Indolent clonal lymphoid disorders

Indolent clonal lymphoid disorders are not recognized as lymphomas as they generally need no systemic treatment, and depending on the lesion, need only limited clinical follow-up. These lesions are usually incidentally diagnosed during the work up for other disease. The recognition of indolent clonal lymphoid disorders is important to avoid misdiagnosis as lymphoma and unnecessary treatment. Notwithstanding, some indolent disorders, especially B-cell disorders, may give important morbidity that is not related to disease burden but related to auto-immune disease which may need treatment. Further, some of these lesions may, at various rates, ultimately progress to lymphoma. As such, the indolent clonal lymphoid disorders also give an insight into the earliest stages of clonal lymphoid disease that may increase our understanding of lymphoma, although much needs yet to be elucidated. In this article both B- and T-cell indolent clonal lymphoid disorders are reviewed. Not included in this review are lymphoid lesions that may be mistaken for lymphoma, but are not clonal, such as indolent T-lymphoblastic proliferation or marginal zone hyperplasia with immunoglobulin light chain restriction. Further, an emphasis has been given to clonal lymphoid lesions and therefore indolent plasma cell lesions have not been included. Also excluded is indolent lymphoma that may not need treatment but nonetheless requires more regular follow up. One may rightfully argue that there may be a gray zone between what constitutes an indolent clonal lymphoid disorder and an indolent lymphoma. This discussion is reflected in the different terminology used for some entities between editions of the WHO classification and between the Fifth Edition of the WHO Classification and the International Consensus Classification (ICC). The former has been used as a selection basis for this review, but cross-reference has been made to the ICC nomenclature when that differs as well as to the earlier Revised Fourth Edition of the WHO Classification (WHO-r4). For this reason, indolent T-cell lymphoma of the gastrointestinal tract (ICC: indolent clonal T-cell lymphoproliferative disorder of the gastrointestinal tract) is not included in this review.