The term 'mastocytosis' denotes a heterogeneous group of disorders characte
rized by abnormal growth and accumulation of mast cells (MC) in one or more
organ systems. Over the last 20 years, there has been an evolution in acce
pted classification systems for this disease. In light of such developments
and novel useful markers, it seems appropriate now to re-evaluate and upda
te the classification of mastocytosis. Here, we propose criteria to delinea
te categories of mastocytosis together with an updated consensus classifica
tion system. In this proposal, the diagnosis cutaneous mastocytosis (CM) is
based on typical clinical and histological skin lesions and absence of def
initive signs (criteria) of systemic involvement. Most patients with CM are
children and present with maculopapular cutaneous mastocytosis (= urticari
a pigmentosa, UP). Other less frequent forms of CM are diffuse cutaneous ma
stocytosis (DCM) and mastocytoma of skin. Systemic mastocytosis (SM) is com
monly seen in adults and defined by multifocal histological lesions in the
bone marrow (affected almost invariably) or other extracutaneous organs (ma
jor criteria) together with cytological and biochemical signs (minor criter
ia) of systemic disease (SM-criteria). SM is further divided into the follo
wing categories: indolent systemic mastocytosis (ISM), SM with an associate
d clonal hematologic non-mast cell lineage disease (AKNMD), aggressive syst
emic mastocytosis (ASM), and mast cell leukemia (MCL). Patients with ISM us
ually have maculopapular skin lesions and a good prognosis. In the group wi
th associated hematologic disease, the AHNMD should be classified according
to FAB/WHO criteria. ASM is characterized by impaired organ-function due t
o infiltration of the bone marrow, liver, spleen, GI-tract, or skeletal sys
tem, by pathologic MC. MCL is a 'high-grade' leukemic disease defined by in
creased numbers of MC in bone marrow smears (greater than or equal to 20%)
and peripheral blood, absence of skin lesions, multiorgan failure, and a sh
ort survival. In typical cases, circulating MC amount to greater than or eq
ual to 10% of leukocytes (classical form of MCL). Mast cell sarcoma is a un
ifocal tumor that consists of atypical MC and shows a destructive growth wi
thout (primary) systemic involvement. This high-grade malignant MC disease
has to be distinguished from a localized benign mastocytoma in either extra
cutaneous organs (= extracutaneous mastocytoma) or skin. Depending on the c
linical course of mastocytosis and development of an AHNMD, patients can sh
ift from one category of MC disease into another. In all categories, mediat
or-related symptoms may occur and may represent a serious clinical problem.
All categories of mastocytosis should be distinctively separated from reac
tive MC hyperplasia, MC activation syndromes, and a more or less pronounced
increase in MC in myelogenous malignancies other than mastocytosis. Criter
ia proposed in this article should be helpful in this regard. (C) 2001 Else
vier Science Ltd. All rights reserved.