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- In situ lymphoid neoplasia (ISLN, also termed in situ lymphoma) is a precancerous condition newly classified by the World Health Organization in 2016. The Organization recognized two subtypes of ISLN: in situ follicular neoplasia (ISFN) and in situ mantle cell neoplasia (ISMCL). ISFN and ISMCL are pathological accumulations of lymphocytes in the germinal centers and mantle zones, respectively, of the follicles that populate lymphoid organs such as lymph nodes. These lymphocytes are monoclonal (i.e. descendants of a single ancestral cell) B-cells that may develop into follicular (FL) and mantle cell (MCL) lymphomas, respectively. When used to characterize a neoplasm, in situ has referred to a localized, non-destructive accumulation in a tissue of cells that bear resemblances to the malignant cells of one of the cancers that can develop in this tissue. The in situ accumulations can progress to become the malignancy that their cells resemble. The term, while readily applicable to abnormal cell accumulations in solid tissues such as those of the cervix, has been difficult to apply to lymphatic tissue because many of these tissue's cells normally move through blood and lymphatic vessels to occupy other tissues. Recently, however, monoclonal B-cells with some key characteristics of the malignant B-cells in FL or MCL have been found to accumulate in one or more lymphoid tissues. These accumulations are localized, non-destructive (i.e. not effacing a tissue's normal architecture), premalignant, and therefore now regarded as in situ disorders similar to those in solid tissues. ISFN and ISMCL are usually indolent, asymptomatic disorders that rarely progress to malignancy. Typically, they are diagnosed based on the findings in lymphoid tissues examined for other reasons. ISLN bear similarities to monoclonal B-cell lymphocytosis (MBL). MLB consists of four subtypes: chronic lymphocytic leukemia/small lymphocyte MBL (i.e. CLL/SLL-MBL), atypical CLL/SLL-MBL, non-CLL/SLL-MBL, and monoclonal B-cell lymphocytosis of the marginal zone (CBL-MZ). These MBL subtypes are indolent, asymptomatic, monoclonal B-cell disorders diagnosed, generally incidentally, by finding the circulation of relatively large numbers of monoclonal B-cells that correspond in type to the malignancies to which they may progress. ISLN differs from MCL in that its B-cells are found mainly in lymphoid tissue, it involves different monoclonal B-cell types, and it usually progresses to a set of different types of lymphoid malignancies. However, 1) MBL disorders can progress to FL or MCL, 2) small numbers of the B-cells involved in ISFN may circulate in individuals who have or will develop ISFN, and 3) the B-cells in MBL may accumulate in lymphoid tissues. (en)
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