Original Article Mass and non-mass breast MRI patterns: a radiologic approach to sick lobe theory Selda Guven 1 , Irmak Durur-Subasi 1,2,3 , Elif Demirci 4 , Ata Turker Arikok 5 , Adem Karaman 2 , Unsal Han 5 and Baki Hekimoglu 1 Abstract Background: According to sick lobe theory, one or more lobes of the breast are more prone to the development of carcinoma. However, the implications of this theory in breast magnetic resonance imaging (MRI) are unknown. Purpose: To evaluate the MRI appearance of mass type (multifocal and multicentric diseases) and non-mass type (non- mass enhancements) sick lobe patterns, together with the histopathology results. Material and Methods: MRI reports of 2015 patients in two tertiary breast imaging centers between June 2012 and June 2018 were retrospectively reviewed for multifocal–multicentric diseases and segmental, linear, and regional enhancements. A total of 113 patients were included. The specimens obtained by thick needle, vacuum, excisional biopsy/lumpectomy or mastectomy after breast MRI scans were pathologically assessed. The pathologic results were categorized as invasive carcinoma, precursor, and benign proliferative lesions according to the 2012 World Health Organization Classification of Tumors. Results: The percentage of underlying benign and precursor invasive lesions was significantly different in patients with mass and non-mass MRI patterns. While the pathology results of mass type patterns were premalignant and malignant in all cases, nearly half of the underlying histologies were benign proliferative subtypes in patients with non-mass type patterns. Conclusion: In this study, the mass and non-mass patterns derived from sick lobe theory were related to different risks of malignancy in the pathological examinations. Keywords Breast, magnetic resonance imaging, dynamic contrast-enhanced imaging, pathology, cancer Date received: 18 March 2020; accepted: 12 June 2020 Introduction The breast has a lobar organization consisting of ductal segmentation systems. One particular lactiferous canal opens at the nipple, branches into several segmental and subsegmental canals, and ends in numerous termi- nal canals and lobules. Terminal canal and lobules form terminal ductal-lobular units. In women, the size and number of lobes show great variations and change during lifetime as well (1,2). For an individual breast, one or more lobes are different by being more prone to the development of carcinoma than the others (sick lobe theory). The cancerous renovation may occur in a sole locus within a sick lobe, or it can involve multiple loci simultaneously. The sick lobe would have in situ or invasive tumor foci (2–5). 1 University of Health Sciences, Diskapi Yildirim Beyazit Training and Research Hospital, Clinic of Radiology, Ankara, Turkey 2 Ataturk University, Faculty of Medicine, Department of Radiology, Erzurum, Turkey 3 Istanbul Medipol University, Faculty of Medicine, Department of Radiology, Istanbul, Turkey 4 Ataturk University, Faculty of Medicine, Department of Pathology, Erzurum, Turkey 5 University of Health Sciences, Diskapi Yildirim Beyazit Training and Research Hospital, Clinic of Pathology, Ankara, Turkey Corresponding author: Selda Guven, University of Health Sciences, Diskapi Yildirim Beyazit Training and Research Hospital, Clinic of Radiology, Ankara, Turkey. Email: seldaaatezel@hotmail.com Acta Radiologica 0(0) 1–7 ! The Foundation Acta Radiologica 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0284185120941825 journals.sagepub.com/home/acr