Page 15 - Screening for Cervical Cancer: Systematic Evidence Review
P. 15
Chapter I. Introduction
Inter-related host factors such as age, nutritional status, immune function, smoking, and
possibly silent genetic polymorphisms modulate incorporation of viral DNA. Studies of the time
required from infection to incorporation are challenging to interpret because assays for viral
18
DNA integration are difficult to perform. Taken as a whole, however, nearly 100% of cases of
carcinoma in situ and cancer are estimated to have integrated HPV DNA compared to a small
15
minority of low-grade lesions.
The transition time from simple viral infection to integration of DNA is unknown and
may be influenced by the risk profile of population studied. For instance, although the
prevalence of HPV infection is higher among immunocompromised hosts such as HIV-infected
women, the speed of progression to cervical cancer is not increased. Natural history studies
confirm that in the vast majority of cases, the course of infection and cervical abnormalities that
progress do so in an orderly fashion from less severe to more severe lesions; de novo HSIL with
HPV incorporation appearing in a short interval is rare. Thus, the sequence associated with HPV
infection and development of cervical cancer is as amenable to surveillance as are cytologic
changes.
In the United States, peak incidence and prevalence of HPV infection occur among
23
women under age 25. More than 30% of postmenopausal women, however, have detectable
HPV DNA using polymerase chain reaction (PCR) detection methods.
Screening Failures
In the United States, incident cases of squamous cell carcinoma can be attributed to
different categories of failures of screening. Between 50% and 70% of cancer cases occur
among women who have never been screened or who have not been screened within the past 5
years. 24,25 Among women who have been screened, failures may occur in 3 ways. First,
5