Ductal carcinoma in situ (DCIS) is a heterogeneous entity, with a wide range of histological appearances including
different architectural growth patterns, different nuclear morphology ranging from minimal to severe nuclear
atypia, and the presence or absence of necrosis and calcification. Diagnostic criteria for DCIS depend on the degree of cytologic
atypia, but in general include cytonuclear and architectural features, clonality of the cell population and extent of
the lesion. Numerous classification systems have been proposed for DCIS in order to predict disease recurrence after surgical
resection, and most systems are based primarily on nuclear grade and secondarily on cell polarization and the absence
or presence of necrosis. Low grade DCIS needs to be distinguished from usual hyperplasia and atypical ductal hyperplasia
(ADH). The main criteria for distinguishing usual hyperplasia from neoplasia (ADH and DCIS) are based on the
identification of a clonal cell process, histologically recognized by the uniformity of cytonuclear features and immunophenotype.
Distinction of low grade DCIS from ADH is mainly based on the extent of the lesion. Differentiation of DCIS
from in situ lobular neoplasia may pose difficulties, however, it is important because of its therapeutic implications. Cancerization
of lobules with associated sclerosis and distortion or involvement of complex sclerosing lesions and sclerosing
adenosis by DCIS may mimic invasive carcinoma. DCIS may also involve a papilloma and these cases should be distinguished
from DCIS with a papillary growth pattern (papillary DCIS), encapsulated papillary carcinoma and invasive papillary
carcinoma. The goal of pathologic examination of breast specimens containing DCIS is to establish the diagnosis
and determine relevant tumor features such as size, extent, grade and margin status.