WHO classification of mastocytosis
| Category . | Comment . |
|---|---|
| Cutaneous mastocytosis | Most common category in children. Subtypes include MPCM, mastocytoma, and diffuse CM. |
| Systemic mastocytosis | Presence of neoplastic mast cell infiltrates in extracutanous tissues. Most common in adults. |
| Indolent systemic mastocytosis (ISM) | The most common category of SM, representing >80% of patients with SM. Patients in this category have mast cell collections meeting WHO criteria for systemic disease in the bone marrow but do not have an associated hematologic neoplasm or advanced disease findings or 2 or more B findings of SSM. Patients with ISM have a life expectancy that is comparable to the general age-matched population with less than 5% risk of progression to advanced disease. |
| Bone marrow mastocytosis (BMM) | This category was added in the most recent WHO document and was formerly included in ISM. BMM is diagnosed in a patient without skin lesions, tryptase level <125, no mast cell infiltrates in extramedullary tissue, and otherwise fits the criteria for ISM. This category is believed to have a lower risk of progression than ISM and SSM. |
| Smoldering systemic mastocytosis (SSM) | This is a rare category of nonadvanced SM with higher mast cell burden as evidenced by 2 or more B findings: i. tryptase level ≥200 ng/mL or bone marrow biopsy infiltration by mast cells of ≥30% or KIT D816V variant allele fraction of ≥10% in bone marrow or peripheral blood; ii. splenomegaly without hypersplenism and/or hepatomegaly without liver dysfunction and/or lymphadenopathy >2 cm; iii. myeloproliferation or subtle morphologic abnormalities in myeloid cells without meeting the criteria for a WHO-classified neoplasm. Patients with SSM may have a higher rate of progression to advSM, which still remains <10%. |
| Systemic mastocytosis with associated hematologic non–mast cell neoplasm (SM-AHN) | These patients have an AHN (usually an MPN or MDS) meeting the WHO criteria in addition to SM. Prognosis is determined by the AHN but is generally poor, with a median survival time of about 2 years after diagnosis. |
| Aggressive systemic mastocytosis (ASM) | Patients with ASM have high-level mast cell burden and tissue dysfunction due to infiltrating mast cells (also known as C findings). One or more C findings attributable to MC infiltration are required for diagnosis: cytopenias (hemoglobin <10 g/dL, platelets <100 000, and neutropenia <1000), liver dysfunction with portal hypertension, elevated liver function tests, ascites, malabsorption, and diarrhea with extensive GI infiltrates, splenomegaly with hypersplenism, large (≥2 cm) lytic bone lesions with pathological bone fractures. It should be noted that osteoporosis and smaller lytic and sclerotic bone lesions are common in all categories of SM and are not considered a C finding. Patients with ASM have a reduced life expectancy, with <3 years of average survival after diagnosis. |
| Mast cell leukemia (MCL) | MCL is diagnosed when ≥10% mast cells are found in peripheral circulation or ≥20% in bone marrow aspirate smears in a nonspicular area. It should be noted that 20% infiltration grade refers to bone marrow aspirate and not to bone marrow biopsy. Patients without circulating mast cells are referred to as aleukemic MCL. Patients with typical MCL also have C findings similar to ASM and carry a very poor prognosis. A chronic form of MCL without C-findings or cytopenias have recently been recognized with more favorable survival rates. |
| Mast cell sarcoma | Rare invasive solid mast cell tumor with poor prognosis. |
| Category . | Comment . |
|---|---|
| Cutaneous mastocytosis | Most common category in children. Subtypes include MPCM, mastocytoma, and diffuse CM. |
| Systemic mastocytosis | Presence of neoplastic mast cell infiltrates in extracutanous tissues. Most common in adults. |
| Indolent systemic mastocytosis (ISM) | The most common category of SM, representing >80% of patients with SM. Patients in this category have mast cell collections meeting WHO criteria for systemic disease in the bone marrow but do not have an associated hematologic neoplasm or advanced disease findings or 2 or more B findings of SSM. Patients with ISM have a life expectancy that is comparable to the general age-matched population with less than 5% risk of progression to advanced disease. |
| Bone marrow mastocytosis (BMM) | This category was added in the most recent WHO document and was formerly included in ISM. BMM is diagnosed in a patient without skin lesions, tryptase level <125, no mast cell infiltrates in extramedullary tissue, and otherwise fits the criteria for ISM. This category is believed to have a lower risk of progression than ISM and SSM. |
| Smoldering systemic mastocytosis (SSM) | This is a rare category of nonadvanced SM with higher mast cell burden as evidenced by 2 or more B findings: i. tryptase level ≥200 ng/mL or bone marrow biopsy infiltration by mast cells of ≥30% or KIT D816V variant allele fraction of ≥10% in bone marrow or peripheral blood; ii. splenomegaly without hypersplenism and/or hepatomegaly without liver dysfunction and/or lymphadenopathy >2 cm; iii. myeloproliferation or subtle morphologic abnormalities in myeloid cells without meeting the criteria for a WHO-classified neoplasm. Patients with SSM may have a higher rate of progression to advSM, which still remains <10%. |
| Systemic mastocytosis with associated hematologic non–mast cell neoplasm (SM-AHN) | These patients have an AHN (usually an MPN or MDS) meeting the WHO criteria in addition to SM. Prognosis is determined by the AHN but is generally poor, with a median survival time of about 2 years after diagnosis. |
| Aggressive systemic mastocytosis (ASM) | Patients with ASM have high-level mast cell burden and tissue dysfunction due to infiltrating mast cells (also known as C findings). One or more C findings attributable to MC infiltration are required for diagnosis: cytopenias (hemoglobin <10 g/dL, platelets <100 000, and neutropenia <1000), liver dysfunction with portal hypertension, elevated liver function tests, ascites, malabsorption, and diarrhea with extensive GI infiltrates, splenomegaly with hypersplenism, large (≥2 cm) lytic bone lesions with pathological bone fractures. It should be noted that osteoporosis and smaller lytic and sclerotic bone lesions are common in all categories of SM and are not considered a C finding. Patients with ASM have a reduced life expectancy, with <3 years of average survival after diagnosis. |
| Mast cell leukemia (MCL) | MCL is diagnosed when ≥10% mast cells are found in peripheral circulation or ≥20% in bone marrow aspirate smears in a nonspicular area. It should be noted that 20% infiltration grade refers to bone marrow aspirate and not to bone marrow biopsy. Patients without circulating mast cells are referred to as aleukemic MCL. Patients with typical MCL also have C findings similar to ASM and carry a very poor prognosis. A chronic form of MCL without C-findings or cytopenias have recently been recognized with more favorable survival rates. |
| Mast cell sarcoma | Rare invasive solid mast cell tumor with poor prognosis. |