Progression of Cervical Disease after HPV infection

Cervical cancer does not happen over night or out of sudden.  Many studies have shown that it may take 10 - 15 years from the exposure of persistent HPV infection before it progress to become an invasive cervical carcinoma. That is why it is said that cervical cancer is 100% preventable and curable. This is because the stages of cervical disease caused by the HPV infection can be easily detected and treated if a woman comes for her routine cervical smear screening.

Following the HPV infection, the virus get embedded into the DNA of the cervical epithelium and cause changes into the genetic material of the normal epithelium. After a certain period, the normal cervical epithelium will be replaced by the abnormal cells and this cervical disease is called Cervical Intraepithelial Neoplasia (CIN) or Squamous Intraepithelial Neoplasia (SIL). There are 3 grades of CIN, depending on the thickness of the abnormal cell replacing the normal cervical epithelium.
CIN 1- 1/3 of the normal epithelium is replaced; CIN 2 - 2/3 of the normal epithelium is replaced;
CIN 3- whole thickness of the cervical epithelium is replaced. Once the abnormal cells invade the deeper layer below the basement membrane of the cervical epithelium it become an invasive cervical carcinoma. It is not clear whether there is a progression from a lower grade CIN to a higher grade before it becomes an invasive cancer but many studies have shown that higher grade SIL (CIN 2 &CIN 3) has a higher tendency to progress into invasive cancer at a shorter period of time compared to the lower grade SIL (CIN 1).

Most of women infected by HPV would clear the infection naturally but women infected by HPV type 16, 18 and other oncogenic HPV types are likely to have persistent infection. The good news is almost all of these condition are curable. Hence, the role of pap smear is to detect HPV infected cervical epithelium and cervical pre-cancerous lesion before it progress into an invasive cancer.