What is anal cancer?
Anal cancer, like cervical cancer, is associated with sexually transmitted viral Human Papilloma Virus (HPV) infections. Some HPV subtypes are more problematic than others and types 16 and 18 together account for most of the anal and genital precancers and cancers. Anal cancers in men who have sex with other men are at a rate which is similar to the rate that triggered the mass cervical screening programme for cervical cancer in females, ie approximately 35 cases per 100,000 population.
Overall, anal cancer is rare and occurs at a rate of 1 or 2 cases per 100,000 in the general population. However, rates are much higher in some populations such as HIV positive men and women, people with other chronic immune-suppression such as transplant patients and women with other genital pre-cancers or cancers such as cervical cancer. In HIV positive people, rates can be as high as 170 cases per 100,000.
Screening for anal pre-cancer and cancer
Several techniques are available to screen for anal cancers and lesions which may develop into anal cancers. Pre-cancerous lesions are generally called Anal Intra-epithelial Neoplasia (or AIN for short). There are several grades of AIN – from 1 to 3. AIN 1 is low grade and generally has a low potential to develop into a cancer. AIN 3 is high grade and shows high probability of progressing to anal cancer. All types of AIN need monitoring and obviously anal cancers need High Resolution Anoscopy to obtain a better visual image and also for biopsies to be taken to define exactly what the lesion is. The good news is that all types of AIN or anal precancers are very susceptible to treatment and have a good prognosis.
Anal PAP Screening
Anal PAP screening is a technique where we take a smear from the anal canal using a special soft brush. This collect cells, rather like a cervical smear in females. These cells are sent to the laboratory and screened for abnormal cells.
HPV DNA Screening
Most HPV infections are temporary and transient. A small but significant number of HPV infections progress to anal cancer in susceptible groups. HPV DNA based detection methods can identify the presence of the human papilloma virus but they cannot identify their “oncogenic” or cancer forming activity. It is the involvement of special viral proteins, E6 and E7 which affect cell growth control that starts the anal cancer process. The detection of the genetic component of these special viral cancer causing proteins – called “oncoproteins” – allows us to confirm that the HPV detected is likely to be problematic. Detection of the five HPV subtypes (16, 18, 31, 33 and 45) will identify most of the caricnoma cases worldwide.
These HPV subtypes are identified using a soft cotton tipped swab which is taken from the anal canal. In combination with an anal PAP smear as above, it gives a good indication as to whether High Resolution Anoscopy is needed.