In situ neoplasia in lymph node pathology.

In situ neoplasia represents the earliest form of malignant progression and is characterized by localization limited to the compartment corresponding to the cell of origin. Like other cancers, lymphoid neoplasms are considered to develop by multistep pathogenetic mechanisms. However, because of the circulating nature of lymphocytes, in situ lymphoid neoplasia may be difficult to identify histopathologically, and the compartment to which it is restricted may be physiological rather than strictly anatomical. The 2016 WHO classification of lymphoid neoplasms recognizes two in situ entities: in situ follicular neoplasia (ISFN) and in situ mantle cell neoplasia (ISMCN). This review summarizes the clinical features, histopathology, genetics, and differential diagnoses of these two entities, including distinction from both their overly malignant counterparts and a variety of reactive processes.