activity. There are many types of HPV, and most cause no problems at all. However, certain high-risk types, especially HPV type 16, can cause lasting infections that eventually lead to cancer in the cells they infect.
Here is how the process usually works. HPV infects the thin, moist lining of the anus. In most people, the immune system finds and clears the virus within one to two years, and no lasting harm is done. But in some people, the infection does not go away. Over many years, this persistent infection can cause the infected cells to grow abnormally. This abnormal growth is called anal intraepithelial neoplasia, or AIN, which is considered a precancerous condition. Explained more in the next sections, AIN can sometimes progress into true anal cancer if it is not found and treated.
Not every case of anal cancer is caused by HPV, though it is the leading cause in the United States according to the Centers for Disease Control and Prevention (CDC). Other factors can also damage the DNA in anal cells and raise the risk of cancer forming. These include long-term smoking, having a weakened immune system, and chronic inflammation in the area.
A weakened immune system plays a big role in how anal cancer starts. The immune system is normally very good at finding and destroying infected or abnormal cells before they can turn into cancer. People with HIV, people who have had organ transplants, and people taking medications that suppress the immune system have a harder time clearing HPV infections. This gives the virus more time to cause lasting damage to anal cells.
Smoking is another major factor. Cigarette smoke contains many harmful chemicals that travel through the bloodstream and can damage cells throughout the body, including cells in the anus. Smokers with HPV infections appear to have a higher chance of the infection progressing to cancer compared to nonsmokers with the same infection.
The process from a normal cell to a cancer cell usually takes many years. This is actually good news because it means there is often a long window of time when precancerous changes, like AIN, can be found and treated before they ever become cancer. This is why regular checkups and being honest with a doctor about symptoms and risk factors matter so much.
Once a cell becomes fully cancerous, it can grow into a tumor and, if not treated, invade nearby tissues. Left untreated for a long time, cancer cells can also break away and travel through the lymphatic system or bloodstream to other parts of the body, a process called metastasis.
Where does Anal Cancer Start?
Anal cancer can start in a few different specific locations within this small area of the body, and where it starts affects what type of cancer it becomes. Most anal cancers begin in the anal canal, the short tube connecting the rectum to the outside of the body.
The most common starting point is the lining of the anal canal itself, called the mucosa. This lining contains squamous cells, which are thin, flat cells. Most anal cancers form in these squamous cells, which is why the most common type of anal cancer is called squamous cell carcinoma.
Cancer can also start right at the dentate line, the zone where the intestinal type lining meets the skin type lining. This transition zone contains a mix of cell types, so cancers starting here can sometimes have mixed characteristics.
Another possible starting point is the perianal skin, the skin on the outside surrounding the anal opening. Skin cancers, including certain rare types of melanoma and basal cell carcinoma, can develop here, though this is less common than cancer starting inside the canal.
Some anal cancers start in glandular tissue. There are small glands inside the anal canal that produce mucus to help stool pass smoothly. When cancer forms in these gland cells, it is called adenocarcinoma. This is much less common than squamous cell carcinoma but is an important type to know about because it is treated differently.
Rare cancers can also start in other specialized cells within the anal area, including pigment-producing cells, leading to anal melanoma, or in specialized nerve cells found in the wall of the digestive tract, leading to a rare tumor called a gastrointestinal stromal tumor, or GIST.
Doctors determine exactly where a cancer started by looking at tissue samples, called biopsies, under a microscope. This information helps doctors classify the type of cancer and choose the most effective treatment plan.
Knowing where in the anus a cancer began also helps doctors predict how it might spread. Cancers that start higher up in the anal canal tend to drain toward lymph nodes near the rectum and inside the pelvis, while cancers starting closer to the outer skin tend to drain toward lymph nodes in the groin.
How does Anal Cancer Affect the Body?
Anal cancer can affect the body in several ways, starting with local symptoms right at the site of the tumor. The most common early symptom is bleeding from the anus or rectum, which many people mistake for hemorrhoids. Because of this overlap in symptoms, anal cancer is sometimes not caught right away.
Other common symptoms include itching around the anus, pain or a feeling of fullness in the area, and a lump or mass that can sometimes be felt near the anal opening. A cancerous lump often feels hard, irregular, or rough, unlike the soft, smooth feel of a typical hemorrhoid.
As a tumor grows larger, it can begin to affect normal bowel function. This might show up as changes in bowel habits, such as stools that are narrower than usual, straining during bowel movements, or a new sense of urgency. Some people also experience unusual discharge from the anus or, in more advanced cases, loss of control over bowel movements, called fecal incontinence.
If cancer cells spread beyond the anus, they most often travel first to nearby lymph nodes in the groin or pelvis. Swollen lymph nodes in these areas can sometimes be felt as new lumps. This is an important warning sign that the cancer may have moved beyond its original location.
In more advanced cases, anal cancer can spread through the bloodstream to distant organs, most commonly the liver and lungs. When cancer spreads this far, it is called metastatic or stage 4 anal cancer, and treatment focuses on controlling the disease and managing symptoms throughout the body rather than curing it completely.
Treatments themselves can also affect the body. Chemotherapy and radiation, which are common anal cancer treatments, can cause side effects such as skin irritation in the treated area, fatigue, and changes in bowel or bladder function. In some cases, surgery to remove the tumor may affect how the sphincter muscles work, which can change bowel control.
The good news, according to the American Cancer Society, is that anal cancer treatment is often very effective, especially when the cancer is found early, before it has spread far from where it started.
Types of Anal Cancer
Squamous cell carcinoma: The most common type of anal cancer by far is squamous cell carcinoma. This type makes up the large majority of anal cancer cases and starts in the thin, flat squamous cells that line the anal canal. In the United States, squamous cell carcinoma of the anus is very strongly linked to HPV infection.
Carcinoma in situ: Sometimes described as stage 0 anal cancer. This means abnormal squamous cells are present only in the surface layer of tissue and have not grown any deeper. It is considered a precancerous or very early cancerous state, closely related to high-grade anal intraepithelial neoplasia, discussed more in the next section.
Anal Adenocarcinoma: A much rarer type forms in the glandular cells inside the anal canal, the cells responsible for producing mucus. According to MD Anderson Cancer Center, adenocarcinoma behaves more like colorectal cancer and is often treated with an approach similar to rectal cancer treatment, rather than the approach used for squamous cell anal cancer.
Anal melanoma: Another rare type, forming in the pigment-producing cells called melanocytes that are present in small numbers in the anal lining. Anal melanoma tends to be more aggressive and is often found at a later stage because it can be mistaken for a hemorrhoid or polyp and does not always look like typical skin melanoma.
Basal cell carcinoma: A type of skin cancer that is quite common elsewhere on the body, can rarely occur on the perianal skin surrounding the anal opening. This form tends to grow slowly and is less likely to spread compared to squamous cell carcinoma or melanoma.
Gastrointestinal stromal tumors: Known as GISTs, are an extremely rare type of tumor that can occasionally occur in the anal or rectal area. GISTs start in specialized cells in the wall of the digestive tract called interstitial cells of Cajal, rather than in the lining cells. GISTs of the anus are considered quite uncommon and are treated differently from other anal cancers, often with targeted drug therapy in addition to surgery.
Doctors determine the exact type of anal cancer through a biopsy, where a small piece of tissue is removed and examined under a microscope by a specialist called a pathologist. Knowing the exact type is essential because treatment approaches differ significantly between squamous cell carcinoma, adenocarcinoma, melanoma, and GIST.
Because squamous cell carcinoma is so much more common than the other types, most research, screening guidelines, and treatment recommendations for anal cancer are built around this type specifically.
Related Conditions to Anal Cancer
Anal intraepithelial neoplasia: One of the most important related conditions to understand, often shortened to AIN. According to The Cleveland Clinic, AIN, also called anal dysplasia, means there are abnormal cells in the lining of the anus that are not cancer yet, but could become cancer over time if left untreated. AIN is graded as low grade or high grade depending on how abnormal the cells look.
Low-grade AIN often goes away on its own as the immune system clears the underlying HPV infection, and it rarely progresses to cancer. High grade AIN is more concerning. Research cited by the Cleveland Clinic suggests that roughly 3 percent to 14 percent of people with high-grade anal dysplasia eventually develop anal cancer if it is not treated, which is why doctors recommend monitoring and, often, treatment to remove or destroy these abnormal cells.
Anal warts: Also called condyloma, are another HPV related condition connected to anal cancer. These are small, flesh colored growths caused by certain types of HPV. While anal warts themselves are usually caused by lower-risk HPV types and are not cancerous, having a history of anal warts is considered a risk factor for anal cancer because it shows a person has been exposed to HPV in this area.
Hemorrhoids: A very common condition that is not related to cancer but is frequently confused with it because the symptoms, like bleeding and discomfort, can look similar. Hemorrhoids are swollen veins in or around the anus, often caused by straining during bowel movements, pregnancy, or chronic constipation. Unlike cancer, hemorrhoids are almost always harmless, though they can be painful.
Anal fissures: Another benign but related condition. A fissure is a small tear in the lining of the anus, often caused by passing hard stool. Fissures can cause sharp pain and bleeding during bowel movements, symptoms that overlap with anal cancer, which is why any persistent anal pain or bleeding should be checked by a doctor rather than assumed to be a fissure.
Anal fistulas: Abnormal tunnels that can form between the anal canal and the skin near the anus, often following an infection in a small gland inside the anal canal. While fistulas are not cancer, chronic, long-lasting fistulas, especially those related to conditions like Crohn's disease, have in rare cases been linked to a higher risk of cancer developing in the surrounding tissue over many years.
HIV infection: Closely related to anal cancer risk, though it is not the same disease. Because HIV weakens the immune system over time, people living with HIV are far less able to clear HPV infections naturally, which raises their risk of developing AIN and eventually anal cancer.
Doctors sometimes recommend a test called an anal Pap smear, similar to the cervical Pap smear used to screen for cervical cancer, along with a procedure called high-resolution anoscopy, to look closely for AIN or early cancer in people at high risk, such as those living with HIV or men who have sex with men.
What Are the Genetic and Risk Factors?
Unlike some other cancers, anal cancer is not usually thought of as a strongly inherited or genetic disease that runs directly in families. Instead, most of the known risk factors are related to infections, lifestyle, and immune system health rather than genes passed down from parents.
HPV: The single biggest risk factor for anal cancer is infection with human papillomavirus, particularly HPV type 16. HPV is responsible for more than 90 percent of anal cancers in the United States. HPV spreads mainly through skin-to-skin sexual contact, and because it is so common, most sexually active people will be exposed to some type of HPV during their lifetime.
A weakened immune system: Another major risk factor. This includes people living with HIV or AIDS, people who have received an organ transplant and take medications to prevent organ rejection, and people with certain autoimmune diseases who take immune-suppressing drugs. A weaker immune system has a harder time clearing HPV infections, allowing the virus more time to cause cell damage.
Smoking cigarettes: Significantly raises the risk of anal cancer. According to the American Cancer Society, current smokers are more likely to develop anal cancer than people who have never smoked, and the risk appears to increase with a longer smoking history.
Sexual history: Plays a role as well. Having many sexual partners over a lifetime and engaging in receptive anal intercourse both increase the chances of HPV exposure in the anal area. Men who have sex with men have a higher rate of anal cancer, largely connected to HPV exposure patterns.
Age and biological sex: Are factors, though they are not truly genetic. According to ACS statistics, anal cancer is rare before age 35 and becomes more common with increasing age, with many cases diagnosed in people age 65 and older. In the United States, anal cancer is diagnosed almost twice as often in women as in men overall, though rates vary by specific risk group.
History of other cancers: Raises risk too. Women who have had cervical, vaginal, or vulvar cancer, all of which can also be caused by HPV, have a higher chance of developing anal cancer later in life. This connection reflects a shared cause, HPV, rather than a shared inherited gene.
While anal cancer is not classified as a hereditary cancer syndrome the way some breast or colon cancers are, ongoing research continues to study whether certain inherited traits might affect how a person's immune system responds to HPV, which could someday help explain why some people with HPV develop cancer while most do not.
What Are the Related Cancers to Anal Cancer?
Anal cancer belongs to a family of cancers that share HPV as a common cause, and understanding these related cancers helps show the bigger picture of how this virus affects the body. More than 39,000 people in the United States are diagnosed with an HPV related cancer every year.
Cervical cancer: Is the most well known HPV related cancer and shares the closest link to anal cancer. HPV causes nearly all cervical cancers, and women with a history of cervical cancer have an increased risk of also developing anal cancer, since both are triggered by the same high-risk HPV types.
Vaginal and vulvar: Cancers are also part of this related group. These cancers affect the internal and external female genital organs and are frequently linked to HPV infection, particularly HPV type 16, the same type most responsible for anal cancer.
Penile cancer: Which affects men, is another cancer in the HPV related family. The CDC notes that HPV is thought to cause roughly 60 percent of penile cancers. Like anal cancer, penile cancer often develops slowly from precancerous changes in the skin.
Oropharyngeal cancer: Which affects the back of the throat, the base of the tongue, and the tonsils, is one of the fastest growing HPV related cancers in the United States. According to the CDC, a majority of oropharyngeal cancers are now linked to HPV rather than to traditional causes like smoking and alcohol alone, and this cancer has become more common than cervical cancer in recent years.
Beyond HPV related cancers, colorectal cancer is worth mentioning because it is often confused with anal cancer due to their close location in the body. However, colorectal cancer usually starts from polyps in the colon or rectum and is generally not caused by HPV, making it a genuinely different disease from anal cancer, with different screening tests, like colonoscopy, and different typical risk factors.
Because so many cancers share HPV as a root cause, the HPV vaccine is one of the most powerful prevention tools available. According to the American Cancer Society, HPV vaccines can prevent more than 90 percent of HPV related cancers when given at the recommended ages, making vaccination an important step in reducing the future burden of anal cancer along with cervical, vaginal, vulvar, penile, and oropharyngeal cancers across the United States.