Anal cancer is a rare tumor. In 2017, the estimated incidence rate for anal cancer in the United States is 8200, and the number of deaths from anal cancer is 1100.
There is a clear correlation between human papillomavirus (HPV) infection, mainly HPV 16, and anal cancer. Hence, there is an increased incidence in certain populations such as in young men in whom have genital viral infections. A large study highlighted a correlation between both the amount of sexual activity and venereal infections and anal cancer. Earlier low-powered studies linked anal-receptive intercourse to an increased risk of anal cancer, but this has not been confirmed in a large trial. In addition to HPV, condylomata infections have been implicated in anal cancer in the general population and homosexual men. A study showed that in women with genital warts, anal cancer was associated with herpes simplex virus (HSV) 1 infection and Chlamydia trachomatis. Conversely, in men with no history of genital warts, there was an association with gonorrhea infection. In addition to the etiologies mentioned above, there has been an association between acquired immunodeficiency syndrome (AIDS) and anal cancer. In patients infected with the human immunodeficiency virus (HIV), the risk of anal cancer is 40 times higher compared to the general population.
Anal cancer is a rare tumor comprising only 2.5% of all digestive system malignancies in the United States. Per data from the National Cancer Institute's Surveillance, Epidemiology, and End Results Program (SEER) the incidence of new cases in the United States was 8200 with 1100 estimated deaths in 2017. The five-year overall survival between 2007 and 2013 was 66.9%. Also, approximately 0.2% of men and women will be diagnosed with anal cancer at some point during their lifetime based on acquired data from 2012 through 2014. The men to woman ratio is approximately 2 to 3.5. Anal cancer is more prevalent in patients with viral genital infection such as HSV and HIV. Also, there is increased incidence in individuals with multiple sexual partners, practice anal-receptive intercourse, have condyloma infections, or have AIDS.
The pathophysiology of the development of anal cancer is believed to be directly linked to a complex inflammatory process secondary to infections such as HSV (particularly serotypes 16 and 18). In a Scandinavian study, serotype 16 was detected in 73% of anal cancer specimens, and serotypes 16, 18, or both were present in 84% of the specimens. The progression of this inflammatory process heralds the development of anal intraepithelial neoplasia (AIN), or squamous cell carcinoma in situ (Bowen disease), which is a pre-malignant condition. Anal intraepithelial neoplasia can be graded from I to III depending on the abnormalities in differentiation and maturation of the squamous layers, mitotic activity, nuclear membranes changes, and depth of those abnormalities. AIN can subsequently progress to invasive squamous cell carcinoma (SCC), and this occurs in 10% to 11% of cases. HIV-positive patients have a higher rate of conversion to squamous cell carcinoma even if AIDS is not manifested. Most anal cancers are squamous or colitogenic cancers with a few adenocarcinomas and rarely melanoma. Tumors tend to spread by local extension but also have the potential to metastasize.
A complete history and physical examination are quintessential in patients with anal cancer. Patients may have no symptoms or present with anal bleeding, anal or pelvic pain, weight loss, the sensation of a mass in the anus or rectum, anal irritation, prolapse of tissue, incontinence of flatus or stools, and obstipation. Due to the negative stigma associated with this diagnosis, patients usually present late for medical care. Approximately 19% of patients wait six months or more after the onset of symptoms before seeking medical attention. Moreover, misdiagnosis does occur with some cases misdiagnosed as hemorrhoids, and in one review this occurred in 27% of cases. A complete history is important such as past medical history, sexual activity history, and any history of venereal diseases. A complete physical examination is imperative including digital rectal examination, inguinal lymph node palpation, and gynecologic examination in females including screening tests for cervical cancer.
In addition to complete history and physical examination, laboratory and radiologic examinations are important. Laboratory studies include a complete blood count (CBC), chemistry panel, HIV testing in specific cases, anoscopy, and anal mass biopsy or fine-needle aspiration, as well as imaging as indicated including CT or PET scan or an MRI. Imaging is imperative specifically when metastatic disease is suspected. Moreover, if the diagnosis of anal cancer is confirmed testing for HSV, gonorrhea, and chlamydia is recommended.
In the past, the treatment of anal cancer was surgical resection utilizing the anterior-posterior approach. This treatment was curative in 50% of cases and was associated with significant morbidity. The current treatment modality for localized anal cancer is chemotherapy and radiation. The treatment of anal cancer, akin to many other malignancies, is dependent on the stage of disease at presentation. Tumors that are less than 2 cm with no lymph node involvement and are well differentiated (T1N0, stage I) are treated with local excision. If adequate negative margins are attained, patients are usually observed. Conversely, if tumors are stage I (T1N0) but are poorly differentiated or are a higher stage but not metastatic (poorly differentiated T1N0, T2-T4, and N0, or any T and N positive) are treated with chemotherapy and radiation. The regimen most widely used currently is mitomycin-C (12 mg/m intravenous (IV) bolus on day one), fluorouracil (1000mg/m/day IV continuous infusion over four days, during the first and last week of radiation), and radiation (total dose 45 Gy). This regimen afforded patients a 46% reduction in the risk of local failure. The expectations for patients is a complete response rate (CR) in 70% to 80% of cases with an overall five-year survival rate of greater than 65%. A large randomized phase III trial that involved 682 patients and compared the regimen above to an intensified regimen consisting of induction with cisplatin and fluorouracil followed by cisplatin, fluorouracil, and radiation. The latter regimen did not show superiority, and in fact, the five-year local-regional recurrence rate, distant metastatic rate, and the cumulative rate of colostomy favored the mitomycin-C regimen. Thus, in clinical practice, the established regimen is mitomycin-C, fluorouracil, and radiation. However, if mitomycin-C is contradicted then the cisplatin, fluorouracil, and radiation can be used. Patients presenting with inguinal lymph node involvement can receive additional radiation therapy to area or lymph nodes dissection although the latter is rarely used. For patients presenting with metastatic disease treatment is palliative and regimens used include fluorouracil, mitomycin-C, cisplatin, paclitaxel, in addition to other regimens. Trials evaluating the role of immunotherapy in anal squamous cell carcinoma is currently ongoing.
Localized anal cancer has good prognosis while the metastatic disease has a poor prognosis. The quadrivalent HPV vaccine (6, 11, 16, and 18) has shown efficacy in reducing the rate of the premalignant intraepithelial neoplasia related to HPV infection by more than 50% in a double-blind, randomized trial. This prompted the Advisory Committee on Immunization Practice in October 2011 to recommend the routine use of the quadrivalent HPV vaccine in boys aged 11 to 12.