Systemic mastocytosis (SM) is a primary hematologic disorder marked by the excessive proliferation of mast cells.
Neoplastic nature of mastocytosis:
- Mast cells produced in this disease are neoplastic and may have some or all of the following markers: presence of somatic gain-of-function mutation at codon 816 of CKIT (KIT), usually, but not always, the D816V mutation; expression of CD2 or CD25 on mast cell surface; atypical spindled morphology of mast cells[i].
- Mastocytosis is a neoplastic condition that is not described exclusively by excessive population of mast cells. Mast cell hyperplasia can occur in response to a number of conditions including chronic urticaria[ii], irritable bowel syndrome[iii], and other hematologic neoplasia, including chronic lymphocytic leukemia, non-Hodgkin lymphoma, and myeloproliferative conditions[iv].
- To meet criteria for SM, mast cell infiltration must be dense with at least 15 mast cells per cluster. In many instances, there is not a validated range of mast cells/hpf in healthy controls[iv].
|Table 1: Diagnostic criteria for systemic mastocytosis[v]
1 major and 1 minor criterion; or 3 minor criteria
|Major||Multifocal dense infiltrates of mast cells (15 or more in aggregate) detected in sections of bone marrow and/or extracutaneous organ|
|Minor||In biopsy sections, more than 25% of mast cells in infiltrated space are spindle-shaped or otherwise morphologically abnormal; or, of all mast cells in bone marrow aspirate smears, more than 25% mast cells are immature or abnormal.||Detection of CKIT mutation at codon 816 in bone marrow, blood or extracutaneous organ||Mast cells in bone marrow, blood or other extracutaneous organ that co-expresses CD-117 with CD2 and/or CD25||Baseline serum tryptase of 20 ng/ml or higher.|
Presence of dense infiltrates:
- The hallmark sign of systemic mastocytosis is multifocal dense infiltration of an organ that is not the skin. Despite this fact, it is possible to biopsy negative while still having SM. A 2004 study reported the pathological findings of bilateral bone marrow biopsies for 23 patients. 83% of patients demonstrated positive biopsy for SM bilaterally while 17% of patients had only one positive biopsy[vi].
- One study found that 20% of ISM patients did not have dense infiltration of mast cells in bone marrow[vii].
Tryptase level in systemic mastocytosis:
- Tryptase ≥20 ng/mL is a minor criterion for SM. In order to meet this criterion, tryptase must be ≥20 ng/mL at baseline, not during or following a reactive or anaphylactic event. Per Phadia, producer of ImmunoCAP® Tryptase test, it can take up to fourteen days for tryptase to return to baseline[viii]. However, other sources recommend shorter time to baseline, as low as “24 hours after clinical signs and symptoms have completely subsided”[ix].
- 20-30% of SM patients do not meet the minor criterion of tryptase level ≥20 ng/mL[xiii].
Detection of CKIT D816V mutation:
- The CKIT D816V mutation may not be detected in peripheral blood in a positive patient. Bone marrow aspirate is the preferred sample type for reliable testing for this mutation[xii].
- One study reported as few as 78% of ISM patients were positive for the CKIT D816V mutation in bone marrow[xiii].
Natural history of indolent systemic mastocytosis:
- Indolent systemic mastocytosis (ISM) is SM that does not meet criteria for smoldering systemic mastocytosis, aggressive systemic mastocytosis or mast cell leukemia.
- ISM is largely described by mediator release symptoms and increased risk of anaphylaxis. Mast cell infiltration does not cause appreciable organ dysfunction in this variant[x].
- Progression from ISM to SSM occurred in about 8% of patients in a cohort of 74. In this same cohort, 4% ISM patients progressed to ASM[xi]. The risk of leukemic transformation from ISM was 0.6% in a cohort of 159[xii].
- Organomegaly can present without loss of function at any level of hematologic disease in SM. Organ swelling may be stable over long periods of time without progression to aggressive systemic mastocytosis (ASM)[x].
- Lifespan for indolent systemic mastocytosis is normal[x].
|Table 2: Diagnostic criteria for smoldering systemic mastocytosis
(2 or 3 B findings in addition to meeting criteria for systemic mastocytosis)[i]
|B findings||Increased mast cell burden (>30% mast cell aggregates on bone marrow biopsy and/or serum tryptase >200 ng/mL)||Hypercellular marrow, signs of myelodysplasia or myeloproliferation in absence of MDS or MPN||Organ swelling without deficit of organ function (hepatomegaly without ascites, palpable splenomegaly, lymphadenopathy >2 cm)|
Natural history of smoldering systemic mastocytosis:
- Smoldering systemic mastocytosis (SSM) is defined by increased systemic mast cell burden, presence of markers associated with progression toward ASM (B findings), and potential need for cytoreduction[xiii].
- SSM can remain stable for many years, even decadesix. In a cohort of 22 patients with SSM, 1 transformed to acute leukemia and 3 progressed to ASM[xiv].
- Lifespan may be shortened in SSM. A widely reported study found an average lifespan of 10 years but reported that death was often unrelated to mastocytosis and in some cases was of natural old age[xiii].
|Table 3: Diagnostic criteria for aggressive systemic mastocytosis
(1 or more C finding in addition to meeting criteria for systemic mastocytosis)[i]
|C findings||One or more cytopenias (absolute neutrophil count <1000/µl; Hemoglobin <10g/dl; platelets <100000/µl)||Hepatomegaly with ascites, elevated liver enzymes with or without portal hypertension||Splenomegaly with hypersplenism||Malabsorption evidenced by low albumin and weight loss||Large osteolysis and/or severe osteoporosis and pathologic fractures (2 or more fractures as direct result of mast cell activity)|
Natural history of aggressive systemic mastocytosis:
- Aggressive systemic mastocytosis (ASM) is defined by significant organ damage and failure as a direct result of mast cell infiltrationxv. Lifespan is often significantly shortened and can be as short as three years[ix] .
- ASM generally follows one of two paths: a slow progressing form that resembles SSM but has C findings; or a rapidly progressing form that resembles mast cell leukemia. In rapidly progressing ASM, the patient may lose the CKIT D816V mutation[ix] .
- ASM is managed with cytoreduction but patient response is often short lived. Tyrosine kinase inhibitors and other kinase inhibitors are also used in this population[ix] .
- In treatment resistant cases, hematopoietic stem cell transplant offers an experimental option. One study on HSCT in advanced systemic mastocytosis included seven ASM patients. 3 (43%) achieved complete remission; 3 (43%) demonstrated progression free survival at the three year mark[xv].
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[iii] Guilarte M, et al. Diarrhoea-predominant IBS patients show mast cell activation and hyperplasia in the jejunum. Gut, 56, 203-209.
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[v] Molderings GJ, et al. (2011). Mast cell activation disease: a concise practical guide for diagnostic workup and therapeutic options. Journal of Hematology & Oncology, 4(10), 10.1186/1756-8722-4-10
[vi] Butterfield JH, Li, CY. (2004). Bone marrow biopsies for the diagnosis of systemic mastocytosis: is one biopsy sufficient? Hematopathology, Am J Clin Pathol, 121: 264-267.
[vii] Sanchez-Munoz L, et al. (2011). Evaluation of the WHO criteria for the classification of patients with mastocytosis. Mod Pathol, 24(9), 1157-1168.
[viii] Phadia AB. ImmunoCAP® Tryptase: Clinical utility of Total Tryptase. Retrieved from: http://www.phadia.com/Global/Market%20Companies/Sweden/Best%C3%A4ll%20information/Filer%20(pdf)/ImmunoCAP_Tryptase_Clin_Util.pdf
[ix] Schwartz LB. (2006). Diagnostic value of tryptase in anaphylaxis and mastocytosis. Immunology and Allergy Clinics of North America, 26(3), 451-463.
[x] Valent P, et al. (2010). How I treat patients with advanced systemic mastocytosis. Blood, 116(26), 5812-5817.
[xi] Matito A, et al. (2013). Serum tryptase monitoring in indolent systemic mastocytosis: association with disease features and patient outcome. PLoS One, 8(10), e76116.
[xii] Lim KH, et al. (2009). Systemic mastocytosis in 342 consecutive adults: survival studies and prognostic factors. Blood, 113(23), 5727-5736.
[xiii] Pardanini A. (2013). How I treat patients with indolent and smoldering mastocytosis (rare conditions but difficult to manage). Blood, 121, 3085-3094.
[xiv] Pardanini A. (2010). WHO subvariants of indolent mastocytosis: clinical details and prognostic evaluation in 159 consecutive adults. Blood, 115, 150-151.
[xv] Ustun C, et al. (2014). Hematopoietic stem-cell transplantation for advanced systemic mastocytosis. J Clin Oncol, 32(29), 3264-3274.
[xvi] Pardanini A. (2013). Systemic mastocytosis in adults: 2013 update on diagnosis, risk stratification, and management. American Journal of Hematology, 88(7, 612-624).
[xvii] Valent P, et al. (2003). Aggressive systemic mastocytosis and related mast cell disorders: current treatment options and proposed response criteria. Leuk Res, 27(7), 635-641.