The tryptase positive compact round cell infiltrate of the bone marrow (TROCI-BM): a novel histopathological finding requiring the application of lineage specific markers

Aims: Compact tryptase-positive round cell infiltrates of the bone marrow (TROCI-BM) are very rare histopathological findings and may pose challenging problems with regard to the cell type involved (either mast cells or basophilic granulocytes) and the exact diagnosis. Methods: A selected panel of immunohistochemical markers against mast cell and basophil related antigens, including CD25, CD34, CD117/Kit, and the 2D7 antigen (which is found only in basophilic granulocytes) on a total of 410 routinely processed bone marrow biopsy specimens (including 88 cases of systemic mastocytosis (SM), 20 cases of chronic myeloid leukaemia (CML), 92 cases of myeloid neoplasms other than CML, and 210 controls with normal/reactive bone marrows). Results: In total, 17 cases with TROCI-BM could be identified: 11 SM (including two cases of well-differentiated SM and two mast cell leukaemias; MCL), 2 myelomastocytic leukaemia (MML), 2 CML with excess of basophils (secondary basophilic leukaemia (CMLba)), and 2 tryptase positive acute myeloid leukaemia (AML). Regarding the cell types involved, TROCI-BM cells were found to express CD117/Kit in all cases of SM and MCL. In MML and tryptase postitive AML, TROCI-BM cells were found to coexpress CD34 and Kit. The basophil specific antigen 2D7 was only detected in CD34/Kit negative TROCI-BM cells in two patients with CMLba. The activating point mutation D816V was detected in 8/11 patients with SM but not in any of the other haematological malignancies. Conclusions: In summary, a total of six rare myeloid neoplasms may present with a novel immunohistochemical phenomenon tentatively termed TROCI-BM. ------------------------ Abbreviations: AML, acute myeloid leukaemia; CML, chronic myeloid leukaemia; CMLba, chronic myeloid leukaemia with excess of basophils; ISM, indolent systemic mastocytosis; MCL, mast cell leukaemias; MML, myelomastocytic leukaemia; SM, systemic mastocytosis; TROCI-BM, tryptase-positive compact round cell infiltrate of the bone marro