Mar 29, 2010
Anal cancers are generally rare with an incidence in the general population in the UK of less than 1 case in every 100,000 people.
Anal cancers though are very significant for a number of reasons. Obviously they can be disfiguring, painful and unpleasant as all cancers but of particular significance is that very many of these cancers are caused by the Human Papilloma Virus (HPV), especially HPV types 16 and 18.
The incidence of anal cancer in females is higher than that in males and this is thought to be related in some way to the presence of HPV causing cervical pre-cancer and cancers in these women.
Anal cancers are thought to develop in much the same way with anal pre-cancers - Anal Squamous Intraepithelial Lesions (ASIL) - developing as a consequence, we think, of contamination of the anal canal with the high risk subtypes of HPV.
People at particular risk of ASIL and possible progression to anal cancers are:-
1) HIV negative males who may have had anal warts and also have receptive anal sex
2) HIV negative women who may have had anal warts; may have had receptive anal sex; have previous or co-existing high grade cervical intra-epithelial lesions or cancer of the cervix, vulva or vagina
3) HIV positive men or women.
HIV negative males who have receptive anal sex and or anal warts have a markedly increased risk of anal pre-cancers or ASIL with the incidence increasing from less than 1 in 100,000 to 35 per 100,000. This is the same incidence figure that led to the wholescale introduction of cervical cancer screening in women.
HIV positive males have a very marked increase in incidence of anal precancer, ASIL and also cancer with a rate of 70 to 100 cases per 100,000.
We know that approximately 70 or so per cent of these lesions are caused by HPV and most of these by HPV 16 and 18.
We can effectively prevent many of these cancers by using the HPV vaccines Cervarix which immunises against HPV 16 and 18 or by using the vaccine Gardasil which immunises against HPV 16, 18, 6 and 11. HPV subtypes 6 and 11 cause benign warty growths and are low risk and almost never cause cancers alone. However, they may be accompanied by the high risk subtypes 16, 18 and the other high risk subtypes HPV 31, 33 and 45.
At Freedomhealth it is our advice that all males who have anal sex whould be screened and immunised against HPV.
Screening for anal pre-cancerous lesions is straightforward but not commonly available.
Our routine at Freedomhealth is to physically examine the anus and anal canal. We then take a gentle PCR swab from the anal canal. This will identify the presence of high risk HPV subtypes, but many HPV infections are transient, with what is called passenger HPV present very temporarily. Most HPV detections will correctly identify the virus but not whether that high risk subtype is actually a danger to the person carrying it.
In determining activity of the HPV it is the expression of viral oncoproteins E6 and E7, affecting cell cycle control, that initiates the anal cancer process. The detection of E6/E7 mRNA confirms the persistent expression of viral oncoproteins in human cells. Detection of any of the five HPV types 16.18.31. 33 and 45 will identify most of the cancer cases worldwide.
When we receive the results of the PCR oncogene HPV swab a negative result is highly reassuring meaning that a patient is at virtually no risk of developing anal cancer disease.
We can augment the screening process by taking anal PAP smears using an anal sampling brush. The cells are then sent for ThinPrep cytology.
Again, if we receive a negative PCR HPV swab and a negative Thinprep anal PAP smear then we can be very reassured that nothing sinister is happening.
If we do find a suspicious area on clinical examination or if the PCR HPV swab suggests oncogenic HPV present in the anal canal or if the anal PAP smear suggests anal intraepithelial lesions or a combination of any of the above then we will perform a high resolution anoscopy on the patient with biopsy of the suspicious areas.
Anal screening for suspicious lesions and anal pre-cancers is a developing are and a technique which should be offered to all those in the high risk sub-groups.