Supplementary Materials and Methods

Background Radiotherapy (RT) following breast-conserving surgery (BCS) for ductal carcinoma in situ (DCIS) reduces ipsilateral breast event rates in clinical trials. This study assessed the impact of DCIS treatment on a 20-year risk of ipsilateral DCIS (iDCIS) and ipsilateral invasive breast cancer (iIBC) in a population-based cohort. Methods The cohort comprised all women diagnosed with DCIS in the Netherlands during 1989–2004 with follow-up until 2017. Cumulative incidence of iDCIS and iIBC following BCS and BCS + RT were assessed. Associations of DCIS treatment with iDCIS and iIBC risk were estimated in multivariable Cox models. Results The 20-year cumulative incidence of any ipsilateral breast event was 30.6% (95% confidence interval (CI): 28.9–32.6) after BCS compared to 18.2% (95% CI 16.3–20.3) following BCS + RT. Women treated with BCS compared to BCS + RT had higher risk of developing iDCIS and iIBC within 5 years after DCIS diagnosis (for iDCIS: hazard ratio (HR) age < 50 3.2 (95% CI 1.6–6.6); HR age ≥ 50 3.6 (95% CI 2.6–4.8) and for iIBC: HR age<50 2.1 (95% CI 1.4–3.2); HR age ≥ 50 4.3 (95% CI 3.0–6.0)). After 10 years, the risk of iDCIS and iIBC no longer differed for BCS versus BCS + RT (for iDCIS: HR age < 50 0.7 (95% CI 0.3–1.5); HR age ≥ 50 0.7 (95% CI 0.4–1.3) and for iIBC: HR age < 50 0.6 (95% CI 0.4–0.9); HR age ≥ 50 1.2 (95% CI 0.9–1.6)). Conclusion RT is associated with lower iDCIS and iIBC risk up to 10 years after BCS, but this effect wanes thereafter.

Background. Ductal carcinoma in situ (DCIS) is a potential precursor of invasive breast cancer (IBC). However, the natural course of a particular DCIS lesion is unknown, because almost all women with DCIS are treated. Furthermore, most studies are biased because these comprise DCIS adjacent to IBC, also known as synchronous DCIS and IBC, indicating that such DCIS lesions already have the capacity to progress to IBC. It is still unknown which proportion and type of subsequent ipsilateral IBCs (iIBC) are related to the initial primary "pure" DCIS lesion. Therefore, we performed an extensive molecular characterization of DCIS and matched subsequent iIBC, to better understand the natural course of DCIS.Patients and methods. We used a unique series of 78 women diagnosed with DCIS and treated by breast conserving surgery (BCS) alone, which subsequently developed iIBC. Mean time to iIBC event was 6.3 years (range 0.5-17.0). These 78 women are a representative sample of a case-control series, nested in a nation-wide, population-based cohort including all patients diagnosed with DCIS between 1989 and 2005 in the Netherlands (Visser, et al Clin Can Res 2018). Data on tumor location (ICD-10) was available for all lesions. DNA and RNA was simultaneously extracted for 78 DCIS lesions and 78 matched subsequent iIBC (DNA >20ng; RNA >100ng), and RNA sequencing (RNAseq) and low coverage whole genome sequencing (CNVseq) was performed. Panel sequencing (PanelSeq), using a custom panel of 53 breast cancer driver genes, was performed with the remaining DNA of 42 DCIS and iIBC matched pairs. We determined if the iIBC lesion and DCIS lesion were related, by comparing tumor location and genomic features.Results. Based on tumor location and histological grade, >95% of the subsequent iIBC reflected outgrowth of residual disease. Based on RNAseq data, 77% of all DCIS and IBC lesions classified into the same PAM50 subtypes. The CNVseq data showed that the DCIS lesions contained copy number aberrations on typical breast cancer-associated loci, such as 1q gain, 8q gain, 16q loss, 20q gain. However, when we compared DCIS with their matched iIBC, we observed in 41% of the cases very distinct copy number profiles, indicating either outgrow of a different tumor (minority) subclone or a second primary tumor. Analysis of PanelSeq mutation data supported this clonal or independent origin of the subsequent iIBC.Conclusion. To our knowledge, our study is the first to investigate if subsequent iIBC is likely to originate from the initial primary "pure" DCIS in a large series with long-term follow-up.Surprisingly, our CNVseq and PanelSeq results indicate that more than one third of the subsequent iIBCs after BCS alone treated primary DCIS are likely to be second primary tumors or represent selective outgrowth of a minority DCIS subclone.Citation Format: Lindy Visser, Marlous Hoogstraat, Tycho Bismeijer, Lotte Elshof, Koen van de Vijver, Emilie Groen, Mathilde Almekinders, Joyce Sanders, Carolien Bierman, Dennis Peters, Ingrid Hofland, Frank Nieboer, Michiel de Maaker, Petra Kristel, Lennart Mulder, Annegien Broeks, Michael Schaapveld, Marjanka Schmidt, Lodewyk Wessels, Esther Lips, Jelle Wesseling, On behalf of the PRECISION team. Ductal carcinoma in situ of the breast: Cancer precursor or not [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 751.

Ductal carcinoma in situ (DCIS) is considered a potential precursor of invasive breast carcinoma (IBC). Studies aiming to find markers involved in DCIS progression generally have compared characteristics of IBC lesions with those of adjacent synchronous DCIS lesions. The question remains whether synchronous DCIS and IBC comparisons are a good surrogate for primary DCIS and subsequent IBC. In this study, we compared both primary DCIS and synchronous DCIS with the associated IBC lesion, on the basis of immunohistochemical marker expression. Immunohistochemical analysis of ER, PR, HER2, p53, and cyclo-oxygenase 2 (COX-2) was performed for 143 primary DCIS and subsequent IBC lesions, including 81 IBC lesions with synchronous DCIS. Agreement between DCIS and IBC was assessed using kappa, and symmetry tests were performed to assess the pattern in marker conversion. The primary DCIS and subsequent IBC more often showed discordant marker expression than synchronous DCIS and IBC. Strikingly, 18 of 49 (36%) women with HER2-positive primary DCIS developed an HER2-negative IBC. Such a difference in HER2 expression was not observed when comparing synchronous DCIS and IBC. The frequency of discordant marker expression did not increase with longer time between primary DCIS and IBC. In conclusion, comparison of primary DCIS and subsequent IBC yields different results than a comparison of synchronous DCIS and IBC, in particular with regard to HER2 status. To gain more insight into the progression of DCIS to IBC, it is essential to focus on the relationship between primary DCIS and subsequent IBC, rather than comparing IBC with synchronous DCIS.

Introduction Ductal carcinoma in situ (DCIS) is a pre-invasive breast lesion, frequently detected by breast cancer screening, with thousands of new cases each year. While DCIS is not life threatening, it does increase a women’s risk of developing invasive breast cancer; this in turn can lead to breast-cancer specific mortality. Therefore, almost all DCIS lesions are treated to prevent progression to invasive disease. However, many DCIS lesion will never develop into invasive breast cancer if left untreated, indicating that many women are overtreated. In this study, we performed RNA sequencing (RNAseq) on a large set of primary DCIS lesions, comparing lesions with a subsequent invasive breast cancer recurrence (IBCR) to those without, to identify factors distinguishing harmless from potentially hazardous DCIS. Material and methods We made use of a DCIS case-control series, nested in a nation-wide population-based cohort of Dutch women diagnosed with primary DCIS and treated with breast conserving surgery alone (standard clinical practice) between 1989–2005. Mean follow up time was 12.0 years. RNA was extracted from FFPE laser-microdissected tissue fragments of 183 primary DCIS lesions: 100 associated with subsequent IBCR and 83 which remained free of invasive recurrence. RNAseq was performed to identify differentially expressed genes between DCIS with and without IBCR. Subsequently, gene set enrichment analysis (GSEA) was performed. Finally, findings were correlated with patients’ clinical and histopathological characteristics. Results and discussions DCIS lesions were categorised into PAM50 intrinsic subtypes: Luminal A 25%, Luminal B 38%, Her2 22%, Basal 10%, Normal 5%. Within each PAM50 subtype, 1 to 64 genes were statistically significant (p Conclusion To our knowledge, we performed the first large-scale transcriptome analysis of primary DCIS lesions with and without subsequent IBCR and long-term follow-up. Our data suggest that immune-related pathways are involved in DCIS progression. The results of our study add to the current understanding of DCIS and to risk stratification of DCIS in the near future.

Abstract Purpose: Ductal carcinoma in situ (DCIS) is treated to prevent progression to invasive breast cancer. Yet, most lesions will never progress, implying that overtreatment exists. Therefore, we aimed to identify factors distinguishing harmless from potentially hazardous DCIS using a nested case–control study. Experimental Design: We conducted a case–control study nested in a population-based cohort of patients with DCIS treated with breast-conserving surgery (BCS) alone (N = 2,658) between 1989 and 2005. We compared clinical, pathologic, and IHC DCIS characteristics of 200 women who subsequently developed ipsilateral invasive breast cancer (iIBC; cases) and 474 women who did not (controls), in a matched setting. Median follow-up time was 12.0 years (interquartile range, 9.0–15.3). Conditional logistic regression models were used to assess associations of various factors with subsequent iIBC risk after primary DCIS. Results: High COX-2 protein expression showed the strongest association with subsequent iIBC [OR = 2.97; 95% confidence interval (95% CI), 1.72–5.10]. In addition, HER2 overexpression (OR = 1.56; 95% CI, 1.05–2.31) and presence of periductal fibrosis (OR = 1.44; 95% CI, 1.01–2.06) were associated with subsequent iIBC risk. Patients with HER2+/COX-2high DCIS had a 4-fold higher risk of subsequent iIBC (vs. HER2−/COX-2low DCIS), and an estimated 22.8% cumulative risk of developing subsequent iIBC at 15 years. Conclusions: With this unbiased study design and representative group of patients with DCIS treated by BCS alone, COX-2, HER2, and periductal fibrosis were revealed as promising markers predicting progression of DCIS into iIBC. Validation will be done in independent datasets. Ultimately, this will aid individual risk stratification of women with primary DCIS. Clin Cancer Res; 24(15); 3593–601. ©2018 AACR.

Introduction Ductal carcinoma in situ (DCIS) is considered a potential precursor lesion of invasive breast cancer and is treated to prevent the development of invasive disease. After DCIS treatment, three scenarios emerge: i) the DCIS lesion has been removed completely, ii) a remnant or new DCIS lesion will progress into invasive breast cancer, or iii) a remnant or new DCIS lesion will be diagnosed as a subsequent DCIS lesion. In the latter case a risk of developing invasive breast cancer still or again exists. Therefore, our aim was to investigate risk factors for subsequent DCIS lesions. Material and methods All patients diagnosed with DCIS between 1989 and 2004 in the Netherlands were selected by the Netherlands Cancer Registry (NCR). Pathology reports were made available by the nationwide network and registry of histo- and cytopathology in the Netherlands (PALGA) with follow-up to 2016. We selected for patients treated by breast conserving surgery with or without radiotherapy, who subsequently presented with a second DCIS lesion according to the pathology report. Afterwards, we linked the datasets of NCR and PALGA to analyse clinical and pathological risk factors for subsequent DCIS lesions. Results and discussions After a median follow-up time of 13 years, 450 of 5584 (8.1%) women with primary DCIS presented with a subsequent DCIS lesion in the ipsilateral breast. The average time to a subsequent ipsilateral DCIS lesion was 4.1 years. Histological factors such as size, grade, margins and clinical factors such as age, radiotherapy yes or no, time of follow-up will be analysed. Results will be compared with the risk factors for developing invasive breast cancer after a primary diagnosis with DCIS (Elshof et al, BCRT 2016). Conclusion According to our results, the risk of a subsequent ipsilateral in situ lesion after primary DCIS is 8.1% after breast conserving surgery. Analysis of clinical variables available from the Netherlands Cancer Registry, histological variables available from the pathology reports, and comparison with risk factors for developing invasive disease is ongoing. Molecular analysis of subsequent in situ lesions will be performed as a follow-up project.

Abstract Background. Ductal carcinoma in situ (DCIS) is a potential precursor of invasive breast cancer (IBC). As we are currently unable to predict which DCIS lesions will progress to IBC, almost all women with DCIS are treated with surgical treatment, adjuvant therapy, or both. Yet, most DCIS lesions will never progress, implying overtreatment in many women. To increase our knowledge of DCIS and improve DCIS risk stratification, we performed a molecular characterization of a large series of DCIS lesions with long-term follow-up. Patients and methods. We made use of a DCIS case-control series, nested in a nation-wide population-based cohort of Dutch women diagnosed with primary DCIS and treated with breast conserving surgery alone between 1989-2005. Mean follow-up time was 12.0 years. DNA and RNA were simultaneously extracted from FFPE laser-microdissected tissue fragments of 183 primary DCIS lesions (100 associated with subsequent invasive recurrences; and 83 which remained free of invasive recurrence during the same follow-up duration) and 78 matched subsequent ipsilateral IBCs. RNA sequencing (RNAseq) and low coverage DNA sequencing (CNVseq) were performed to identify differentially expressed and aberrated genes between DCIS with and without an invasive recurrence and to compare primary DCIS and its subsequent IBC. Results. After exclusion of samples with poor data quality, 158 DCIS lesions and 72 IBCs (88%) were included for CNVseq analysis and 155 DCIS lesions and 68 IBCs (85%) were included in the RNAseq analysis. CNVseq analysis identified copy number aberrations of known breast cancer-related genes. Amplification of FGFR1, CCND1, ZNF217, and ERBB2 was found in 16, 19, 22, and 44 DCIS samples, respectively. Furthermore, loss of CBFB, CDKN2A, CDKN2B, TP53, and MAP2K4 was found in 10, 11, 11, 11, and 14 DCIS samples, respectively. Deletion of CBFB, TP53, and CDH1 was more often detected in DCIS with subsequent IBC as compared of DCIS without invasive recurrence. When comparing DCIS and subsequent IBC, aberrations of breast cancer-related genes were mostly comparable. RNAseq analysis revealed genes, differentially expressed between DCIS with and without subsequent IBC, which were enriched in immune-related pathways. Conclusion. We performed the first large-scale analysis of primary DCIS lesions with and without subsequent invasive recurrences, with long-term follow-up. Our data show differences in aberrated genes between DCIS with and without subsequent IBC, whereas DCIS and subsequent IBC were more alike. Furthermore, our transcriptomic data suggest that immune-related pathways are involved in DCIS progression. More extensive data analysis will be presented at AACR 2018. The results of our unique study add to the current understanding of DCIS and allow optimized risk stratification of DCIS in the near future. Citation Format: Lindy Visser, Marlous Hoogstraat, Asli Kucukosmanogl, Frank Nieboer, Michiel de Maaker, Petra Kristel, Lennart Mulder, Lotte Elshof, Flora van Leeuwen, Emiel Rutgers, Marjanka K. Schmidt, Esther Lips, Jelle Wesseling. In depth molecular analysis of 183 primary DCIS lesions and 78 subsequent invasive breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4348.

Objective: To assess cause-specific mortality in women treated for ductal carcinoma in situ (DCIS). Background: From screening and treatment perspective, it is relevant to weigh the low breast cancer mortality after DCIS against mortality from other causes and expected mortality in the general population. Methods: We conducted a population-based cohort study comprising 9799 Dutch women treated for primary DCIS between 1989 and 2004 and estimated standardized mortality ratios (SMRs). Results: After a median follow up of 9.8 years, 1429 patients had died of whom 284 caused by breast cancer (2.9% of total cohort). DCIS patients <50 years experienced higher mortality compared with women in the general population (SMR 1.7; 95% confidence interval, CI: 1.4–2.0), whereas patients >50 had significantly lower mortality (SMR 0.9; 95% CI: 0.8–0.9). Overall, the risk of dying from general diseases and cancer other than breast cancer was lower than in the general population, whereas breast cancer mortality was increased. The SMR for breast cancer decreased from 7.5 (95% CI: 5.9–9.3) to 2.8 (95% CI: 2.4–3.2) for women aged <50 and >50 years, respectively. The cumulative breast cancer mortality 10 years after DCIS was 2.3% for women <50 years and 1.4% for women >50 years treated for DCIS between 1999 and 2004. Conclusions: DCIS patients >50 years had lower risk of dying from all causes combined compared with the general female population, which may reflect differences in health behavior. Women with DCIS had higher risk of dying from breast cancer than the general population, but absolute 10-year risks were low.

Population screening with mammography has resulted in increased detection of ductal carcinoma in situ (DCIS). The aim of this population-based cohort study was to assess whether the method of detection should be considered when determining prognosis and treatment in women with DCIS.

Abstract The introduction of population-based breast cancer screening and implementation of digital mammography have led to an increased incidence of ductal carcinoma in situ (DCIS) without a decrease in the incidence of advanced breast cancer. This suggests DCIS overdiagnosis exists. We hypothesize that asymptomatic, low-grade DCIS can safely be managed by active surveillance. If progression to invasive breast cancer would still occur, this will be low-grade and hormone receptor positive with excellent survival rates. Also, breast-conserving treatment will still be an option, if no prior radiotherapy has been applied. It also may save many low-grade DCIS patients from intensive treatment. We will compare active surveillance with conventional treatment, being either wide local excision (WLE) only, WLE plus radiotherapy or mastectomy, possibly followed by hormonal therapy for primary low-grade DCIS, detected by calcifications only, in a phase III, open-label, non-inferiority, multi-center, randomized clinical trial legally supported by the Breast Cancer Group of the European Organisation for the Research and Treatment of Cancer (EORTC-1401-BCG). The Dutch Centers are coordinated by the BOOG Study Center (BOOG 2014-04). Randomization will be 1:1 in to one of the following arms: active surveillance or standard treatment per local policy. In total, 1240 women (≥ 45 years) with asymptomatic, pure, low-grade DCIS, based on vacuum-assisted biopsies of calcifications as detected by population-based or opportunistic screening, without prior breast cancer, will be included. Assuming 25% of randomized women qualified to enroll in the study will drop out or will be excluded from per protocol population, at least 1,240 women need to be randomized to obtain the 930 patients required for the evaluation of the primary endpoint. The same follow-up scheme will be applied in both study arms, i.e. annual mammography for a period of 10 years. The primary end-point is ipsilateral invasive breast tumor-free rate at 10 years. Secondary end-points are among others: overall survival, breast cancer-specific survival, mastectomy rate and patient reported outcomes. Accrual has started in January 2017. Acknowledgements: This trial is funded by Pink Ribbon Netherlands, the Dutch Cancer Society and Dutch Cancer Society/Alpe d’HuZes. Citation Format: Jelle Wesseling, Frederieke van Duijnhoven, Konstantinos Tryfonidis, Aleksandra Peric, Elise van Leeuwen-Stok, Nina Bijker, Lotte Elshof, Emiel Rutgers. Standard therapy versus active surveillance for low grade DCIS: the LORD trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT137. doi:10.1158/1538-7445.AM2017-CT137

Abstract Background. The incidence of DCIS has increased since the introduction of population-based screening. This has not resulted in a decrease in invasive breast cancer incidence, implying overdiagnosis exists. All women with DCIS are still intensively treated, by surgery, radiotherapy, and/or hormonal treatment, although only a minority will develop a subsequent invasive breast cancer. As we cannot discriminate such hazardous from harmless DCIS lesions, accurate prognostic biomarkers are urgently needed. In the current study we aim to identify molecular markers for DCIS aggressiveness, using a large population-based cohort. Patients and methods. We used a population-based, nation-wide cohort consisting of 10,090 women treated for primary DCIS between 1989 and 2004 with a median follow-up time of 10.7 years. Within this cohort, a case-control study was set up to analyse which markers are associated with progression to invasive breast cancer. Formalin-fixed paraffin embedded (FFPE) tissue blocks were retrieved from 1580 DCIS patients who were treated by breast conserving surgery without radiotherapy (316 DCIS patients with a subsequent ipsilateral invasive breast cancer (iiBC): i.e. the "cases"; and 1264 DCIS patients without subsequent invasive breast cancer: i.e. the "controls"). A first study using this population-based cohort will involve immunohistochemistry (IHC) on 200 "cases" and 500 "controls" for an 8-marker IHC panel (ER, PR, HER2, Ki67, p16, p53, COX-2, and Annexin A1). Molecular subtypes of the DCIS and invasive breast cancer lesions will be determined and intra-individual heterogeneity will be assessed. IHC marker expression will be both compared between "cases" and " controls" as well as between DCIS lesions and its subsequent invasive breast cancer. In a second study, DNA and RNA will be isolated from these specimens, using laser microdissection, and extensive molecular profiling will be performed. Results. We have collected FFPE tissue blocks of 287 "cases" and 1149 "controls" (86% of requested material) from 56 participating hospitals. At present, the specimens of 223 "cases" (matched DCIS and iiBC specimen) and 103 "controls" have been centrally revised for extensive morphological characteristics. Only a small part (14%) of the specimens had to be excluded from the study population. IHC staining of the tissue specimens, using the 8-marker IHC panel is ongoing. Conclusion. Within a nation-wide cohort of 10,090 patients diagnosed with primary DCIS, we were able to collect tissue material of a representative case-control series of 200 "cases" with subsequent invasive breast cancer and 500 invasive breast cancer-free "controls". This is the first time such a large unique, unbiased DCIS series, with long-term follow-up is analysed integrating clinical, histological, and immunohistochemical data. The results will be presented at SABCS 2015. Citation Format: Visser L, Elshof L, Groen E, van de Vijver K, Lips E, de Maaker M, Nieboer F, Schaapveld M, Rutgers E, Wesseling J. Biomarkers to distinguish hazardous from harmless ductal carcinoma in situ (DCIS) of the breast. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-17-09.

With the widespread adoption of population-based breast cancer screening, ductal carcinoma in situ (DCIS) has come to represent 20–25% of all breast neoplastic lesions diagnosed. Current treatment aims at preventing invasive breast cancer, but the majority of DCIS lesions will never progress to invasive disease. Still, DCIS is treated by surgical excision, followed by radiotherapy as part of breast conserving treatment, and/or endocrine therapy. This implies over-treatment of the majority of DCIS, as less than 1% of DCIS patients will go on to develop invasive breast cancer annually. If we are able to identify which DCIS is likely to progress or recur as invasive breast cancer and which DCIS would remain indolent, we can treat the first group intensively, while sparing the second group from such unnecessary treatment (surgery, radiotherapy, endocrine therapy) preserving the quality of life of these women. This review summarizes our current knowledge on DCIS and the risks involved regarding progression into invasive breast cancer. It also shows current knowledge gaps, areas where profound research is highly necessary for women with DCIS to prevent their over-treatment in case of a harmless DCIS, but provide optimal treatment for potentially hazardous DCIS.

Since population-based breast cancer screening was implemented in the Netherlands, the incidence of Ductal Carcinoma In Situ of the breast, regarded as a non-obligate precursor lesion of breast cancer, has strongly increased. However, the incidence of invasive breast cancer has not decreased. This suggests that a percentage of all the DCIS lesions would never have become symptomatic if no screening was performed. This phenomenon is known as 'overdiagnosis'. Nonetheless, almost all DCIS lesions are managed surgically, often followed by radiotherapy in those having breast-conserving treatment, to avoid potential progression to breast cancer. To prevent potential over- or undertreatment, further studies are required to distinguish low from high risk DCIS. Ultimately this could help avoid non-beneficial intensive treatment for women with low risk DCIS.

Abstract Background Population-based mammographic screening programs have led to a substantial increase in incidence of ductal carcinoma in situ (DCIS). We assessed whether the method of detection provides prognostic information among women with DCIS detected through the Dutch screening program (screen-detected DCIS) and those with DCIS not detected within the national screening program (non-screen-detected DCIS). This could have impact on the treatment strategy of screen-detected DCIS as compared to symptomatic DCIS. Methods We studied a population-based retrospective cohort comprising 7,106 women aged 49-76 years with primary pure DCIS, who were treated by mastectomy or breast conserving surgery with or without radiotherapy between 1989 and 2004 in the Netherlands. Risk of subsequent ipsilateral and contralateral invasive breast cancer and overall survival among women with screen-detected (n=4,905) and non-screen-detected (n=2,201) DCIS were compared using Cox regression, adjusting for treatment (time-dependent), age (time-scale), diagnosis period and follow-up duration. Because of gradual implementation of the screening program in the Netherlands, we defined two periods based on year of DCIS diagnosis: 1989-1998 (gradual implementation of screening) and 1999-2004 (full coverage of screening). Results With a median follow-up of 10.5 years (interquartile range 7.7-14.0 years) 366 ipsilateral (screen-detected DCIS n=234, non-screen-detected DCIS n=132) and 380 contralateral (screen-detected DCIS n=245, non-screen-detected DCIS n=135) invasive breast cancers were diagnosed, and 1,088 of 7,106 women died (screen-detected DCIS n=603, non-screen-detected DCIS n=485). From 1989 to 2004 the number of non-screen-detected DCIS remained stable (mean 140, range 110-187 per year), whereas the number of screen-detected primary pure DCIS increased from 8 in 1989 to 596 in 2004. Ipsilateral invasive breast cancer risk was lower for screen-detected DCIS compared to DCIS not detected within the national screening program, irrespective of DCIS treatment, period of diagnosis, and follow-up duration (adjusted hazard ratio [HR] 0.74, 95% confidence interval [CI] 0.59-0.92, p < 0.01). The prognostic value of method of detection was similar across categories of treatment, period of diagnosis, and follow-up duration. The risk of contralateral invasive breast cancer did not differ between screen-detected DCIS and non-screen-detected DCIS (adjusted HR 0.89, 95% CI 0.71-1.11, p = 0.3) and neither did all-cause mortality (adjusted HR 0.91, 95% CI 0.79-1.04, p = 0.2). Conclusion Women with primary pure DCIS detected through the Dutch screening program had lower risk of subsequent ipsilateral invasive breast cancer, irrespective of DCIS treatment, compared to women whose DCIS was not detected within the national screening program. However, the magnitude of this risk difference does not warrant a different treatment strategy of screen-detected DCIS as compared to non-screen-detected DCIS. Having a screen-detected DCIS was not associated with risk of subsequent contralateral invasive breast cancer and all-cause mortality. Citation Format: Elshof LE, Schaapveld M, Schmidt MK, van Leeuwen FE, Rutgers EJTh, Wesseling J. Prognostic value of method of detection in primary pure DCIS. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-17-06.