General Approach to Diagnosing Lymphoid Proliferations
Diagnosing lymphoid proliferations can be challenging and might require the combined power of multiple methods. We view the following methods as complementary and advocate a step-wise diagnostic approach.
Morphology - The first step is morphologic assessment of specimens, by cytology or histopathology.
Cytology: For initial assessment of enlarged lymph nodes or effusions, cytology is the test of choice because it is minimally invasive, cost-effective, and can identify lymphoid neoplasia in most cases in dogs, and about half of cats and horses. Tissue architecture cannot be assessed by cytology.
Biopsy: Histopathology is the appropriate test when cytologic findings are equivocal, when concurrent assessment of tissue architecture is necessary (gastrointestinal and skin lesions, spleen), and to classify lymphoid neoplasia.
Immunophenotyping is indicated when cytology and/or histopathology are equivocal or to classify lymphoid neoplasia.
Flow cytometry (FC): Cells in fresh tumor samples are analyzed for light scatter and expression of a range of markers. Cells are not assessed microscopically in FC; therefore the test should only be used in conjunction with cytology.
Immunohistochemistry (IHC): In formalin-fixed paraffin-embedded tissues, IHC is used to identify the 2 major lymphocyte subsets, B and T lymphocytes. This aids in the assessment of complex lesions and it is required for morphologic classification of lymphoid neoplasms and for the interpretation of clonality testing results.
Clonality testing can help distinguish reactive from neoplastic lymphoid proliferations if other methods are equivocal. Clonality results must be interpreted in conjunction with clinical, morphologic, and immunophenotypic data, and should hence be done as the last step in the diagnostic algorithm. Clonality testing is not suitable for immunophenotyping. Clonality testing from cytology samples is feasible if a biopsy cannot be obtained readily (e.g., CSF, effusions without primary mass, etc.). Ideally, additional material should be obtained for immunophenotyping (flow cytometry, immunochemistry). The latter information is required for the interpretation of clonality results. Samples without immunophenotyping will not be accepted.