Liquid Based Cytology (LBC) for Cervical Screening

Published Date: 17 Jan 2018

Explain the basis of Liquid-based Cytology (LBC). Include in your essay reference to the automation of LBC and to the national cervical screening programme.

Introduction

Cervical cancer attributes to one of the leading causes of cancer death with more than 273,000 deaths occurring each year; this malignant neoplasm arises from cells that are located in the cervix uteri. The high prevalence and mortality rates have dramatically declined since the advent of the Papanicolaou (Pap) smear in the 1940’s however issues with consistency have overshadowed the benefits. This essay will discuss the induction of Liquid-based Cytology and the resultant impact this technique has had on screening for cervical cancer. (Issah et al, 2011)

Cervical Neoplasia

The primary underlying cause of cervical cancer is due to infection with a sexually transmitted virus termed the human papillomavirus (HPV). These infections arise spontaneously and can induce changes when squamous metaplasia is occurring during adolescence and pregnancy. This virus incorporates viral particles into the DNA of these cells which if left untreated can progress to pre-cancer (dysplasia) which ultimately can develop into cancer, as depicted in figure 1. Cancer is the term used to describe the uncontrolled growth of cells which lead to tumours forming. These tumours can invade and destroy tissues and compete and inhibit tissues from receiving essential nutrients and gases. Metastases can also occur when new tumours arise by cancerous cells being carried to distant sites via the blood and lymph nodes. (He et al, 2013).

This transformation zone on the ectocervix has shown to be extended in cases of pregnancy, puberty and when women use oral contraceptives which results in a higher possibility of infection if exposed to HPV infections. Subsequently, the key determinants of cervical cancer are early sexual activity, multiple pregnancies. Furthermore, a factor such as a high number of sexual partners or having multiple partners has also been associated. (Martinez-Mesa et al, 2013). Cervical cancer is an easily preventable disease due to the fact that usually takes 10-20 years for mild dysplasia to develop into carcinoma. This factor and the fact that cervical cancer is usually always fatal illustrate the rationale for screening.

Current Service Provision

The mortality rates of cervical cancer vary seventeen fold between the different regions of the world. For instance, a staggering 2.4 million years of life are lost among women between the ages of 25-65 in developing areas in contrast to a 0.3 million years of life lost in developed countries. (Cancer Research UK, 2014). Although both economic and political factors contribute to this figure, perhaps this significant contrast may be due to the introduction of interventions like the National Cervical Screening Programme which have been implemented in the UK since 1989 and have subsequently resulted in a dramatic decrease in the mortality rates of cervical cancer. In fact, it has been predicted that it has been responsible to prevent 63% of cervical cancers. In this programme, women aged between 25-49 years are requested to attend a screen every 3 years and then every 5 years until the age of 64. (NHS Cervical Screening Programme, 2008). These age guidelines are the result of scientific studies showing that women under the age of 25 years do not require screening nor do women over the age of 64 providing that there is no history of cancers. This is because in this age category, there has been very low prevalence of cervical cancers and thus testing has led to unnecessary procedures being performed and needless anxiety, along with increased cost of care. It also placed these women at risk of problems associated with testing. These age guidelines have been debuted following careful deliberation by taking into account the time span it takes for a HPV infection to develop. (Flynn and Lewis, 2013).

Screening involves obtaining a collection of cells from the surface of the cervix and evaluating their appearance microscopically. In the UK, the British Society for Clinical Cytologists (BSCC) categorises these smears using guidelines such as negative, borderline, mild, moderate, severe, glandular neoplasia, invasive or inadequate. Women exhibiting dyskaryotic smears are monitored and additional testing such as a colposcopy may be performed. Current techniques can detect any cell abnormalities before the cells can become cancerous and thus with early detection, people can be offered information, further tests or the relevant treatment and can be administered and prevent the onslaught of cervical cancer completely. ( Herbert and Turnbull, 2006).

Screening currently involves the Pap test where the sample is prepared from the cervix uteri by using a spatula broom-type device or a cyto-brush to obtain a sample of cells, obtained under direct vision using a vaginal speculum. Cervical cytology techniques such as this make use of the fact that tumour cells lose their cohesive property early on in the disease process which encourages the shedding of cells from tissue surfaces. Thus if there are any cancerous cells present when the cervix is scraped then they would be easily dislodged. This method then involves directly smearing the cervical cells on to a glass slide for subsequent analysis via microscopy. The cells must be transferred to the slide quickly in order to try and preserve the sample. (Malm et al, 2013).

However, screening in itself is only a preventative measure rather than a cure. The diagnostic tests that are implemented within the screening programme are not always accurate as it is possible that pre-cancerous cells will not be detected in some instances. However, screening programmes like the NHSCSP and Cervical Screening Wales which screen women at regular intervals tackle this issue as subsequent cervical smear tests would yield a more accurate result.

Approximately, 8% of Pap smear tests yield inaccurate results; this may be due to insufficient cervical cells being present in the sample or presence of contamination and inflammatory cells obscuring the sample. This then results in women being invited to attend a repeat test which is an inconvenience and causes unnecessary anxiety. (Chankong et al, 2014).

Liquid Based Cytology

Advances in screening technology has led to the gradual introduction of Liquid based cytology in the 1990’s which tackles some of the limitations of the conventional Pap smear methodology. Although recent studies are actually suggesting that there is little difference in the effectiveness of each diagnostic, the advent of liquid based cytology has successfully reduced the screening time whilst simultaneously reducing the chance that the sample is unsatisfactory or limited. Examples of this technology being utilised are methods like the Cytoscreen or The Labonard Easy Prep method. (Smith, 2009) In contrast to the customary pap smear, in this technique the cells are collected from the cervix using plastic devices and then placed in a preservative liquid (instead of directly on a slide) and then sent for transport to the laboratory. These cells are then smeared onto glass slides and analysed under a microscope. The preservative liquid aids in the removal of some of the impurities like pus, mucus, bacteria and yeast that can potentially cause the sample to be obscured whilst also preventing the cells from drying out. Furthermore, the liquid enables the cells to be spread out more on the slide thus preventing distortion and can also be tested for HPV (human papillomavirus). These advantages mean that it is more likely that a more representative transfer of cells is collected. This has numerous benefits: it prevents anxiety and inconvenience of the patient having to be retested because of a poor sample, the higher turnover rate and the fact that the preparations are easier to analyse means that it is more productive and thus less time consuming. However, there is still controversy relating to the specificity of the two approaches. (Smith, 2011).

Automation

Despite the aforementioned benefits of liquid-based cytology, it is evident that human error can be a problem as it can lead to inadequate samples which arise when insufficient squamous epithelial cells are present. Therefore to bypass these limitations, automation of this technique has been sought. Prior to the advent of liquid-based cytology, automation was not a feasible option: visualisation of cells was difficult due to obscuration from factors like blood, mucus and overlapping of cells. It was difficult to distinguish between normal cells and neoplastic cells. A single Pap smear would give rise to approximately 300,000 cells, the analysis of which was unachievable due to limited computer capacity. The Liquid-based cytology technique addressed these limitations as it enables a near-monolayer of cells to be analysed that were largely devoid of obscuration. This together with progresses in computerised image analysis has now resulted in the advent of semi-automated slide scanning. (Smith, 2011).

The food and drug administration have recognised these benefits and have approved the use of two liquid-based cytology systems termed The SurePath System and The Thin Prep Technique. The thin prep technique omits the stage of initially smearing the sample on to a microscopic slide. Instead it employs a more efficient way of obtaining a clearer and easier to read sample. In contrast to the conventional Pap smear, virtually the entire sample is captured by rinsing the collection device in a proprietary preservative solution. This is then sent to the laboratory where an automated processor filters and disperses the sample and then places a representative sample on the slide. This technique therefore minimises contamination and cellular preservation is said to be enhanced. The result is improved fixation which allows consistent staining and has improved quality and is thus more appropriate for diagnostic review. Furthermore, the cell suspension can be retained and utilised later for further biological tests like chlamydia or human papilloma virus (HPV). The ThinPrep 2000 is a semi-automated machine which allows one specimen to be processed at a time but the ThinPrep 3000 and 5000 machines have been made available which allow 80 and 160 specimens respectively to be analysed at one time leading to significantly rapid turnover of results. (Zhao and Austin, 2007) Similarly, the Surepath method also employs the use of a preservative solution but the collection device is also retained and placed in a SurePath collection vial that contains the transport fluid. This ensures that all the cervical cells that are collected are sent to the laboratory where they are prepared using a Prepstain machine which processes 48 samples at one time. A major limitation of automation in LBC is the cost of the techniques. However, this can be offset by savings from the reduction in the number of inadequate samples and quicker diagnosis rates. Another limitation is that it is very labour intensive and thus training would need to be administered to healthcare professionals to completely eradicate the conventional methodology employed in the Pap smear. (Mina Desai, 2009).

The BD FocalPoint^TM and The ThinPrep^TM imaging system have both been approved by the US Food and Drug Administration on account of having the ability to be more efficient at detecting high-grade cytological abnormalities when compared to manual screening methodology. Using algorithms, the BD FocalPoint^TM system assigns a score between 0.0-1.0 based on the probability of abnormality. If a slide is scored that does not meet a certain value, it is disposed of which leads to a significant reduction in the workload. For example, customarily 25% of a population of cervical screening samples can be discarded, eliminating the need to microscopically analyse them. This system has been developed by utilising an automated microscope to further save time. The ThinPrep^TM imaging system also effectively reduces the time and labour taken to process samples by rapidly scanning the entire microscopic slide and identifying 22 fields of view that contains cells that are likely to be significant. With the aid of an automated microscope, any abnormal cells are then identified. If the slide yields no abnormal cells then the slide is considered as negative thus no further diagnostics are required. However, automation-assisted reading cannot be currently implemented in England for primary cervical screening as studies have demonstrated that these methods are inferior to manual methods due to issues with sensitivity and specificity. (Smith, 2011).

HPV DNA Testing

In recent years, HPV DNA testing has been proposed as a viable diagnostic option for cervical cancer as it has been proven to provide greater sensitivity and increase reproducibility. Although it is still in its initial stages, it is a promising diagnostic technique that can be used for primary screening in conjunction with cervical cytology for women aged 30 onwards and has the potential to allow for longer screening intervals. (El-Morsi, 2013).

Conclusion

Due to its invasive nature women are reluctant to attend their screens because of reasons like embarrassment. Since attendance is only an optional measure, maybe an increase in both the education on the rationale behind screening or mandatory screens should be implemented. Furthermore, it is evident that the laboratories have vast volumes of samples to diagnose and thus further studies on cost effectiveness should be carried out so that automated and alternative approaches can be incorporated into the current screening regime. In conclusion, although screening has resulted in a dramatic decline in the prevalence of cervical cancer, it is only a diagnostic and preventative measure and not a cure. Since HPV has been demonstrated to be the initial virus that spurs on the development of cervical cancer, it’s reasonable to suggest that we target this virus in an attempt to stop altogether the onset of cervical cancer. Furthermore, recent studies have suggested a vaccine may be able to reduce the onset of HPV and subsequently cancer which would bypass the problems of screening. Taking into account that cervical cancer is one of the worlds’ leading cancers; it is evident that there is still a lot of leeway for improvements in the current methodology of preventing cervical neoplasia.

References

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FLYNN, H and LEWIS, P. (2013). Rational care or rationing care? The case of cervical screening across the united kingdom. Health Policy. October. Pp 197-201.

GREAT BRITAIN, CANCER RESEARCH UK. (2014) Lets beat cancer soon. United Kingdom: London

GREAT BRITAIN, WORLD HEALTH ORGANISATION. (2014) Screening for cervical cancer. Department of Chronic Diseases and Health Promotion. Switzerland: Geneva.

HE et al. (2013) Applications of the CELLDETECT staining technique in diagnosis of human cervical cancer. Gynaecologic Oncology. December.

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MORSI et al. (2013) Clinical validation of high risk HPV DNA testing versus ThinPrep cytology for primary cervical cancer screening. Middle East Fertility Society Journal. Januray, Pp 2

Smith, H.F (2011) Cytology, liquid based cytology and automation. Best practise & research clinical obstetrics & gynaecology. October. Pp585-596.

SMITH, J (2009) The future of cervical screening in the UK. Diagnostic histopathology, July, Pp 330-334.

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