Cytological screening and management of abnormalities in prevention of cervical cancer: an overview with stochastic modelling.

Abstract

Aims-To develop a mathematical model of the histological changes of precancer and the development of invasive cancer and how these are related to cytological findings. To use this to investigate the effects on incidence of cervical cancer, number of smear tests and colposcopies, of different schedules for cervical screening, and the clinical management policies for dyskaryosis. Methods-A stochastic model was developed relating the available data on tissue progression to the cytological findings. Two strategies, A and B, were compared: under A, women with any abnormal smear receive immediate colposcopy and treatment; under B, women with mild or borderline dyskaryosis have repeated smears at six monthly intervals with colposcopy only for persistent abnormalities. Results-The model predicted an incidence of invasive cervical cancer in an unscreened population of women aged over 18 years of 5 9 per 10 000 per year. With 70% coverage and three yearly screening under strategy A, the incidence fell to 2-00 and under B to 2*10. The number of smears required was similar but A required two to three times as many colposcopies as B. Raising the coverage to 90% reduced the incidence to around 1 per 10 000 per year but changing the screening interval, the specificity or sensitivity of cytology had much less effect. Conclusion-The model has been tested under a wide range of possible variations in natural history, specificity and sensitivity of cytology. For low grade smear abnormalities, open colposcopic referral is predicted to reduce invasive cancer only slightly more than repeat cytology, at the expense of much additional colposcopy. Improving cytological coverage is suggested as more effective in reducing invasive cancer than increased use of colposcopy or more frequent screening. ( Clin Pathol 1994;47:430-435) Cervical screening is important and costly. Evaluating the overall effectiveness of cervical screening in relation to the contribution of different components is difficult. Determining overall policy requires the evaluation of many possible alternative combinations of cytology, colposcopy, and treatment for their effectiveness in preventing the progression of precancer to invasion and death.There has been debate, reflected by wide variations of practice,34 concerning the relative merits of screening at intervals of less than five years, of achieving more complete coverage of women at risk, and of colposcopy or repeat cytology for low grade smear abnormalities.5-9 This has arisen mainly from concern about the limited accuracy of cytology for the detection and grading of tissue changes of precancer and invasive cancer, and uncertainty about the natural history of precancer.There is detailed information about the effectiveness of colposcopy and the sensitivity and specificity of cervical cytology for detecting cervical abnormality and for predicting its histological grade.'1"2 There are also areas of uncertainty. High grade precancer (CIN 3) has a high risk of progressing to invasion'3 but much borderline or...

The predicted effect of changes in cervical screening practice in the UK: results from a modelling study

“…Therefore, the natural history of cervical disease lends itself to a modelling approach. Modelling can assist with decision making by exploring the implications of a range of possible interventions (Royston, 1999) and has been previously used to evaluate the natural history of CIN and changes in screening practice (Sherlaw-Johnson et al, 1994, 1999. The model we have described builds on previous work to provide a tool that encompasses the current understanding of the natural history of cervical neoplasia and is specific to the UK setting.…”

Cost-Effectiveness of Rescreening Conventionally Prepared Cervical Smears by PAPNET Testing

“…The key parameter values assumed in this model are given in Table I. Eddy 5,6 and SherlawJohnson et al 7 are the sources for most parameters not taken from Muller et al 2 As such, the cervical neoplasia-related events that occur in the simulation should closely resemble those which would be found in a representative sample of U.S. women aged 20-64.…”

Challenging the role of calibration, validation and sensitivity analysis in relation to models of health care processes

“…This paper arose from a request to provide advice concerning what sensitivity analysis procedures to adopt for testing the forecasts of a model of cervical cancer screening, an area of modelling of which the author has experience [1][2][3][4]. Having agreed to write some notes on this, it seemed appropriate to include a brief preamble discussing the question 'why do we do sensitivity analysis?'.…”