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WHO Classification of Tumours
Indolent systemic mastocytosis
Tumours of haematopoietic and lymphoid tissues


Indolent systemic mastocytosis (ISM) meets criteria for systemic mastocytosis (SM). There are no "C" findings (see below), and no evidence of systemic mastocytosis with associated clonal haematological non-mast-cell lineage disease (AHNMD) . In this variant, the mast cell burden is low and skin lesions are almost invariably present (Table 2.12).

> Subtypes of indolent systemic mastocytosis
Bone marrow mastocytosis
As above (ISM) with bone marrow involvement, but no skin lesions.
Smouldering systemic mastocytosis
As above (ISM), but with 2 or more "B" findings but no "C" findings.

"B" findings
1. Bone marrow biopsy showing >30% infiltration by mast cells (focal, dense aggregates) and/or serum total tryptase level >20 ng/mL.
2. Signs of dysplasia or myeloproliferation, in non-mast cell lineage(s), but insufficient criteria for definitive diagnosis of a haematopoietic neoplasm (AHNMD), with normal or only slightly abnormal blood counts.
3. Hepatomegaly without impairment of liver function, and/or palpable splenomegaly without hypersplenism, and/or lymphadenopathy on palpation or imaging.

"C" findings
1. Bone marrow dysfunction manifested by one or more cytopenia (ANC <1.0x109/L, Hb <10 g/dL, or
platelets <100x109/L), but no obvious non-mast cell haematopoietic malignancy.
2. Palpable hepatomegaly with impairment of liver function, ascites and/or portal hypertension.
3. Skeletal involvement with large osteolytic lesions and/or pathological fractures.
4. Palpable splenomegaly with hypersplenism.
5. Malabsorption with weight loss due to GI mast cell infiltrates.

> Related topics
Overview: Mastocytosis