Detection of early-stage occult cancers

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Detection of occult invasive cancers in asymptomatic women may increase the incidence of cancer in early rounds organised screening.

An audit of 382 cancers from 1985 to 1996 in Southampton, during the introduction of organised screening in the background of opportunistic screening with poor quality control, showed a trend from symptomatic to screen-detected cancers with an increase in overall incidence during the prevalence round (Figure 6.7).

Screen-detected cancers are more likely to be diagnosed at stage I: Andrae et al. (2008) reported 91% of screen-detected cancers to be equally divided between stage IA and IB compared with 51% of symptomatic cancers (12% IA, 39% IB).

Screen-detected cancers

Screen-detected cancers may increase incidence of cancer in early rounds of organised screening
Screen-detected cancers are usually diagnosed as IA or IB1 cancers

Figure 6.7. Trend from symptomatic to screen-detected cancer in Southampton (Figure 1 from Herbert et al. 2009)

In summary, data from well-organised programmes reported more than 25 years ago as well as from those organised more recently demonstrate multiple factors to be considered when assessing the effectiveness or otherwise of cervical screening in terms of incidence and mortality.

Measurement of the effect of centrally organised screening depends on

Accuracy of cancer registration
Effect of spontaneous screening carried out in addition to organised screening
Age range of women screened and screening intervals
Compliance with recommendations
Risk of disease in different birth cohorts
Distinction between screen-detected and symptomatic cancers
Quality control of all aspects of the programme

Learning points from Chapter 6

Organised population screening can decrease the cervical cancer incidence and mortality by 80%
Changes in cervical cancer incidence and mortality may be masked by changes in prevalence of HPV in the population
Estimation of the effect of screening depends on accurate cancer registration
The apparent effectiveness of organised screening may be affected by spontaneous screening in the same population
Effectiveness of screening depends on compliance with recommendations
Effectiveness of screening depends on age range of women screened and intervals between tests
Quality control of all aspects of the programme is essential for successful screening
Early rounds of effective screening may increase incidence through detection of early occult invasive cancers in asymptomatic women