The World Health Organisation (WHO) classification of haematological malignancies published in 2001 represents the first true international consensus on the classification of haematolymphoid neoplasms. Based on the cell of origin, three major categories of lymphoid neoplasms are recognised:

(a) B-cell lymphoma,
(b) T and NK cell lymphoma
(c) Hodgkin's lymphoma.

Each category is further stratified into distinct disease entities, which are defined by a constellation of clinical, morphological, immunophenotypic, and genetic features. The relative importance of each of these features varies with disease. Although morphology (looks under the microscope) remains the most basic approach, immunophenotypic and genetic studies are helpful in many cases and improve interobserver reproducibility.

Some lymphomas are defined by morphology, whereas others are diagnosed by specific immunophenotype. In some others, the crucial defining feature is a specific genetic abnormality, such as t(2;5) in anaplastic large-cell lymphoma. In yet others, such as mediastinal large B-cell lymphoma, the clinical presentation gives clues to important biological distinctions.

Each lymph-node biopsy specimen should be evaluated individually. The selection and apportionment of tissue for ancillary studies beyond traditional morphology is based on the potential diagnostic accuracy and specificity to be gained from these methods. The adjuvant methods include immunohistochemistry, flow cytometry, cytogenetic analysis, and molecular analysis.

Diagnostic immunohistochemistry relies on a large collection of monoclonal antibodies (Mabs) that bind to surface molecules involved in adhesion or signalling of lymphoid cells, histiocytes, and their subsets. The abbreviation CD (cluster of differentiation) refers to a group of Mabs made in laboratories around the world that recognise the same or different epitopes on a particular cell-surface molecule. Immune markers are useful in delineating the cell lineage of lymphoid cells, evaluation of lymph-node architecture, and classification of lymphomas.

Immunohistochemistry is important and recommended for lymphomas and lymphoid proliferations that are indeterminate between reactive or malignant processes. In most instances, a limited panel of markers may suffice. Additional antibodies may be used as guided by the differential diagnoses or after the results with the initial panel are known.

Flow cytometry of cell suspensions is another technique that detects cell-surface molecules and has the advantage of measuring multiple antigens on each cell simultaneously. However, the results can sometimes be misleading because morphological correlation is not possible.

Specific chromosomal translocations have been identified in some lymphomas and they can be detected by standard cytogenetic techniques and fluorescence in situ hybridisation (FISH). Molecular studies are usually applied in routine practice only when morphological and immunohistochemical studies yield inconclusive results.

The usefulness of molecular tests include assignment of cell lineage, determination of clonality, detection of chromosomal translocation, and identification of oncogene involvement, such as Epstein-Barr virus in Burkitt's lymphoma. Although cytogenetic and molecular studies play important diagnostic and prognostic roles with identification of unique genetic profiles, which have direct bearing on disease behaviour and pathogenesis, practical considerations preclude a purely genetic approach.

Processing of fresh lymph-node biopsy specimen

 

Dr Ng Siok Bian
Associate Consultant
Dept of Pathology, SGH