Page 15 - Screening for Cervical Cancer: Systematic Evidence Review
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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

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               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

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               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

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               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,





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