Quantifying the potential problem of overdiagnosis of ductal carcinoma in situ in breast cancer screening

The relevance of detection of ductal carcinoma in situ (DCIS) in a breast cancer screening programme, and the extent of overdiagnosis of non-progressive lesions, remains controversial. It was the purpose of this paper to estimate the incidence of non-progressive, 'overdiagnosed' DCIS. We defined non-progressive DCIS (DCIS₀) as DCIS which could not have progressed to invasive disease if left untreated. Progressive DCIS (DCIS₁) was defined as DCIS which has the propensity to progress to invasive disease. We fitted a Markov process model of the incidence of progressive and non-progressive DCIS, the transition of the former to preclinical invasive disease and the subsequent progression to clinical symptomatic cancer. We used data from the Swedish Two-County Trial and from service screening programmes in the UK, Netherlands, Australia and the USA to estimate the incidence of progressive and non-progressive DCIS, and the detection rates of each at the first and subsequent screening. Average incidence of non-progressive DCIS was 1.11 per 100 000 per year. Average incidence of progressive DCIS was 2.1 per 1000 per year. At prevalence screen, 37% of DCIS cases were estimated to be non-progressive. A woman attending prevalence screen has a 19 times greater chance of having a progressive DCIS or an invasive tumour diagnosed than of having a non-progressive DCIS diagnosed. At incidence screen, only 4% of DCIS cases were estimated to be non-progressive. A woman attending an incidence screen has a 166 times higher probability of having a progressive DCIS or invasive lesion diagnosed than of having a non-progressive DCIS diagnosed. There is an element of overdiagnosis of DCIS in breast cancer screening, but the phenomenon is small in both relative and absolute terms.