copyrighted material, used by arrangement with john wiley & sons limited. for personal use only, must not be reproduced or shared with third parties. anyone wishing to reproduce this content in whole or in part, in print or in electronic format, should contact digitalrightsuk@wiley.com browse the abc series at www.wiley.com ### Overview Two main types of non-invasive (in situ) cancer can be recognised from the histological pattern of disease and cell type (Table 1). Ductal carcinoma in situ is the most common form of non-invasive carcinoma, making up 3–4% of symptomatic and 20–25% of screen-detected cancers. It has increased in frequency because of the widespread use of screening mammography (Figure 1). The increase is across all age groups, with a 12% annual increase in the 30–39-year age group and an 18.1% annual increase in women over the age of 50. Ductal carcinoma in situ is characterised by distortion, distention and complete involvement by a similar and neoplastic population of cells of adjacent ducts and lobular units (Figure 2). By contrast, lobular carcinoma in situ, now known as lobular intraepithelial neoplasia (LIN), which incorporates what was previously known as lobular carcinoma in situ (LCIS) and atypical lobular hyperplasia (ALH), is rare (<1% of screen-detected cancers) and presents as relatively uniform expansion of the whole lobule by regular cells …