Lack of consensus in calculation of interval cancer rates for cervical cancer screening

In 2018, nondisclosure of results of retrospective audits of cytology in interval cancers precipitated a crisis in the Irish national cervical screening programme. In response, an Expert Reference Group was convened which recommended a collaborative approach to the development of a new key performance indicator, the interval cancer rate. The Expert Reference Group also recommended that the Irish programme should collaborate with international colleagues to reach consensus on (i) the definition of an interval cervical cancer, (ii) the methodology to calculate the interval cancer rate, and (iii) benchmarking with other international programs. This study was undertaken to determine if a consensus regarding the definition of an interval cervical cancer and the calculation of an interval cancer rate exists.

A web-based questionnaire was sent to 18 population-based cervical screening programs. Inclusion criteria involved (1) a national or regional population-based cervical screening prograe; (2) a country or region with a population ≥population of Ireland; (3) programs located in Europe, Australia, or Canada; (4) programs that had responded to a previously published international survey on the disclosure of retrospective cytology reviews in cervical cancer cases.

The response rate was nine out of 18. Of nine respondents, six had an agreed definition of interval cervical cancer, and four of these calculated an interval cancer rate. Three programs neither calculated interval cancer rates nor had any guidelines related to this. Of the six with an agreed definition, all respondents defined the numerator as invasive cancers in the screening age group, with four including microinvasive disease. Respondents included cancers diagnosed 3-5 years after the last screening test had been taken. Three respondents also included cancers diagnosed in women up to 3.5 years after they exited the screening program. Countries use different denominators, including (i) per women years, (ii) per number of screens, and (iii) per total cancers in screened population.

There is variation in the parameters used in interval cancer rate calculation. To allow benchmarking of cervical screening program performance, there is a need for consensus on a standardized method of interval cancer definition and interval cancer rate calculation.