Introduction
Cervical cancer is one of the most frequent female cancers in the world, mainly in the lower-resource countries. It is preventable through vaccination before starting sexual activity. The progression from infection with high-risk (HR) strains of HPV to invasive cancer takes time, from 10 to 13 years or even longer. During this long natural history of precancer phase, the early detection and the local treatment of the screened population could stop the progression to invasive cancer and would make this infectious neoplasia a preventable disease.
In 2018, about 570,000 women were diagnosed with cervical cancer and 31,100 died due to this disease. The highest incidence of invasive cervical cancer (84%) and the highest death rate from the same disease are found in lower-resource countries, with no preventive care programs(1).
The key of success is an organized screening program that could dramatically reduce the incidence and mortality from cervical cancer through early detection and local treatment of early precancer lesions. A very convincing example is offered by the results of the national screening program in UK and Finland. In 1988, the national screening program for cervical cancer was introduced in UK. In 2017, the statistics showed a reduction by 24% of the incidence and a drop of mortality from 8/100,000 to 3/100,000 when compared to 1988(2).
More impressive figures are presented by Finland. After the introduction of the population-based screening, the incidence and mortality dropped by 80%, being nowadays 4/100,000 and 1/100,000, respectively(3).
Traditionally, the screening program was based on exfoliative cytology (Pap smear test). In the last decade, the traditional cytology screening was improved by liquid-based cytology. Also, within the last decade, major improvements in cervical cancer prevention have been registered thanks to the introduction of HPV DNA detection tests.
The value of exfoliative cytology in the cervical cancer screening
In recent years, a new technique was introduced, liquid-based cytology (LBC). The conventional cytology is progressively replaced by LBC, because the latter has more advantages. First, it is a semiautomated process, removing inflammatory cells, debris, erythrocytes and other artefacts, allowing a thin and uniform spread of epithelial cells on slide, facilitating the reading by machine and/or cytopathologists. This technique reduces the rate of unsatisfactory smears from 9.1% to 1.6% and even more, and the solution can be used for HPV DNA testing and for other markers(4).
A comparison of conventional and liquid-based cytology shoved that both have the same sensitivity and specificity for the diagnosis of CIN2 or higher, excepting ASC-US, where LBC is less specific.
In the Bethesda system for reporting cervical cytology, the majority of cervical cells anomalies are in squamous cells. The incidence of these abnormalities are: 3-5% ASC-US, 1-2% LSIL, and 0.5-1% HSIL. The abnormalities of glandular cells are less than 0.2% of all smears(5).
A negative impact of the cytology screening is the high incidence of false-negative results, estimated to 20-25%. Thirty percent of women attending regular screening have a false-negative result due to incorrect sampling or misinterpretation(6). Moreover, exfoliative cytology has limitations in the diagnosis of glandular intraepithelial neoplasia localized in the cervical canal(7). This is important because the incidence of adenocarcinoma of the cervix has increased in the last decades, representing 20% to 30% of all cervical cancers. This cancer has a delay in the diagnosis, being detected in more advanced stages, with a poor prognosis, partly reflecting the diagnostic delay.
The value of HPV DNA testing
in the primary cervical cancer screening
It is well known that the persistent infection with a high-risk human papillomavirus (HR-HPV) strain is necessary for the development of high-grade cervical intraepithelial neoplasia (HG-CIN or CIN3), adenocarcinoma in situ and invasive, and invasive squamous cervical cancer(8). When compared to conventional cytology, the molecular testing for HR-HPV has an increased sensitivity, but a lower specificity for the diagnosis of CIN 3+(9). HPV testing is recommended in Europe, alone, as primary screening method and for triage women with ASC-US, in the follow-up, after the treatment of CIN lesions(6). In the USA, HPV testing is recommended for triage women with ASC-US and as an additional test of cytology for women older than 30 years of age, this last indication being labeled as “co-testing”(10-12).
Today, many countries are using HPV primary screening for the detection of precursor lesions of cervical cancer, clearly proving that HPV primary screening, maintaining a high sensitivity, could reduce the cost and the complexity related to co-testing(13,14). Most of the countries using HPV primary screening do not use routine co-testing, therefore reducing the cost of the screening while maintaining a high sensitivity. Prospective randomized trials and even retrospective studies clearly showed that HPV primary screening has a higher sensitivity, when compared to cytology, in identifying earlier CIN3 and higher(9,15).
One of the questions waiting for an answer is the selection criteria for HPV-positive women who should be referred to colposcopy.
Convincing results about how to build a national screening program for the detection of cervical cancer, in a country that has not one yet, are offered by the results of the Athena study(16).
Three models of screening strategies were compared after more than 42,000 women ≥25 years old were screened during a three-year prospective study.
HPV primary screening strategy
Most of the tests used for HPV detection are PCR-based. Most amplified HPV-DNA tests detect a group of 13 HR-HPV types (HPV-HR 16, 18 and other HPV-HR: 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68).
If HR-HPV was negative, women were rescreened in three years. If HPV 16/18 was positive, the woman was referred to colposcopy. In case of a positive result for the other 12 HR-HPV, reflex cytology and colposcopy were offered if cytology was ASC-US or more than ASC-US. If cytology was negative, the woman was rescreened for HPV and cytology in one year.
Cytology strategy
The cytology strategy was based on liquid-based exfoliative cytology examination. HPV was tested only when cytology was ASC-US or more.
Hybrid strategy
The hybrid strategy consists in the association between HPV and cytology testing. If HR-HPV was positive and cytology was negative, the woman was rescreened in one year.
Comparative evaluation of the three screening strategies
In women ≥25 years old, HPV primary strategy is the best screening method for the detection of CIN3+, even better than hybrid strategy. It proved to be the best screening strategy at baseline, detecting with 37.8% more cases of CIN 3+ when compared to either hybrid or cytology strategy.
HPV primary strategy detected with 22.5% more cases of CIN3+ than cytology alone and with 64.2% more cases than hybrid strategy(16). These better results were associated with a higher number of colposcopies.
When analyzing the age group ≥30 years old, HPV primary strategy continued to offer better results compared to cytology, similar for women ≥25 years old. When compared to hybrid strategy, HPV primary method detected the same number of CIN3+, requiring the same number of colposcopies. HPV primary strategy has the advantage of detecting more cases at baseline when compared to hybrid strategy.
Discussion
There is a large number of prospective screening trials clearly proving that HPV primary screening is more sensitive than cytological screening(9,15,17,18).
HPV primary screening offers a 60% to 70% better protection against invasive cervical cancer in women ≥30 years old when compared to conventional cytology(7,19).
The advantage is clearly seen in glandular disease. High-risk HPV primary test is an objective test determined in central laboratories, with no need for technicians training, assuring a good quality control and the most credible results. All these strategies would allow longer screening intervals of 5 years compared to 3 years for cytology, and would reduce the number of unsatisfactory results.
High-risk HPV-DNA testing can be done on vaginal self-samples, which offers opportunities to include in population screened women who did not participate in the regular screening programs. Meta-analyses have shown that clinically validated PCR-based assays are as accurate on self-taken taken cervical samples when compared to clinician-taken cervical samples(20,21).
Co-testing is more expensive than HPV primary screening and offers minimal protection against evolution to CIN2+ or CIN3+(15,20,23). When compared to cytology, HPV primary screening detected more CIN3+ in women ≥25 years old (28.3% increased sensitivity) and ≥30 years old (24.3% increased sensitivity).
HPV primary screening has the advantage that women with HPV 16/18 positive will be referred to colposcopy, allowing gynecologists to detect about 50% of all cases of CIN3+ lesions.
Screening guidelines recommend a balance between costs and benefits. One potential excess of HPV primary screening is an increased number of unnecessary colposcopies. These will increase the cost of screening and will generate patient anxiety. This is especially evident for women ≥25 years old, where 10.5% of them are transient HPV-positive, without a true carcinogenic potential, and could be referred for an unnecessary colposcopy or even unnecessary surgical interventions(10,24,25).
To improve specificity and reduce overreferral to colposcopy, triage tests are needed to identify the persistent infections and the potential association with the development of cancer.
A triage approach would include reflex cytology for all women positive for the other 12 HR-HPV (Cobas test) in order to select women who will be referred for colposcopy instead of sending all HR-HPV positive women. Without a triage approach, HPV primary screening would recommend approximately a double number of colposcopies compared to cytology.
Another limitation of HPV primary screening is the lack of follow-up for women with CIN2+ at baseline. Many studies offer reassuring results, showing that 20% of untreated CIN2+ regress after the first year of diagnosis and 60% after three years(24,26), while CIN3+ lesions infrequently regress(27).
As a prospective screening study, Athena clearly supports the use of HPV primary screening with triage of HPV-positive women, using an association of genotyping for HPV 16/18 and reflex cytology, and starting the action at the age of 25 years old. The results of this study strongly proved that HPV primary detects more CIN3+ than either cytology or hybrid strategy. A limitation of this study is represented by a higher number of colposcopies compared to cytology, but if the target is the detection of CIN3+, the number of colposcopies is the same as with hybrid screening.
Conclusions and recommendations
Romania has the highest rate of mortality due to cervical cancer in Europe and has no national screening program to prevent cervical cancer. The opportunist screening is substantial in urban area, but does not cover the rural areas. The explanations for this situation are represented by lack of coordinating infrastructure, the lack of human resources (cytopathologists), the subjective reading of cytology slides by cytopathologists, the lack of quality control, the lack of involvement of family doctors in screening, and the lack of report and data collection.
Romania urgently needs a nationwide screening program to detect precursor lesions of the cervical cancer.
The 2nd edition of the European Guidelines for Quality Assurance in Cervical Cancer Screening established the principles of an organized screening program(6,28). These principles are: defining a target population (women of 25-65 years of age) and screening intervals (3-5 years if using cytology and 5-7 years if using HR-HPV-test); the use of a population-based registry and appropriate recruitment measures (personal invitations, place and date where the screening test is taken); the definition of the screening test; adequate facilities to perform this test; defined management algorithms, and the monitoring and evaluation of the process and of the impact of screening(28).
There are many examples proving that HPV primary screening could be preferred to the other models of screening. Among the advantages of HPV primary strategy, there can be mentioned: the test is automated and the interpretation is objective; it is cheaper than cytology; extended screening interval to 5 years; higher sensitivity; higher negative predictive value.
The old infrastructure based on the Pap smears is no more functional, there are fewer cytopathologists and no more public warning campaigns against cervical cancer. The best example of implementation of a successful screening program based on HPV primary testing is offered by Turkey(29-31).
The decision to implement a new policy of cervical cancer screening, replacing the existing one based on cytology, is a political one. The first step is to design and write a project describing the protocol of HPV primary screening.
One of the most important decisions is to have one or two nationwide centralized diagnostics laboratories and a sustainable agreement with the diagnosis industry.
Family doctors must be involved in the screening program and trained accordingly (theoretical training and hands-out practice).
Well-defined algorithms must be written in the protocol. A network of referral centers for colposcopy and treatment of precursors lesions must be established.
All the post-screening data (colposcopy, final pathology) must be collected in the institutional registry and sent to regional centers and then to the national registry for cancer. The success of a population-based screening program is grounded on the quality control of the program and particularly on high population coverage.
HPV primary population-based screening should be adopted as a national cervical cancer prevention program(13,14).