ABC of breast diseases: Breast cancer

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Abstract
Breast cancers are derived from the epithelial cells that line the terminal duct lobular unit. Cancer cells that remain within the basement membrane of the elements of the terminal duct lobular unit and the draining duct are classified as in situ or non-invasive. An invasive breast cancer is one in which there is dissemination of cancer cells outside the basement membrane of the ducts and lobules into the surrounding adjacent normal tissue. Both in situ and invasive cancers have characteristic patterns by which they can be classified. Carcinoma in situ affecting a breast lobule The most commonly used classification of invasive breast cancers divides them into ductal and lobular types. This classification was based on the belief that ductal carcinomas arose from ducts and lobular carcinomas from lobules. We now know that invasive ductal and lobular breast cancers both arise from the terminal duct lobular unit, and this terminology is no longer appropriate. Some tumours show distinct patterns of growth and cellular morphology, and on this basis certain types of breast cancer can be identified. Those with specific features are called invasive carcinomas of special type, while the remainder are considered to be of no special type. This classification has clinical relevance in that certain special type tumours have a much better prognosis than tumours that are of no special type. #### Classification of invasive breast cancers ##### Special types • Tubular • Cribriform • Medullary • Mucoid • Papillary • Classic lobular ##### No special type • Commonly known as NST or NOS (not otherwise specified) • Useful prognostic information can be gained by grading such cancers Invasive carcinomas showing diffuse infiltration through breast tissue: grade I (left), grade II (centre), and grade III (right) ### Tumour differentiation Among the cancers of no special type, prognostic information can be gained by grading the degree of differentiation of the tumour. Degrees of glandular formation, nuclear pleomorphism, …

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