HPV infection, risk factors and natural history

Several risk factors for cervical cancer have been investigated over the years. There is now a strong and consistent body of evidence demonstrating that persistent infection by oncogenic high‐risk HPV subtypes is responsible for instigating the neoplastic process and that this might be potentiated by other cofactors (Fig. 61.3). With more than 200 HPV subtypes recognized today, it is only a fraction of these that has been found to have carcinogenic potential. Of these, subtypes HPV‐16 and HPV‐18 are most commonly associated with invasive cancers and are thought to cause approximately 65–75% of cases, depending on the continent. The most frequent low‐risk subtypes are HPV‐6 and HPV‐11, which also lead to the development of anogenital warts. The commonest types are classified according to their oncogenic potential as follows:

• low‐risk subtypes HPV‐6, HPV‐11;
  
• high‐risk subtypes HPV‐16, HPV‐18, HPV‐31, HPV‐33, HPV‐45, HPV‐56.

The lifetime risk of acquiring any HPV infection probably exceeds 80%. With more sensitive testing available, studies show that HPV infection is more commonly the rule, not the exception. HPV types associated with the alpha species predominate in the anogenital area but other HPV types such as the beta and gamma HPV types, once thought to be predominantly cutaneous only, can also be commonly detected. Since the lifetime risk of developing invasive cervical cancer is much lower at 0.6%, cervical cancer should be regarded as a rare complication of a very common infection by HPV.

One of the early observations in most epidemiology studies was the high frequency of detection of the viral DNA. The more HPV types tested, not surprisingly, the higher the frequency of detection. In most studies, but not all, age influenced prevalence with young age being associated with higher rates, some as high as 45% in Western societies. This vulnerability is thought to be due to immature or naive immune responses as well as the biological vulnerability of the immature cervical epithelium seen in adolescents. The prevalence of HPV infection declines to about 5% in women above the age of 30.

Most HPV infections are transient. Approximately 60–80% clear spontaneously within 1–2 years, although the rest may result in CIN lesions within 2–4 years. The progression rate of CIN in women with high‐risk HPV positivity is about 5% per year. In women over 30 years with high‐risk HPV positivity and a normal cytology sample, the risk of developing CIN 3 is 116 times higher than in women with HPV but cytology negative. Even for women who develop CIN, the regression rates are high and depend, amongst other factors, on the grade of CIN and the woman’s age. Above the age of 30 years, the regression rate is lower. Taken together, in the absence of intervention, roughly one‐third of early precursor lesions disappear spontaneously, one‐third persist and one‐third progress to CIN 3 or invasive cancer.

HPV is a non‐lytic infection, so that the inflammatory response to HPV is much more subtle than other mucosal infections, such as Chlamydia trachomatis. The initial immune response to acute HPV infections is most likely mediated by the local innate immune system, probably involving mechanisms such as activation of Toll‐like receptors and natural killers cells. Persistent infections are cleared by the development of adaptive immune responses, which are dependent on antigen‐presenting cells. HPV‐16 is thought to downregulate both innate and adaptive immune responses. Final pathways to cancer result in interference with telomerase activity and viral integration, although a proportion of cancers are found to have episomal HPV DNA only. HPV E6 and E7 are known oncoproteins that control fundamental carcinogenic events including proliferation, senescence, and apoptosis. Cellular targets include p53, E6AP, CBP, p300, Bak, hTERT, MAGUK, cIAP, survivin, p107, pRB and p130.

Observational data show that the estimated time from infection to the development of invasive disease is approximately 15 years, although there may be swift progression in rare cases (Fig. 61.4). Cervical carcinogenesis normally has a lengthy precancerous phase that has been well defined through different grades of cervical pre‐invasive disease, although the continuum of the carcinogenic process has been questioned in some cases. Despite these major advances in our current understanding of the disease, the exact factors that determine infection and/or disease that will persist, progress or, conversely, spontaneously resolve are incompletely understood.

Several other risk factors of cervical disease have been described previously and have been correlated with heightened or reduced risk of cervical cancer in epidemiological studies. These include:

• low socioeconomic status;
  
• tobacco smoking (twofold);
  
• oral contraceptives (2.5‐fold);
  
• early sexual debut;
  
• multiple sexual partners (of woman or of the partner);
  
• other sexually transmitted infections (e.g. herpes simplex virus, Chlamydia) and bacterial vaginosis might influence HPV persistence and the probability of progression of HPV infection to dyskaryosis;
  
• immunocompromise, including HIV (fivefold).

The most consistent factors are tobacco smoking, oral contraceptive use and parity. All have biological plausibility. Nicotine and its carcinogenic metabolites can be detected in cervical mucus and smoking has been associated with a dampening of local immune markers. Both oestrogen and progesterone increase cell proliferation and hence vulnerability to DNA damage. Higher parity may be associated with high levels of hormone exposure and/or repeated trauma.

Most of the risk factors reporting a positive association are surrogates for sexual activity. They relate to increased risk for HPV infection and do not have a causal independent relation to cervical cancer. Others are potentially determinants of progression rather than prime aetiological agents.

Classification of cervical intraepithelial neoplasia

Cervical cancer screening

Traditionally, cytology relied on the assessment of exfoliative smears on glass slides under the microscope, as first described by Papanicolaou in the 1940s. Liquid‐based cytology has largely replaced conventional cytology in recent years. The technique has many advantages. It is semi‐automated, creates a uniform spread of epithelial cells that are easier to read by cytotechnicians and cytopathologists, and reduces the rate of unsatisfactory samples. The fluid can also be used for reflex testing for HPV DNA and other biomarkers.

It is expected that cervical screening based on HPV DNA tests will replace cytology‐based screening in the future, at least in women above the age of 30. Large randomized controlled trials and a meta‐analysis of these documented the superiority of HPV‐based screening when compared with cytology, with a 60–70% better protection against invasive disease. The optimal way of triaging women that test positive for high‐risk HPV remains to be determined and some options include reflex cytology, HPV‐16/HPV‐18 genotype positivity or newer biomarkers. The use of HPV mRNA tests in primary screening has also been suggested but longitudinal data are required.

The age of screening initiation and the frequency of screening intervals vary across countries [2,4]. In the UK, for example, screening starts at the age of 25. Screening does not start earlier as the risk of invasive cancer at younger ages is small, while many lesions spontaneously regress without treatment in this age group [3]. Sentinel pilot sites are testing the introduction of HPV‐based screening with reflex cytology for those that test positive for HPV. This may allow further extension of the screening interval to 5 years if women test negative (Table 61.3).

The current indications for referral for colposcopy in the UK are:

• one cervical sample showing borderline nuclear changes in squamous cells that are high‐risk HPV positive;
  
• one cervical sample showing mild dyskaryosis changes in squamous cells that are high‐risk HPV positive;
  
• one cervical sample showing mild dyskaryosis changes in squamous cells with unreliable, inadequate or no results for HPV test;
  
• three cervical samples showing borderline nuclear changes in squamous cells with unreliable, inadequate or no results for HPV test;
  
• one cervical sample showing borderline nuclear changes in endocervical cells;
  
• one cervical sample showing moderate or severe dyskaryosis;
  
• one cervical sample showing possible invasion;
  
• one cervical sample showing glandular neoplasia;
  
• three consecutive inadequate cervical samples;
  
• any grade of dyskaryosis following treatment for CIN;
  
• three abnormal cervical samples of any grade over a 10‐year period;
  
• suspicious symptoms and abnormal cervix.

Colposcopy

Hinselman first introduced colposcopy early in the last century (1925). Colposcopy comprises low‐power magnification and illumination of the lower genital tract after application of various stains: acetic acid (3–5%) and Lugol’s iodine. Apart from refinements to the optical and illumination systems and the introduction of a green filter that can enhance the vascular appearance, there has been little technological advancement since. New technologies (such as DySiS and Zedscan) are being assessed and have potential to improve the accuracy and reproducibility of the colposcopic assessment.

The objectives of colposcopic assessment are to:

• assess the presence and severity of the abnormalities detected on cytology;
  
• guide colposcopically directed biopsies from the area with the most severe changes;
  
• exclude invasive disease;
  
• aid in outpatient management and treatment of CIN;
  
• assist follow‐up after treatment.

Colposcopy is deemed satisfactory when the entire squamocolumnar junction is visualized and the upper limit of any lesion is seen. The size and topography of the lesion should be ascertained, especially if there is any extension of the lesion into the cervical canal or onto the vagina. Both of these clinical scenarios are important in relation to appropriate treatment. Colposcopic abnormalities maybe graded according to a variety of indices, such as the appearance of the aceto‐white epithelium, iodine negativity and vascular patterns such as mosaic, punctation and atypical vessels. The area between the original and the new squamocolumnar junction after replacement of the columnar epithelium after puberty by metaplastic squamous epithelium is called the transformation zone and should be described as type I, II or III [6].

The colposcopic assessment is subjective and prone to intra‐observer variability, particularly in the diagnosis of low‐grade as opposed to high‐grade lesions. The predictive accuracy improves with the increased severity of the anticipated lesion. Agreement between the colposcopic impression and histology is achieved in only 37% of cases, while agreement within one grade is 75%. Formal training and a period of apprenticeship leads to certified expertise in this technique.

Management and treatment

One in ten women in the UK have an abnormal result at screening. Of these, 2–3% will present with high‐grade findings at cytology and the remaining 7% with cervical samples classified as ASCUS or LSIL or their British terminology equivalents of borderline and mild dyskaryosis [7].

The management of the minor cytological abnormalities that are common in young women has challenges. Although the majority represent clinically insignificant lesions, some can harbour high‐grade disease. Their management consumes a disproportionate amount of health resources, although their significance remains debatable. An HPV DNA test has recently been introduced in many countries to assist the selection of women with these minor abnormalities that should be further investigated with colposcopy. Women who test negative for high‐risk oncogenic HPV types return to routine recall. Further management of confirmed CIN 1 lesions varies and depends on the woman’s age, the length of persistence of the disease and her fertility wishes. Young women who have not yet completed their families are usually managed conservatively with surveillance. Older women or women at high risk of non‐compliance with persistent disease may undergo treatment.

Most women with histologically high‐grade lesions (defined as CIN 2+) will undergo treatment and CIN 2 on a biopsy is often considered the clinical threshold to proceed to treatment. Exceptions may apply in selected cases of young women with small CIN 2 lesions. Increasingly, clinicians manage carefully selected cases with close surveillance, as the regression rate in young women with small lesions is high.

Local conservative cervical treatment for CIN aims to remove or ablate the entire transformation zone and lesion with a cone‐shaped part of the cervix to a depth of more than 7 mm to ensure eradication of CIN that may involve the gland crypt. The majority of the techniques are easy to perform, of low cost and are usually performed under local anaesthesia in an outpatient setting. The peak age of CIN is similar to the mean age of having the first child in Western societies. It is therefore important that the proposed treatment will eradicate the pre‐invasive disease with the minimum disruption in cervical function.

The treatment methods are divided into excisional and ablative (Table 61.4). Both have high cure rates of over 90% and have similar treatment failure rates, with the exception of cryotherapy for high‐grade disease. The choice of technique relies on patient characteristics, the colposcopic appearance, the depth, severity and size of the lesion, the type of transformation zone, the age and fertility wishes, the clinician’s experience/preference and the equipment available.

The need to maximize clinical resources, achieve quicker and more effective management of patients, limit postoperative complications and preserve reproductive function has led to the popularity of local excisional methods for cervical premalignancy. Most centres use large loop excision of the transformation zone (LLETZ), as this is quick, easy to learn, of low cost and well tolerated. Excisional techniques provide a histological sample that allows the precise grading of the lesion, assessment of the excision margins, and the ability to confirm with certainty the absence of microinvasive or glandular disease. This further allows clinicians to adopt a ‘see and treat’ approach in appropriate cases at the initial visit. In contrast, ablative techniques destroy the cervical epithelium and demand pretreatment biopsies at a separate initial visit with the risk of non‐compliance. Furthermore, punch biopsies under‐diagnose the severity of the lesion in 20% of cases compared with the histology of the large loop.

Complications of treatment

The complications of CIN treatment are rare, and are divided into early and late complications.

Follow‐up after treatment

After CIN treatment women remain at higher risk of recurrent/residual disease and invasive cervical cancer as compared with the general population. Several risk factors have been associated with high risk of recurrence and these include positive margins for disease, particularly in the endocervix, glandular disease, large lesions, age over 40 and grade of the disease. Although the risk for high‐grade recurrence is substantially increased in women with involved as opposed to clear margins (18% vs. 3%), repeat conization is not recommended, with the exception of women over 50 who have positive endocervical margins for high‐grade disease [11].

The majority of therapy failures (90%) are detected within 24 months from treatment. Traditionally, women with a history of treatment were followed up closely for 10–20 years after their treatment. More recently, HPV DNA test with or without cytology has been introduced in most countries as a ‘test of cure’ and allows early discharge of women who test negative back to routine recall and better detection of those at high risk of recurrence.

Cytology after treatment is less accurate and sampling should ensure good representation of endocervical cells, particularly when treatment was conducted for glandular disease. In the case of HPV positivity or cytological abnormality, a colposcopic assessment should be undertaken. Colposcopic assessment is technically more difficult in women who have undergone treatment. Foci of CIN and/or invasive disease may be buried under an apparently normal epithelium. The transformation zone may be difficult to visualize in its entirety due to scarring and because it often retracts deep in the endocervical canal.

Challenging clinical groups

HPV vaccines

The discovery that HPV is causally associated with cervical cancer led to the development of HPV vaccines. The introduction of prophylactic vaccination is the latest important landmark in the history of prevention of cervical cancer. To date, two vaccines have been developed and clinically evaluated, the quadrivalent vaccine (HPV 16/18/6/11, Gardasil) and the bivalent vaccine (HPV 16/18, Cervarix). Results from trials indicate that the vaccine is safe, well tolerated and highly efficacious in HPV‐naive women. The optimal target age is the prepubertal woman before coitarche, while it will remain an individual decision for older women. The first country to introduce a national HPV immunization programme was Australia in both males and females in 2007. In the UK, this was initiated in September 2008 in females only. Data from Australia, Denmark and other countries that implemented the vaccination revealed an 80–90% reduction in the incidence of anogenital warts (for quadrivalent vaccine). More recently, vaccinated populations were found to have significant reduction in the incidence of HPV infection and high‐grade pre‐invasive disease. The nonavalent vaccine (Gardasil 9) that includes a further five high‐risk subtypes in the vaccine cocktail (31, 33, 45, 56, 58) was launched in 2016. Several studies are exploring the use of therapeutic vaccines in women with existing high‐grade disease.

Vaccination and screening are complementary strategies and synergy in a cost‐effective manner will be required for the next few decades. Further research to assess screening strategies in vaccinated cohorts is needed. The introduction of vaccination is especially important in the developing countries, but affordability remains a major issue.