Causes of cervical cancer

The development of cervical cancer precursors and later cancer is linked to infection of the cervix with high-risk types of the human papillomavirus (HPV).

The most common means of acquiring HPV infection is through sexual intercourse. Human papillomavirus is also transmitted vertically – that is, from mother to child as the child passes through the birth canal and becomes infected from cells in the vagina and cervix that harbour HPV.

Smoking increases the risk to develop cancer cells in the cervix. The risk is dose related; the more smoking the higher the risk.
Conditions that lower immunity like Aids or certain medicines for transplant patients also increase the risk for cervical cancer.

Cell changes that can lead to cancer

Scientists know that abnormal changes in cells on the cervix are the first step in a series of slow changes that can lead to cancer years later. That is, some changes are pre-cancerous; they may become cancerous with time. However, not all cellular abnormalities will automatically turn into cancer.

Over the years, doctors have used different terms to refer to pre-cancerous changes in the epithelial cells. One term now used in cytological screening of Pap smears is "squamous intraepithelial lesion" (SIL). (The word "lesion" refers to an area of abnormal cells; "intraepithelial" means that the abnormal cells are present only in the epithelial layer covering the cervix.) Changes in these cells can be divided into two categories:

•    Low-grade SIL
refers to early changes in the size, shape and number of cells. Some low-grade lesions go away on their own. However others may, with time, become more abnormal, forming a high-grade lesion. Pre-cancerous low-grade lesions also may be called mild dysplasia or cervical intraepithelial neoplasia grade 1 (CIN 1). On a biopsy specimen taken from the cervix, these dysplastic cells would only be present in the lower third of the epithelial thickness which covers the cervical surface. It is usually not necessary to treat or investigate further when LSIL is present on a Pap smear. This is however an indication that the smear should be repeated a few months later and if the LSIL is still present on the repeat smear it will need further investigation.

•    High-grade SIL means there are a larger number of pre-cancerous cells. On a biopsy specimen, dysplastic cells would be present in the lower and middle third of the epithelial thickness (representing cervical intraepithelial neoplasia: CIN 2) or in the lower, middle and upper third (representing CIN 3). In the past, these lesions were referred to as moderate or severe dysplasia, or carcinoma in situ. When HSIL is found on a Pap smear, a further test like a colposcopy or Lletz is necessary.

If intraepithelial neoplastic cells break through the base membrane, which is the border between the epithelial layer and the deeper tissue of the cervix, the disease is called cervical cancer, or invasive cervical cancer. It occurs most often in women over the age of 40.

(Reviewed by Professor Lynette Denny, Gynaecology Oncology Unit, Department Obstetrics & Gynaecology, University of Cape Town/Groote Schuur Hospital)
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