Systemic mastocytosis is a myeloproliferative neoplasm (MPN) with prominent involvement of mast cells [1-4]. Some cases have an associated myelodysplastic syndrome or a specific myeloproliferative or myelodysplastic/myeloproliferative neoplasm involving other lineages; among these associated conditions chronic myelomonocytic leukaemia (CMML) is most often observed. The disease may terminate in acute myeloid leukaemia (AML), most often not acute mast cell leukaemia. There is an unexplained association of systemic mastocytosis with AML with a t(8;21)(q22;q22).
Presentation is in adults, usually from the third decade onwards, with a slight male predominance. Patients may present with clinical symptoms related to release of mast cell products  or with features such as hepatomegaly and splenomegaly characteristic of MPD . Organomegaly is not usually marked. Mast cell release of histamine and other inflammatory mediators can cause flushing, tachycardia and collapse, sometimes triggered by insect bites or exposure to certain drugs. There may be gastrointestinal symptoms (diarrhoea or peptic ulceration) as a result of infiltration or chronic histamine release. Some patients have bone pain. Some have urticaria pigmentosa, a skin lesion resulting from cutaneous mast cell infiltration; this is characterized by pigmentation due to melanin deposition and urticaria on stroking. Striking systemic symptoms due to release of inflammatory mediators can occur in patients whose mastocytosis is indolent.
Haematological and pathological features
The blood count and film may be normal or there may be anaemia, leucocytosis, monocytosis or eosinophilia. With more advanced disease there may be leucopenia, neutropenia and thrombocytopenia. Sometimes there are small numbers of circulating mast cells (Figure 9.1). Eosinophils are sometimes degranulated (Figure 9.2). The bone marrow is infiltrated to a variable extent in the great majority of patients, with infiltration often being preferentially paratrabecular or periarteriolar (Figures 9.3, 9.4, 9.5). Infiltrates are multifocal and cohesive in comparison with the scattered mast cells present in reactive conditions. The mast cells are often morphologically abnormal, being spindle-shaped with elongated nuclei, hypogranularity or both. They can be readily detected on a Giemsa stain (Figure 9.6) and their nature can be confirmed by immunohistochemistry (see below). Granulocytic hyperplasia, neutrophilic and eosinophilic, is common. On trephine biopsy sections, eosinophils may be apparent around the periphery of the mast cell infiltrates. Lymphocytes are sometimes also increased and may similarly surround the mast cell infiltrates (Figure 9.7). There may be increased macrophages and fibroblasts. Bones may show osteoporosis, osteosclerosis or both (Figure 9.8). The bone marrow aspirate is considerable less useful in diagnosis than the trephine biopsy but may show dysplastic mast cells, often in low numbers and within or adjacent to fragments (Figure 9.9). These are often oval or spindle-shaped, sometimes with cytoplasmic tails (Figure 9.10) with fewer granules than are seen in normal mast cells. They are positive for napthol AS-D chloroacetate ...