Cancer begins with a change in the structure of the DNA that’s present in all human cells. DNA provides the cells with a basic set of instructions, including when to grow and reproduce.
A change in the DNA’s structure is known as a mutation. It can alter the instructions that control cell growth. This means that the cells continue growing instead of stopping when they should. If the cells reproduce uncontrollably they produce a lump of tissue called a tumour.
Human papillomavirus (HPV)
Almost all cases of cervical cancer occur in women who have been previously infected with the human papillomavirus (HPV). HPV is actually a group of viruses, rather than a single virus. There are more than 100 different types.
HPV is spread during sexual intercourse and is thought to be very common. An estimated 1 in 3 women will develop a HPV infection within two years of starting to have regular sex.
Some types of HPV do not cause any noticeable symptoms and the infection will pass without the need for treatment. Other types of HPV can cause genital warts.
Two types of HPV are known to have the highest risk of causing cervical cancer. They are:
- HPV16 – which accounts for around 50-55% of all cases
- HPV18 – which accounts for around 15-20% of all cases
A further 11 types of HPV also have a higher chance of causing cancer.
High-risk types of HPV are thought to contain genetic material that can be passed into the cells of the cervix. This material begins to disrupt the normal workings of the cells (see below), which can eventually cause them to reproduce uncontrollably, leading to the growth of a cancerous tumour.
Cervical intraepithelial neoplasia
Cancer of the cervix usually takes many years to develop. Before it does, the cells in the cervix often show changes, known as cervical intraepithelial neoplasia (CIN).
CIN is a pre-cancerous condition. Pre-cancerous conditions do not pose an immediate threat to a person’s health, but they can potentially develop into ‘fully-blown’ cancer in the future. However, most women who have CIN won’t develop cervical cancer.
CIN is graded using a number system from one to three. The higher the number, the more abnormalities there are in affected cells. It is estimated that:
- one third of women with CIN 1 will progress to CIN 2
- around half of all women with CIN 2 will progress to CIN 3
- around one third of women with CIN 3 will develop cervical cancer
Therefore if you develop CIN 1, your chances of developing cervical cancer at a later date are less than 1 in 30.
The progression from becoming infected with HPV to developing CIN and then developing cervical cancer is very slow. It usually takes at least six years to progress from an initial infection to CIN 3. It takes up to 10 years for CIN 3 to develop into cervical cancer.
Risk factors
The fact that HPV infection is very common but cervical cancer is relatively uncommon would seem to suggest that only a very small proportion of women are vulnerable to the effects of a HPV infection. There appear to be additional risk factors that affect an individual’s chance of developing cervical cancer.
Known risk factors include:
- smoking – women who smoke are twice as likely to get cervical cancer than non-smokers; this may be due to harmful effects of chemicals found in tobacco on the cells of the cervix
- having a weakened immune system – which can be the result of taking certain medications, such as immunosuppressants, which are used to stop the body rejecting donated organs, or as a result of a condition such as HIV/AIDS
- taking the oral contraceptive pill for more than five years – women who do this are thought to have twice the risk of developing cervical cancer than those who do not take the pill
- having children (the more children you have, the greater your risk) – women who have two children have twice the risk of getting cervical cancer compared to women who do not have any children
The reason for the link between cervical cancer and childbirth is unclear. One theory is that the hormonal changes that occur during pregnancy could make the cervix more vulnerable to the effects of HPV.
The spread of cervical cancer
If cervical cancer is undiagnosed and untreated, it will slowly spread out of the cervix (neck of the womb) and into the surrounding tissue and organs. The cancer can spread down to the vagina and the surrounding muscles that support the bones of the pelvis. Alternatively, it can spread upwards, blocking the tube that runs from your kidneys to your bladder (ureters).
The cancer will then spread into your bladder, rectum (back passage) and eventually into your liver, bones and lungs. Cancerous cells can also spread through your lymphatic system. The lymphatic system is a series of nodes (glands) and channels that are spread throughout your body in a similar way to your blood circulation system.
The lymph nodes produce many of the specialised cells that are needed by your immune system (the body’s natural defence against infection and illness). If you have an infection, the nodes in your neck or under your armpits may be swollen.
In some cases of early cervical cancer, the lymph nodes close to the cervix contain cancerous cells.
And in some cases of advanced cervical cancer, lymph nodes in the chest and abdomen can be affected.
- Immune system
- The immune system is the body’s defence system, which helps protect it from disease, bacteria and viruses.