Usefulness of FICTION Method for the Evaluation of Response after Treatment in Multiple Myeloma

Introduction: According to the new uniform response criteria of International Myeloma Working Group (IMWG), stringent complete response (sCR) is defined as a condition of normal free light chain (FLC) ratio and absence of clonal cells in the bone marrow (BM) by immunohistochemistry or immunofluorescence, in addition to the CR condition. The kappa/lambda ratio is assessed to identify clonal cells in the BM and a minimum of 100 plasma cells is required for analysis of clonal cells. However, the evaluation of kappa/lambda ratio by immunohistochemistry or immunofluorescence may be inaccurate, because it is difficult practically to countthe number of anti-kappa/lambda antibodies-stained cells in the BM section and in case of low percentage of residual plasma cells, flow cytometric evaluation is also difficult. To investigate whether FICTION (Fluorescence Immunophenotyping and interphase Cytogenetics as a Tool for the Investigation Of Neoplasms) technique can be used as a tools for evaluation of clonal cells after treatment in multiple myeloma, we performed FICTION on BM cells in follow-up patients with myeloma and compared the results of FICTION with other parameters.

Method: 18 myeloma patients, whose BM examination and serum free light chain were checked at the same time after treatment, were enrolled in Seoul National University Hospital. We performed FICTION for the fluorescence in situ hybridization (FISH) items that were abnormal at initial BM examination of each patient. The selected probes were LSI 1q25/1p36/1p subtelomere probe (Vysis, Downers Grobe, IL, USA) for trisomy 1q25, LSI 13 (RB1) 13q14 probe (Vysis) for RB1 deletion, LSI IGH probe (Vysis) for IGH rearrangement and LSI p16 (9p21)/CEP 9 probe (Vysis) for p16 deletion. The FICTION results were reported by the percentage of plasma cells with abnormal FISH signals among plasma cells stained with anti-kappa and lambda antibodies labeled with fluorescein isothiocyanate (FITC). We compared FICTION results with response parameters, such as % plasma cells in BM aspirates, serum FLC ratio, M-component in serum and/or urine protein electrophoresis (PEP) and FISH results.

Results: Among 18 patients in follow-up, 5 (28%) showed <5% plasma cells by light microscope based differential count in BM aspirates and normal FISH results below the cut-off level. However, these patients turned out to have clonal cells in BM by FICTION techniques. Among these 5 patients, 3 patients showed abnormal serum FLC ratio and the other 2 patients showed M-component in serum PEP with normal serum FLC ratio. One patient with plasma cells fewer than 5% in BM aspirates, no cytogenetic abnormality in FISH and no M-component in serum, showed abnormal serum FLC ratio and abnormal FICTION results; 2 (25%) of 8 plasma cells showed RB1 deletion in FICTION. After 2 years, this patient progressed to plasma cell leukemia.

Conclusion: Results of FICTTON correlated with FLC ratio and/or M-component in serum, while the percentage of plasma cells in BM aspirates or FISH results did not. The assessment of percentage of plasma cells in bone marrow aspirates might be inaccurate due to poor aspiration technique including dilution and focal infiltration of plasma cells. Also, FISH results based on the percentage of abnormal cells among all bone marrow nucleated cells do not reflect clonal plasma cells. In clonclusion, we suggest that FICTION technique is more sensitive method for identification of residual malignant plasma cells with clonalityalong with the evaluation of response after treatment in multiple myeloma.