Experience with cervical screening in British Columbia

Summary Non-progression and duration of preclinical neoplastic lesions of the cervix uteri were studied using screening data from a previously unscreened population, Maribo County, Denmark (1966-82). To estimate regression rates, the incidence of clinical cancer before the screening programme was related to the prevalence and incidence of preclinical lesions estimated from the detection rates of first smear and third and subsequent smears respectively. Duration was estimated from the time lag between the cumulative incidence of preclinical lesions and the combined cumulative incidence of clinical cancer and the estimated 'incidence of regression'. Of all preclinical lesions in women aged 25-50, 24% progressed, 39% regressed and 38% remained. Even if we assume no onset of preclinical lesions above age 50, we estimated that 48% of the preclinical lesions would not progress to clinical cancer in the women's lifetime. The estimated mean duration of preclinical lesions was 16 years. In Maribo County during the 1970s, the positive rate (1.6%) was low compared with current rates in several countries. We conclude that the detection of non-progressive lesions was outweighed by the prevention of clinical cancer.Keywords: cervical cancer; screening; natural history; Pap smear; dysplasia A thorough understanding of the natural history of a disease is among the basic requirements for the initiation and evaluation of screening programmes (Wilson and Jungner, 1968). However, the natural history of cervical cancer can only be learned directly from the experience of women with positive smears followed without treatment until development of invasive cervical cancer. As observation without treatment has been considered unethical for many years, such data on the natural history of cervical cancer are available only for small groups of women (Ostor, 1993). Nevertheless, data from the first and subsequent rounds of mass screening in a previously unscreened population can give insight into crucial aspects of screening for cervical cancer, such as regression and duration of the preclinical stage (Boyes et al, 1982).Maribo County in Denmark is an area in which cervical smears were not used before an organized screening programme was started in 1967 for all women aged 30-49 years (Berget, 1979). From the beginning, the local pathologists ensured registration of all smears and cervical biopsies taken in the area (Lynge and Poll, 1986a,b). This combination of screening started from scratch and complete registration makes Maribo County an ideal setting for the study of the natural history of cervical cancer.We analysed the data of Maribo County focusing on the estimation of non-progression rates and duration of the preclinical stage. Estimates were obtained by relating detection rates among first smears ('prevalence of preclinical lesions') and detection rates of repeated smears ('incidence of preclinical lesions') to the incidence of clinical cancer in the unscreened population. Nonprogression contributes to the negative side-effects of s...

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Non-progression of cervical intraepithelial neoplasia estimated from population-screening data

Bos

Ballegooijen²,

Oortmarssen³

et al. 1997

“…In the early 1970s, when approximately 85% of women aged 20 years and over in the province were enrolled in the programme (Boyes et al, 1981), an investigation into the true incidence and prevalence of cancer of the cervix was carried out in two cohorts of women (Boyes et al, 1982). Cohort I consisted of women born in the years 1914-18 and cohort II were those born in the years 1929-33.…”

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confidence: 99%

Forty years of repeated screening: the significance of carcinoma in situ

Morrison

Coldman²,

Boyes³

et al. 1996

Summary Two cohorts of women born in 1914-18 and 1929-33 who participated in a cervical screening programme have been followed for over 40 years. Age-specific incidence rates of squamous carcinoma of the cervix by rank of smear and length of interval between smears are reported. The younger cohort, who had undergone more frequent screening, had lower rates of invasive disease. From these incidence rates, estimates of false-negative rates and regression rates for carcinoma in situ have been made. The false-negative rate was estimated to be about 15%. Regression seemed more frequent in younger than in older women. For the younger cohort it was estimated to be 72% and in the older 47%. Before the updating of the cohort data was attempted it was realised that, for many of the women, no post-1969 records would exist, since, before or soon after 1970, some of the women would have died from causes other than cervical cancer, some would have had a hysterectomy and some would have left the province. During the 1970s some culling of the files in the cytology laboratory took place. The files of women who had had nothing but negative smears, no histology and had not had a smear taken in the previous 7 years, or were known to be dead, were deleted. Interpolating from the data on annual terminations of surveillance, it would appear that the files of approximately 10% of the women were removed.Before proceeding with the update an attempt was made to estimate from the smear and pathology records of the original cohorts that were on magnetic tape the number of women who had a very low probability of being linked. They fell into two groups. The first group were women without histology records and only a single smear which was taken before 1967. The authors considered that women below age 55 with a gap without a smear in the years 1967-70 (when many families returned to the eastern provinces) and with only a single smear preceding, was a strong indication of a transient resident. The second group were those who, before 1970, had had a hysterectomy for reasons other than cervical cancer or its precursive abnormalities, or who had died of cervical cancer. In cohort I, 12 923 women fell into these two groups, and in cohort II 14 537. These women, however, were included in all matching attempts since it was easier to leave their records in the files and because there was a slight probability that a linkage might be made.Two methods of matching were used. One was to search the files at the cytology laboratory for each unique smear number included in the original cohort study. A successful match on these numbers enabled us to identify the woman in the cytology files and, from this, update our records. The Correspondence: BJ Morrison

“…It is assumed that the crosssectional clinical incidence data and the longitudinal screening data are comparable. Differences between these data sets could be caused by cohort effects, but cohort differences were not found in the cohort study (Boyes et al, 1982).Regression is supposed to occur only in pre-invasive lesions. The observed detection rates of pre-invasive lesions (dysplasia and carcinoma in situ) of a first smear are included in estimating and testing the assumptions about regression (see Table A-I).…”

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Epidemiological evidence for age-dependent regression of pre-invasive cervical cancer

Oortmarssen

Habbema²

1991

151

In the analysis of the screening data from the British Columbia cohort study, a considerable amount of regression was found. The regression is concentrated in lesions detected at younger ages (Boyes et al., 1982). This finding was based on a comparison of the estimated cumulative incidence of pre-invasive lesions with the prevalence of these lesions and with the cumulative incidence of clinically diagnosed invasive cervical cancer. However, no explicit hypotheses about regression were tested. A model based approach would allow for using the details age-specific data from the cohort study in testing the assumptions. Earlier modelling efforts, e.g. by Coppleson and Brown (1975) Columbia, but only on a very limited scale. The impact on clinical incidence in the first years of screening has presumably been negligible. Thus, the incidence in 1955-57 is considered to represent the situation without screening. We converted the clinical incidence into an incidence rate for the female population at risk of cervical cancer by correcting for the cumulative hysterectomy rates recorded in the two cohort study populations.A distinct advantage of the data from British Columbia is that age-specific rates are available over a broad age-range, both for clinically diagnosed cancers and for screen-detected lesions. The two cohorts together span the age-range 20-54 in the study period. In the overlapping age-group 35-39, results from the younger cohort are used because of the larger numbers in this group. It is assumed that the crosssectional clinical incidence data and the longitudinal screening data are comparable. Differences between these data sets could be caused by cohort effects, but cohort differences were not found in the cohort study (Boyes et al., 1982).Regression is supposed to occur only in pre-invasive lesions. The observed detection rates of pre-invasive lesions (dysplasia and carcinoma in situ) of a first smear are included in estimating and testing the assumptions about regression (see Table A-I). In the results of the second and subsequent smears, the pre-invasive and early invasive lesions are pooled, because of the small numbers in the latter category. On basis of the tables in the report of the cohort study, these results are classified by age at midpoint between the first and second smear, and by the time since the preceding Pap-smear.