Association of human papillomaviruses and cervical cancer
Although the Pap test has proven to be a valuable tool in the prevention of cervical cancer, the analysis of cervical smears is a labour intensive process which can only be undertaken by highly trained cytotechnologists. Moreover the interpretation of the smear is a subjective process and subject to diagnostic error. An objective test based on the detection of high risk HPV DNA would appear to be a practical alternative.
Three settings for HPV DNA testing have been evaluated
- as a primary screening test
- as an adjunct to cytology
- follow up post treatment or as a marker for test of cure
Hybrid capture for primary screening
HPV infection is a common sexually transmitted disease. Studies of the prevalence of HPV in sexually active women using Hybrid Capture 2(HC2) or PCR based methods have shown that women have a 70% lifetime risk of infection. Most women are infected before the age of 30 when most infections are transient and unlikely to be associated with the development of cervical cancer. Only a minority of women develop persistent infection which carried a high risk of malignant change in the cervix. Thus primary screening for HPVDNA is directed at older women who are more likely to have persistent infection which is known to be associated with a high risk of malignant disease.
Several studies have compared the sensitivity and specificity of HPVDNA screening with cervical cytology and colposcopy in women at risk of cervical cancer. A review of 14 such studies by Franco (Franco EL, 2003, J Nat Cancer Inst monograph 31) and a large study in the UK by Cuzick et al (Lancet 2003: 362;1871-1876) showed that HPVDNA testing was more sensitive but less specific than cytology in detecting women at risk. Of cervical cancer. A negative HPVDNA test was shown to have a very high negative predictive values ranging from 97% to 100% (ie the likelihood of the woman having cancer was very low indeed). As a result of these studies, two different approaches to cervical cancer screening using HPVDNA as a primary test have been advocated.
The first approach recommends a combination of HPVDNA testing with cytology testing for primary screening of older women (i.e. over the age of 30). In view of the greater protection provided by this combined approach. Women who are negative for both HPVDNA test and the Pap test could be doubly assured that they were free of disease. The increased cost of dual testing could be offset by less frequent tests.
The alternative approach involves two stage screening of women aged 30 or greater. Initially this group of women would be screened for HPVDNA. Cytology would be used to triage women who were found to be HPVDNA positive. Women would be referred for evaluation only if both tests were positive.
A major problem of HPVDNA testing is the low specificity of the test for CIN and cervical cancer. There is as yet no clear advice on how women with a positive HPVDNA test and negative cytology should be managed. Increased surveillance is recommended since it is possible that these women may have an increased of CIN. Castle et al(Cancer 2002:95;2145-2151) found that 15% of over 2000 women with a positive HPVDNA test and negative cytology developed a significant cervical lesion within a 5 year period. By rising the threshold for a positive HPVDNA test it may be possible to minimise the risk of false positive cases in the future.
HPVDNA testing as an adjunct to cytology
One of the problem areas of cervical cytology is the management of women with ASCUS (borderline lesions) or LSIL. Several studies have shown that women who are classified as have an ASUS lesion may in fact have a significant cervical lesion (CIN2 or worse). The ALTS trial which was coordinated by the National Institute of Cancer (Acta Cytologica 2002:44; 726-742) in 2000 was a large multi site randomised trial specifically designed to evaluate different methods of managing women with ASCUS or LSIL. The management strategies under evaluation were
- immediate colposcopy for all women
- HPV testing and referral for colposcopy if the HPV test was positive
- Repeat cytology with referral to colposcopy if the smear showed HSIL
The study found that HPV testing was not of value in managing women with LSIL. The American Society for Colposcopy and Cervical Pathology (ASCCP) recommended that all women with LSIL undergo colposcopy rather than HPV testing.
The study also showed that about half the women with ASCUS were HPVDNA positive and were referred for colposcopy. It also showed that only about one quarter of women who have ASCUS cytology and who are HPV positive and who undergo colposcopy will have underlying CIN2/CIN3. Thus the specificity of HPVDNA testing even in the context of ASCUS cytology is low.
Consensus management guidelines for the follow up of ASCUS developed under the sponsorship of the ASCCP include repeat cytology, immediate colposcopy and HPV testing as options. However, if liquid based cytology (LBC) was used for the initial Pap test, reflex HPV testing using the residual fluid in the LBC sample is the preferred option as it eliminates the need for a second visit to the clinic.
HPVDNA testing for follow up post treatment or as a test of cure
Women who have been diagnosed with CIN2 or CIN3 and treated by ablative therapy or cone biopsy have to be monitored closely for at least 5 years after their treatment. Although over 90% of women are cured by these treatment regimes, there is a risk of recurrence of CIN or the development of invasive cancer in 5% -10% (Soutter et al Lancet1997: 349 ; 978-980). In most cases follow up involves both colposcopic and cytological investigation at 6 months intervals for the first year; followed by annual cytology and/ or colposcopy at yearly intervals for the remaining 5 years. Thereafter (in the UK at least) the patient returns to a normal screening routine i.e. Pap tests at 3 or 5 yearly intervals).
HPVDNA testing has been investigated as a predictor of residual or recurrent disease in these women. Several authors have evaluated this approach with variable results. Lorincz analysed the results of 10 studies of HPV testing post treatment and reported a combined sensitivity, specificity and negative predictive value of 96.5%, 77.3% and 98%. Paraskevaidis et al (ObstetGynecol 2001: 98; 833-836) reported on 11 other studies of HPV testing post treatment and found that the sensitivity of HPV testing reached 100% in four studies but only 47-67% in two. In view of the conflicting results further studies of the value of HPV testing as a marker of successful treatment are indicated.
Although HPV testing alone is a sensitive method of detecting CIN lesions of the cervix the test is not sufficiently specific to make it a practical method of primary screening for cervical cancer. The significance of a positive HPVDNA test in the absence of underlying disease is not yet known and may cause unnecessary concern in women with normal cervices who harbour the virus. The role of HPVDNA testing for triage of case of ASCUS or LSIL is also controversial as is HPVDNA for the follow up of women treated for CIN2/3.
(For further reading see Denny and Wright in Best Practice and Research in Clinical Obstetrics and Gynaecology2005 vol 19 , no4, pp 501 – 515 Also available on line at www.sciencedirect.com).