Long-standing fistulas can, in rare cases, give rise to cancer. The transformation happens most often when a fistula has been chronically inflamed for years or decades, and it accounts for roughly 3% to 11% of all anal canal cancers depending on the study.1Annals of Coloproctology. Chronic fistula in ano associated with adenocarcinoma: a case report with a review of the literature The risk is low enough that most people with a fistula will never develop a malignancy, but it is real enough that clinicians and patients should be aware of it, especially when a fistula has been present for a long time or when Crohn’s disease is involved.
How Chronic Inflammation Leads to Malignant Change
A fistula is an abnormal tunnel between two body surfaces, most commonly between the inside of the anal canal and the skin near the anus. When one of these tracts stays open and inflamed for years, the cells lining it are under constant stress. They’re repeatedly damaged and repaired, and over time that cycle of destruction and regeneration can push cell growth in a direction that becomes uncontrolled. The process is sometimes called malignant transformation, and it’s the same basic principle behind other chronic-wound cancers: tissue that never fully heals is tissue at higher risk of going wrong.
The cancers that develop within fistula tracts tend to be one of two types. Adenocarcinoma, which arises from glandular cells, is the more common variety. Squamous cell carcinoma, which comes from the flat cells lining skin and mucous membranes, is the second most frequent. A systematic review of cancers arising in perianal fistulas among Crohn’s disease patients found adenocarcinoma in about 59% of cases and squamous cell carcinoma in about 31%.2Journal of Gastrointestinal Surgery. Malignant Transformation in Perianal Fistulas of Crohn’s Disease: a Systematic Review of Literature Among the adenocarcinomas, the mucinous subtype, which produces mucus-like material, is especially common in fistula-associated cases, unlike typical colorectal cancers where ordinary tubular adenocarcinoma dominates.3Modern Pathology. Adenocarcinoma within anorectal fistulae: different clinicopathological characteristics between Crohn’s disease-associated type and the usual type
At the molecular level, researchers have found evidence that a process called epithelial-to-mesenchymal transition plays a role. In plain terms, this is when the cells lining a fistula tract begin behaving more like mobile, invasive cells rather than staying put in an orderly layer. In one case of fistula-associated cancer in a Crohn’s patient, investigators found strong expression of a protein called Slug in the transitional cells lining the fistula tract, a pattern not seen in fistulas without cancer.4European Journal of Gastroenterology & Hepatology. Epithelial-to-mesenchymal transition in a fistula-associated anal adenocarcinoma in a patient with long-standing Crohn’s disease Broader molecular profiling of fistula-associated anal adenocarcinomas has revealed mutations in genes like TP53, NOTCH1, and SMAD4, which are known players in other gastrointestinal cancers.5PubMed. Fistula-Associated Anal Adenocarcinoma: A 20-Year Single-Center Experience
The Crohn’s Disease Connection
If there’s one group for whom fistula-associated cancer is a real clinical concern, it’s people with Crohn’s disease. Perianal fistulas are already common in Crohn’s, affecting somewhere around a quarter to a third of patients over the course of the disease. When those fistulas persist for a long time, the added layer of Crohn’s-related chronic inflammation raises the stakes.
The timeline is revealing. In a systematic review of squamous cell carcinomas arising from perianal fistulas in Crohn’s patients, the average gap between the onset of Crohn’s disease and a cancer diagnosis was about 21 years, and the average gap from fistula appearance to cancer was about 11 years.6PubMed Central. Fistula-Related Cancer in Crohn’s Disease: A Systematic Review A Dutch study tracking patients over 17 years confirmed that malignant transformation in perianal and enterocutaneous fistulas is rare but tends to surface after roughly a decade of fistula presence, with a median age at cancer diagnosis around 48.7PubMed. Malignant transformation of perianal and enterocutaneous fistulas is rare: results of 17 years of follow-up from The Netherlands
A multicentre case-control study comparing Crohn’s patients who developed fistula-associated carcinoma against those whose fistulas remained benign found some telling differences. Patients who went on to develop cancer had lived with Crohn’s disease significantly longer, about 26 years on average compared with roughly 20 years in the control group. Their fistulas were also more complex and chronically active. The study found adenocarcinoma in 33 of 40 cancer cases and squamous cell carcinoma in the remaining seven.8Journal of Crohn’s and Colitis. Perianal Fistula-Associated Carcinoma in Crohn’s Disease: A Multicentre Retrospective Case Control Study The takeaway: the longer and more severely a fistula has been causing trouble, the more seriously any new or worsening symptoms should be investigated.
Why These Cancers Are Easy to Miss
One of the most frustrating aspects of fistula-associated cancer is how well it hides. The early symptoms, pain, discharge, swelling, changes in the tissue around the fistula, look almost identical to a routine flare-up or a benign complication. In the Dutch follow-up study mentioned above, only one of four patients who developed adenocarcinoma actually had symptoms that pointed toward cancer. The other three were found incidentally.7PubMed. Malignant transformation of perianal and enterocutaneous fistulas is rare: results of 17 years of follow-up from The Netherlands
Imaging can help, but even MRI findings can be ambiguous. The mucinous adenocarcinomas that are especially common in fistula tracts tend to appear as multiloculated, cauliflower-shaped masses with thin walls, features that can overlap with complicated abscesses or inflammatory collections. Advanced MRI sequences can distinguish between different tumor types once suspicion is raised, but the problem is getting to that point in the first place.9PubMed Central. Magnetic resonance imaging findings of carcinoma arising from anal fistula: A retrospective study in a single institution The consequence is that many patients already have advanced disease by the time surgery happens.
The clearest red flag is a sudden change in symptoms in a fistula that has been stable for a long time. New pain, increased or unusual discharge, a mass that feels different from the usual fistula tract, or rapid worsening that doesn’t respond to typical treatment should all prompt a biopsy under anesthesia with thorough tissue sampling.10PubMed Central. Cancer in Anal Fistulas Clinicians who manage chronic fistulas in Crohn’s patients are increasingly aware of this, but there’s still no established formal screening program that tells them exactly when and how often to check.11PubMed Central. Perianal disease in inflammatory bowel disease: Broadening treatment and surveillance strategies for anal cancer
HPV as an Additional Risk Factor
Human papillomavirus (HPV) is best known for its role in cervical cancer, but it’s also a recognized cause of anal squamous cell carcinoma in the general population. For people with Crohn’s disease and perianal fistulas, there may be an extra layer of vulnerability. A comparative study found that patients with perianal fistulizing Crohn’s disease had about three times the odds of carrying HPV in their fistula tracts compared with patients without Crohn’s. About a third of the Crohn’s fistula group tested positive, versus roughly 17% of controls. Among the HPV types detected, the high-risk HPV 16 was present in about 9% of the Crohn’s fistula group.12Clinics. Perianal fistulizing Crohn’s disease is associated with a higher prevalence of HPV in the anorectal fistula tract
This is concerning because HPV 16 is one of the strains most strongly linked to cancer. Whether HPV actually drives fistula-associated cancers or is just a bystander that thrives in chronically inflamed tissue isn’t fully settled. But the finding adds weight to the argument that patients with chronic perianal fistulas, particularly those on immunosuppressive medications for Crohn’s disease, deserve vigilant monitoring of the perianal region.
Beyond Anal Fistulas
Perianal fistulas in Crohn’s disease dominate the research, but cancer can arise from other types of chronic fistula tracts as well.
Enterocutaneous fistulas, tunnels connecting the intestine to the skin surface, have been reported as sites of malignant change. A case report documented adenocarcinoma developing in an enterocutaneous fistula in a patient with long-standing Crohn’s disease. The cancer appeared to arise from columnar epithelium that had re-lined the fistula tract over time.13PubMed. Primary adenocarcinoma in an enterocutaneous fistula associated with Crohn’s disease Squamous cell carcinoma has also been found in enterocutaneous fistula tracts, and the clinical profile is similar: a patient with years of chronic, poorly healing disease whose symptoms suddenly worsen.14PubMed Central. Squamous Cell Carcinoma Originating from a Crohn’s Enterocutaneous Fistula Clinical suspicion is often low in these scenarios, which delays diagnosis.
Osteomyelitis fistulas present another, less well-known pathway. When a bone infection drains through a sinus tract to the skin for years, the chronic wound at the skin surface can undergo the same kind of malignant transformation. This is sometimes classified under the umbrella term “Marjolin’s ulcer,” which refers to any cancer arising in a chronic wound, whether from a burn scar, a pressure ulcer, or a draining fistula. One reported case involved a patient who developed squamous cell carcinoma in an osteomyelitis fistula on his leg, 33 years after the original fracture that triggered the infection.15PubMed. Marjolin Ulcer Developed in Osteomyelitis Fistula: A Rare Clinical Entity That Should Not Be Overlooked The warning signs parallel those of fistula-associated cancers elsewhere: a chronic wound that won’t close, rolled-out wound edges, and raised or excessive granulation tissue should all prompt biopsy.16PubMed Central. Marjolin’s Ulcer of the Tibia With Pelvic Lymph Node Metastasis
It’s worth clarifying a related but distinct scenario in gynecological cancers. Fistulas can develop as a complication of cancer treatment, particularly radiation therapy for cervical or vaginal cancers. In that case, the cancer came first and the fistula is a side effect, not the other way around. One study of patients who received brachytherapy for advanced gynecological cancers found that about 5% developed treatment-related fistulas, while an additional group developed fistulas in the setting of recurrent disease.17Journal of Contemporary Brachytherapy. Risk factors for fistula formation after interstitial brachytherapy for locally advanced gynecological cancers involving vagina When people ask “can a fistula cause cancer,” they usually mean the chronic-inflammation-to-cancer direction. But it’s worth knowing that cancer and its treatments can also cause fistulas, and that a new fistula in someone being treated for cancer should raise concerns about disease recurrence.
Treatment When Cancer Is Found
Treating cancer that has developed within a fistula is more complicated than treating either condition alone. The standard surgical approach for fistula-associated anal adenocarcinoma is an extensive procedure called abdominoperineal resection, which removes the rectum, anus, and surrounding tissue. Because these cancers tend to be locally aggressive with a high risk of coming back, surgeons generally take wide margins. Reconstruction of the perineum afterward often requires tissue flaps.
Surgery alone doesn’t appear to be enough for most patients. A multimodal approach combining preoperative chemoradiation (chemotherapy and radiation given before surgery to shrink the tumor), the surgery itself, and sometimes adjuvant chemotherapy afterward has shown promising results in small case series. In one report of three patients treated this way, the two who completed the full course of therapy had no recurrence or distant spread during follow-up.18PubMed Central. Mucinous adenocarcinoma arising from chronic perianal fistula-a multidisciplinary approach Another small series explored adding hyperthermia (targeted heating of the tumor area) to chemoradiation before surgery, with all three patients achieving complete tumor removal at the margins.19PubMed Central. Outcomes of surgical treatment in patients with anorectal fistula cancer
The honest picture, though, is that the evidence base is thin. Sample sizes in published studies are small because fistula-associated cancers are rare events. There’s no consensus on the single best treatment protocol. What clinicians do agree on is that aggressive, combined treatment gives the best chance of long-term survival.10PubMed Central. Cancer in Anal Fistulas
Survival and Prognosis
Outcomes for fistula-associated cancer depend heavily on how early the disease is caught and whether surgery can achieve clear margins. In the multicentre Crohn’s disease study, overall survival averaged about 45 months, and the five-year survival rate was 65%.8Journal of Crohn’s and Colitis. Perianal Fistula-Associated Carcinoma in Crohn’s Disease: A Multicentre Retrospective Case Control Study That’s a reasonable number for a cancer that is usually caught late, and there’s some evidence that outcomes have been improving as awareness grows and treatment gets more aggressive. The factors that predicted worse outcomes were the type of cancer (adenocarcinoma versus squamous cell carcinoma behave differently), whether the cancer had already spread, and whether the surgeon was able to remove it completely.
The biggest modifiable factor is timing. Earlier detection means smaller tumors, cleaner surgical margins, and a better chance that the cancer hasn’t spread. This is why the conversation about establishing formal surveillance programs for people with long-standing Crohn’s fistulas keeps coming up in the gastroenterology community, even though no consensus protocol exists yet.11PubMed Central. Perianal disease in inflammatory bowel disease: Broadening treatment and surveillance strategies for anal cancer
What to Watch For
If you have a chronic fistula, particularly one related to Crohn’s disease that has been present for more than a few years, the risk of cancer is low but not zero. No one is suggesting you should panic, but you should know what to watch for and take unexplained changes seriously.
Symptoms that warrant prompt medical evaluation include:
- New or worsening pain: A fistula that has been stable or mildly bothersome and suddenly becomes significantly more painful.
- Changed discharge: New types of drainage, bloody or mucous discharge that wasn’t there before, or a marked increase in volume.
- A palpable mass: Any new lump or hardness near the fistula opening that feels different from the usual tract.
- Failure to heal: If treatment that previously helped the fistula stops working, or if the fistula worsens despite appropriate therapy.
- Wound-edge changes: In skin-level fistulas, rolled or raised edges around the opening and excessive granulation tissue are classic warning signs of malignant transformation.
These signs should prompt a biopsy, not just a clinical exam. A visual inspection alone can’t distinguish inflammatory tissue from early cancer in most cases.
The Emotional Weight of Living With Chronic Fistulas
An article about cancer risk can add to the already substantial psychological burden of living with a chronic fistula. It’s worth acknowledging how heavy that burden already is. Qualitative research with Crohn’s fistula patients has documented an impact that goes well beyond physical symptoms, affecting intimate relationships, work opportunities, and emotional well-being. Patients describe feelings of being dirty, embarrassed, and anxious.20PubMed Central. Burden of disease and adaptation to life in patients with Crohn’s perianal fistula: a qualitative exploration A large international survey found that patients with Crohn’s perianal fistulas experienced significantly more psychological distress than patients with Crohn’s disease alone, including worse self-concept and higher rates of anxiety and depression.21Crohn’s & Colitis 360. The Impact of Crohn’s Perianal Fistula on Quality of Life: Results of an International Patient Survey
Adding cancer worry to that mix isn’t easy. But knowing the risk factors, understanding that it’s rare, and being aware of the warning signs puts you in a much stronger position than being caught off guard. If you’ve had a perianal fistula for more than a decade, it’s reasonable to discuss long-term monitoring with your gastroenterologist, even if current guidelines don’t mandate a specific screening schedule. The conversation itself is a form of surveillance.
Congenital Fistulas and Cancer Risk in Children
Most of the evidence on fistula-associated cancer involves adults with longstanding acquired disease. But some children are born with congenital anomalies that include fistulas, such as those seen in a cluster of birth defects known as VACTERL association, which can involve vertebral, cardiac, kidney, and limb abnormalities along with anorectal and tracheoesophageal malformations. A recent population-based study of over 21 million births found that children meeting VACTERL criteria had roughly three times the cancer risk of unaffected children, with a particularly elevated risk for embryonal tumors.22PubMed. A Population-Based Assessment of Cancer Risk in Children With VACTERL The mechanism here is different from the chronic-inflammation pathway in adults. The increased cancer risk in these children likely reflects underlying developmental or genetic vulnerabilities rather than the fistula itself acting as the trigger. Still, it’s relevant context for families dealing with congenital anomalies that include fistulous connections. The absolute risk remains small, but the relative increase is enough that pediatricians who manage these conditions factor it into long-term follow-up.