Mastocytosis is a collective term used to describe a heterogenous group of disorders comprising clonal, pathological mast cells. The World Health Organisation 2008 criteria serve an excellent framework for diagnosis, with indolent, intermediate and aggressive forms of the disease, similar to that seen in lymphoma. Physicians need good clinical acumen given these conditions are under diagnosed, and the same is just as true in the laboratory. Synthesis of a complex array of histological, immunohistochemical, immunophenotypic and molecular data is now required. Understanding normal expression patterns on mast cells is also pivotal. With the bone marrow the central tissue involved, these diseases are important for the practising haematopathologist.
The key driver mutation on chromosome 4 is the D816V point mutation in the c-kit gene (the receptor for stem cell factor) seen in over 95% of cases. However, the mutational landscape is growing in complexity in aggressive mastocytosis and, when mastocytosis coexists with an associated haematological clonal non-mast cell disorder (AHNMD) as demonstrated by new molecular techniques such as massive parallel sequencing. These are exciting times, as the likelihood of defining appropriate targeted therapies for our patients moves closer.