Relation between the Incidence of Invasive Cervical Cancer and the Screening Interval: Is a Five Year Interval Too Long?

Abstract

Objective –To examine the incidence of invasive cervical cancer per 100 000 women years at risk and relative risk according to screening history among eligible women aged 25–69 in Southampton and South West Hampshire during the three years after completion of the first round of comprehensive screening. Results –There was a significantly higher incidence of invasive cervical cancer in women who had not been screened during the preceding 0.5–5.5 years than in those who had been screened (relative risk (RR) 2.6; 95% confidence interval (CI) 1.6 to 4.3). Among the latter group of women (with interval cancers) there was a significantly higher incidence in those with a long interval of 3.5–5.5 years since their most recent smear than in those with a short interval of 0.5–3.5 years (RR 2.2; 95% CI 1.3 to 3.8). Among women with non-interval cancers, there was a significantly higher incidence among those who had no cytology record than among those who had been screened but were overdue for a smear (RR 3.0; 95% CI 1.2 to 7.3). When screen detected cancers were excluded from the figures the relative risks for all the comparative groups described above were greater, though the 95% confidence limits were wider because the numbers were smaller. The most pronounced difference in incidence was between symptomatic cancers in women with a short screening interval (5.8 per 100 000 women years at risk) and in women with no cytology record (71.3 per 100 000 years at risk). Most cancers were interval cancers (76%) because of the high screening coverage: 89.2% of eligible women aged 25–69 had been screened during the preceding 0.5–5.5 years. The overall incidence per 100 000 women years at risk approached that of interval cancers, and was nearer to that observed in the short than the long interval because 74.7% of women had been screened within 3.5 years. Conclusion –The results confirm the effectiveness of screening but suggest that a five year screening interval may be too long, at least during the early rounds of screening.