Indolent lymphomas of the gastrointestinal tract: update on classification, morphology and mimics

Indolent lymphomas of the gastrointestinal (GI) tract represent a challenging diagnostic area. Around one third of extranodal non-Hodgkin lymphomas (NHL) involve the GI tract. This may be primary, including several relatively recently described entities such as duodenal-type follicular lymphoma and intestinal T-cell lymphoma of the GI tract, or secondary involvement by systemic lymphoma. As a significant proportion of lymphomas may initially present in the GI tract, associated with non-specific GI symptoms without strong clinical suspicion for haematological malignancy, it is crucial that general and gastrointestinal pathologists are familiar with these entities. The differential diagnosis for an indolent-appearing lymphoid infiltrate in the GI tract ranges from reactive or benign conditions such as nodular follicular hyperplasia or coeliac disease, to highly aggressive lymphomas such as monomorphic epitheliotropic T-cell lymphoma. The aim of this review is to discuss indolent B-, T- and NK-cell lymphomas of the GI tract including the classification of the more recently described entities, with an emphasis on morphological and immunohistochemical features that may help the pathologist differentiate these from benign or aggressive malignant mimics, having significant implications for management and prognosis.