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9 - Grade/differentiation unknown, not stated, or not applicable
Aggressive systemic mastocytosis
Lymphadenopathic (systemic) mastocytosis with eosinophilia
Systemic mastocytosis with associated clonal hematological non-mast cell lineage disease
Systemic tissue mast cell disease
Accumulation of increased numbers of mast cells in the tissues other than skin, such as the liver, spleen, bone marrow, stomach, and small intestines, with or without skin lesions. There are > 15% mast cells in aggregates in the bone marrow and/or other extracutaneous organs. The aggregates of neoplastic spindle-shaped mast cells are compact (not scattered throughout tissue).
Bone marrow is almost always involved. Lymph nodes are rarely involved except for a variant of aggressive of systemic mastocytosis. The white and red pulp of the spleen may be involved. Involvement of liver may present with small granulomatoid foci of mast cells, but severe liver involvement is rare. It may frequently be suspected that the GI tract mucosa is involved based on symptoms, but it is rarely assessed morphologically. Blood abnormalities include anemia, leukocytosis, eosinophilia, neutropenia and thrombocytopenia.
There are several variants of SM reportable as 9741/3 described below. [Indolent systemic mastocytosis (ISM) is NOT reportable; it includes subtypes bone marrow mastocytosis and smoldering systemic mastocytosis and would be coded as 9741/1 in the WHO classification; the mast cell burden is very low (<1% of nucleated bone marrow cells) and life expectancy is normal.]
Aggressive systemic mastocytosis (ASM): Organomegaly and impaired organ function are usually present. There may be weight loss caused by malabsorption in the GI tract due to mast cell infiltrates, or skeletal involvement with large osteolytic lesions and/or pathologic fractures. There are usually no skin lesions. A type of ASM, lymphadenopathic mastocytosis with eosinophilia, features progressive lymphadenopathy with peripheral blood eosinophilia, often with extensive bone involvement and hepatosplenomegaly. Genetically cases classified as ASM have no PDGFRA rearrangement. Survival is a few years.
In systemic mastocytosis with associated clonal hematological non-mast-cell lineage disease (SM-AHNMD), a myeloid or lymphatic malignancy is diagnosed with the SM; CMML (9945/3) is the most common associated disease. The prognosis is usually dominated by the non-mast cell malignancy.
Definitive Diagnostic Methods
Bone marrow biopsy
For ASM: no FDGFRA rearrangement
Somatic point mutation at codon 816 in kit receptor gene
Immunohistochemical markers anti-tryptase/chymase and anti-CD117