Jacky Luiten

26 | Chapter 2 Discussion This population‐based study describes the trends in incidence of both screen‐ detected DCIS and invasive breast cancer in the southern part of the Netherlands from July 2005 to July 2015. When comparing high versus low or intermediate grade, our study shows that screen‐detected DCIS was mainly high grade in contrast to invasive carcinomas which were mainly found to be low or intermediate grade. This observation suggests that treatment of poorly differentiated DCIS detected through mammographic screening could play a role in the relative reduction of the incidence of poorly differentiated invasive carcinoma, assuming that high grade DCIS was removed before it could develop into high grade invasive cancer. 14,15 Taken into account that high grade DCIS may preferably develop into high grade invasive carcinoma, increased screen‐detected DCIS and the subsequent treatment of high grade DCIS may not purely lead to overtreatment, but prevention of the development into high grade carcinomas. However, high grade invasive cancers, on the other hand, may be more frequently presenting as interval cancers. Recently, Duffy et al . investigated the association between detection of DCIS at screening and invasive interval cancers subsequent to the relevant screen. 16 They concluded that a higher rate of screen‐detected DCIS is associated with a lower rate of invasive interval cancers, suggesting that detection and treatment of DCIS is worthwhile in prevention of future invasive disease. Our study, however, was directed to screen‐detected malignancies in consecutive screening cohorts and therefore we disregarded the interval carcinomas. Our study not only provides an overview of the impact of the transition of SFM to FFDM on screening outcome, but it also compares the periods before and after the intervention. In the SFM period as well as in the FFDM period the aforementioned ratio in the distribution of DCIS grade and invasive tumor grade was compared with regard to the type of screening technique. During the transition period from SFM to FDDM a significantly increase in the number of low grade DCIS was observed. This finding may be attributed to the better detection of small abnormalities, especially grouped microcalcifications, at FFDM. 17,18 The clinical value of this increased detection of low grade DCIS and subsequent treatment is currently a matter of debate as the majority of these lesions may remain subclinical. 6,9 The total CDR of DCIS per 1000 screened women increased during the transition period and then decreased to a lower level in the FFDM‐only periods. In the FFDM period (2011‐2015), the total CDR of DCIS per 1000 screened women is almost comparable to this rate in the SFM period (2005‐2009). The rate of low or