By Dr Milton Lum

THE prevention of any disease can be primary or secondary. The former involves taking action on the determinants of the disease to prevent it from occurring. The latter involves the early detection of disease, followed by appropriate interventions to prevent its progression.

There has been considerable media publicity about the prevention of cervical cancer in the past six months. Most of it has focused on the human papilloma virus (HPV) vaccine, which has been called a cervical cancer vaccine, although there is no such vaccine available anywhere in the world.

Cervical smears have resulted in a steady decline in the incidence and mortality of cervical cancer in developed countries which have introduced population wide screening programmes.

Yet cervical smears have scarcely been mentioned in the media focus on cervical cancer prevention. This is despite the fact that only 43% of Malaysian women have ever had a cervical smear in their lives (National Health and Morbidity Survey 2006) although cervical cancer is the second most common cancer in women (National Cancer Registry 2003). There is an overuse of cervical screening by women who are younger and/or who are at low risk.

Natural history of cervical cancer

An understanding of the natural history of cervical cancer is helpful in the planning, implementation, and evaluation of any cervical cancer prevention programme.

Cervical cancer begins with changes in the squamocolumnar junction of the cervix where the flat squamous epithelium of the exocervix meets the columnar epithelium of the endocervix. The ratio of the cell nucleus to the cell size is increased in the epithelium in the pre-cancer phase of the disease.

There is a relationship between the induction of these changes and HPV infection. These pre-cancer changes are called cervical intraepithelial neoplasia (CIN). CIN is graded as mild (CIN 1), moderate (CIN 2) or severe (CIN 3). The CIN progresses from mild to moderate to severe disease and then invasive cancer over seven to 20 years. There are usually no symptoms during this progression, which can be detected by cervical smears.

CIN may spontaneously regress or persist. It has been estimated that, if untreated, 70 to 90% of CIN 1 will regress to normal. By contrast, the rates of persistence or progression to invasive cancer among those with CIN 2 and CIN 3 have been estimated at 57% and 70% respectively (Wheeler CM Obstet Gynecol Clin North Am 2008; 35:519-536).

Various risk factors interact and lead to the development of cervical cancer. The risk factors are:

• sexual activity i.e. early initiation of sexual intercourse, multiple sexual partners, number of current or previous sexual partners of sexual partner, all of which increase the risk of HPV infection
• immunocompetency which affects the body's ability to clear HPV infections
• lower socio-economic status
• high parity (five or more pregnancies)
• smoking
• Behavioural interventions

Cervical cancer has a pre-cancerous phase lasting about seven to 20 years before the normal cells change to cancer cells. As the risk factors of cervical cancer are known, behavioural interventions can be taken to prevent its development.

One or more of the following methods can be utilised:

• starting sexual intercourse only when one gets married
• having only one sexual partner
• knowing that one's sexual partner does not have many sexual partners
• using condoms regularly to prevent the transmission of sexually transmitted viruses like human papillomaviruses (HPV) which play a role in the development of cancer. This is useful when one is unsure whether one's sexual partner has many sexual partners or whose other sexual partner has cervical cancer. Condoms can be used in addition to other contraceptive methods
• avoiding smoking or reducing the number of cigarettes smoked
• Regular pelvic examinations

Regular pelvic examinations and cervical smears would detect most pre-cancerous changes in the cervix. With treatment, the development of invasive cancer would be prevented. Even if there is invasive cancer present, it will be detected at an early, curable stage.

The cervical smear is a screening test that detects pre-cancerous cells. This enables doctors to refer those with abnormal changes in the cervix for further investigation and treatment. It must be emphasised that the cervical smear is not a diagnostic test.

It involves the taking of a sample of cells from the cervix using a brush or spatula. The cells are placed on a glass slide or into a container and sent to the laboratory for microscopic examination.

Cervical smears are recommended for all women, even though if the woman has never had sex. The likelihood of cervical cancer in such women is thought to be low, but it can still occur. Regular pelvic examinations and Pap smears should be done once sexual activity starts. The frequency would depend on the findings and the woman's risk profile, which the doctor will discuss with a patient.

It is important that the doctor's advice be carefully followed, especially if there are increased risk factors.

Despite some limitations, cervical smears are 80-90% effective in detecting cervical pre-cancer. Cervical smears together with early detection and treatment can prevent 75% of cervical cancers from developing.

Cervical screening programmes

The use of cervical smears in widespread population screening in several developed countries has resulted in a marked reduction in the incidence of cervical cancer.

The first evidence of its effectiveness of screening came from the Scandinavian countries. The decrease in mortality rates between 1965 and 1982 was greatest in Iceland (80%) where coverage in the nationwide programme was most extensive. The mortality decreased by 25% in Denmark where 40% of the population was covered by organised screening unlike the decrease in Norway (10%) where only 5% of the population was covered.

The introduction of organised population screening in the Canada, United Kingdom (UK), other European countries, Australia, and New Zealand led to a marked decrease in the incidence and mortality from cervical cancer. For example, the introduction of a national call-recall system in the UK in 1988 resulted in a decrease in the incidence and mortality by about 50%, i.e. the incidence of invasive cervical cancer decreased from 14-16 per 100,000 women in 1971 to 10 per 100,000 in 1995 and to eight per 100,000 women in 2005. The number of deaths from cervical cancer has also fallen from 2000 in 1988 to 921 in 2006.

It is generally accepted that participation in the UK cervical screening programme by a woman aged 35 to 64 reduces her cervical cancer risk in the next five years by 60-80% and the risk of advanced cancer by about 90%.

The benefit of screening for women aged 25 to 34 years is more modest whereas screening in women aged 20 to 24 years has little or no impact on the incidence of cervical cancer under the age of 30 years (Sasieni P, Castanon A and Cuzick J, John Snow BMJ 2009;339:b2968).