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Anal-Rectal Cytology For Anal Cancer Screening

By Chengbao Liu, M.D.


Anal cancer is an uncommon malignant tumor involving the anus, anal canal, or anorectum. About 5900 new cases are diagnosed annually in women in the United States, with a mortality rate of 14% (1). Although it remains a rare type of cancer, the incident rate of anal cancer has been increasing worldwide. According to SEER (surveillance, epidemiology, and end results) datasheet, the incidence of squamous cell carcinoma (SCC), the most common type of anal cancer, increased 2.7 percent per year between 2001 and 2015 (2).

Risk Factors

Multiple factors are associated with the development of anal cancer, including cigarette smoking, chronic immunosuppression, anal receptive intercourse or sexually transmitted disease, a history of cervical, vulvar, or vaginal cancer, and persistent human papillomavirus (HPV) infection. Epidemiology studies have demonstrated that up to 93 percent of anal squamous cell carcinomas are associated with HPV infection3. Similar to what was described in the cervix, HPV16 is the most frequently isolated type in both anal SCC and high-grade anal intraepithelial neoplasia (AIN2-3), a precursor to SCC (3).

Screening Recommendation

Routine screening for AIN or SCC is controversial. Most guidelines do not recommend anal cancer screening even in high-risk individuals. However, some investigators argue there are benefits

with anal cytology screening (4). Screening with Pap smear and high-risk HPV has greatly reduced the incidence of cervical cancer; it’s reasonable to expect that screening could decrease the incident rate of anal cancer due to the similarity between the anus and cervix. Given the low incidence of anal cancer, universal screening is not recommended. Instead, screening should target individuals with increased risk of  developing anal cancer.

Screening Procedure 

Anal exfoliative cytology is the initial screening test for above mentioned high‑risk individuals. Patients should be instructed to avoid anal sex prior to the procedure since it may decrease the yield of cellular contents.

The steps to collect an anal cytology specimen (5):

1. Moisten synthetic swab with tap water or saline
2. Position the patient in the lateral recumbent or dorsal lithotomy position, separate buttocks gently, so anal opening may be clearly viewed
3. Insert swab slowly, until it bypasses the internal sphincter; be certain to find an angle that is not painful or immediately resistant; adjust angle and reinsert if needed
4. Insert as far as possible, until resistance is met and the swab abuts the distal wall of the rectum, usually 5–7 cm (2–3 inches)
5. Slowly remove swab ‑ in a spiral motion, while applying firm, consistent lateral pressure to sample all aspects of the mucosa of the anal canal
6. Count slowly to 10 (or more) while removing the swab and collecting the cellular sample
7. When reaching the anal verge (i.e. distal end of the anal canal), release hold on the buttocks so that the verge is sampled
8. Transfer sample to liquid‑cytology vial by vigorously swirling swab in the preservative fluid, or prepare smear on glass slide for immediate fixation

Screening Results and Follow-up 

The interpretation of an abnormal anal pap smear should prompt the next step testing. There is no established guideline for abnormal anal cytology. It’s proposed that all patients with abnormal results should undergo high‑resolution anoscopy, a procedure allowing the clinician to locate the lesion for targeted biopsy, followed by histopathologic examination. Treatment for a low‑grade squamous intraepithelial lesion (LSIL), including condyloma, is optional since most of them do not progress to invasive carcinoma. However, patients diagnosed with high‑grade squamous intraepithelial lesion (HSIL, AIN2‑3) are recommended to receive treatment (6).

Several HPV vaccines are available in the United States and have been shown to be effective in preventing or reducing the risk of cervical and anal cancers. The Advisory Committee on Immunization Practices (ACIP) recommends routine use of quadrivalent or 9‑valent vaccine in boys and girls, aged 11‑12 years; in females aged 13‑26 years, and in males 13‑21 years; who have not been previously vaccinated. In 2018, the FDA extended the use of the 9‑valent vaccine up to the age of 45 years.


1. Siegel RL, Miller KD, Jemal A. Cancer Statistics, 2020. CA Cancer J Clin. 2020 Jan;70(1):7‑30.
2. Deshmukh AA, Suk R, Shiels MS, Sonawane K, Nyitray AG, Liu Y, Gaisa MM, Palefsky JM, Sigel K. Recent trends in squamous cell carcinoma of the anus incidence and mortality in the United States, 2001‑2015. J Natl Cancer Inst. 2019 Nov 19.
3. Daling JR, Madeleine MM, Johnson LG, Schwartz SM, Shera KA, Wurscher MA, Carter JJ, Porter PL, Galloway DA, McDougall JK. Human papillomavirus, smoking, and sexual practices in the etiology of anal cancer. Cancer. 2004 Jul 15;101(2):270‑80.
4. Scholefield JH, Harris D, Radcliffe A. Guidelines for management of anal intraepithelial neoplasia. Colorectal Dis. 2011 Feb;13 Suppl 1:3‑10.
5. Darragh TM1, Winkler B. Screening for anal neoplasia: anal cytology ‑ sampling, processing and reporting. Sex Health. 2012 Dec; 9(6):556‑61.
6. Joel M Palefsky, M.D.; Ross D Cranston, M.D.; Anal squamous intraepithelial lesions: Diagnosis, screening, prevention,and treatment,, viewed March 18, 2020


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