# Indolent and smouldering systemic mastocytosis

Source: https://onco.cc/cancers/indolent-systemic-mastocytosis/  
OnCo record `indolent-systemic-mastocytosis` (Cancer). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Indolent systemic mastocytosis is the common, slow form of this rare blood disorder, in which KIT-mutant mast cells build up in the marrow and skin and release histamine that causes flushing, itching, stomach pain, bone thinning and sometimes severe allergic reactions. Life expectancy is near normal, antihistamines and adrenaline control symptoms, and low-dose avapritinib was approved in 2023.

## Summary

Systemic mastocytosis is a clonal myeloid neoplasm of mast cells driven in more than nine in ten patients by the KIT D816V mutation. The indolent form, which accounts for most cases, is defined by the WHO and ICC criteria of multifocal mast cell aggregates in the marrow with abnormal CD25 or CD2 or CD30 expression, raised serum tryptase and the KIT mutation, without the organ damage ('C findings') that defines advanced disease; smouldering disease has a higher mast cell burden ('B findings') but still no organ damage, and bone marrow mastocytosis lacks skin lesions. Patients suffer from mediator release: flushing, urticaria pigmentosa, pruritus, abdominal cramps, diarrhoea, brain fog, fatigue and, in a substantial minority, anaphylaxis, classically after insect stings; osteoporosis and fractures are common. Hereditary alpha-tryptasaemia, a common germline duplication of the tryptase gene, raises baseline tryptase and worsens symptoms in some patients and must be accounted for when interpreting tryptase levels. Progression to advanced disease is uncommon, and mutations in SRSF2, ASXL1 or RUNX1 identify the minority at risk.

Management for decades was symptomatic: H1 and H2 antihistamines, cromoglicate, leukotriene antagonists, proton-pump inhibitors, omalizumab for recurrent anaphylaxis, adrenaline autoinjectors for every patient, venom immunotherapy after sting anaphylaxis, and bisphosphonates for osteoporosis, with cladribine or interferon alfa for the few with intolerable symptoms. The KIT D816V-selective inhibitor avapritinib changed that: the PIONEER trial (NEJM Evidence 2023) randomised 212 patients with moderate to severe symptoms to avapritinib 25 mg daily or placebo on top of best supportive care and showed a greater fall in total symptom score, in serum tryptase, in KIT D816V allele burden and in marrow mast cells, and avapritinib was approved for indolent systemic mastocytosis in the United States in May 2023 and in Europe later that year; it does not carry the intracranial bleeding risk seen at higher doses in advanced disease but is avoided when platelets are low. Newer KIT D816V inhibitors, elenestinib (HARBOR) and bezuclastinib (Summit), are in randomised trials aiming for greater selectivity, and the disease is otherwise followed with tryptase, KIT allele burden and bone density.

## Fields

- Kind: Cancer
- Last checked: 2026-09-18
- Also known as: ISM; Indolent SM; Smouldering systemic mastocytosis; Bone marrow mastocytosis; Non-advanced systemic mastocytosis
- Tags: subtype-page; haematologic
- Group: haematologic
- Burden: The large majority of systemic mastocytosis; life expectancy is close to normal, but symptoms from mast cell mediators, anaphylaxis risk and osteoporosis affect quality of life for years.
- Subtypes: Indolent systemic mastocytosis with skin involvement (urticaria pigmentosa); Indolent systemic mastocytosis without skin involvement; Bone marrow mastocytosis (marrow only, low tryptase, anaphylaxis-prone); Smouldering systemic mastocytosis (high burden, B findings, no organ damage); Indolent systemic mastocytosis with hereditary alpha-tryptasaemia; Indolent systemic mastocytosis with recurrent anaphylaxis (venom immunotherapy, omalizumab)
- Biomarkers: Serum tryptase (adjusted for hereditary alpha-tryptasaemia copy number); KIT D816V by high-sensitivity PCR in blood (allele burden tracks response); Marrow mast cell aggregates with CD25, CD2 and CD30 expression; B findings (marrow burden over 30 percent, tryptase over 200 ng/mL, organomegaly) defining smouldering disease; SRSF2, ASXL1 and RUNX1 mutations (progression risk); Bone density scan (osteoporosis)

## Standard of care

- Diagnosis: Serum tryptase, KIT D816V testing in peripheral blood, bone marrow biopsy with flow cytometry, tryptase gene copy number, bone density scan and screening for B and C findings. ([Histopathology & immunohistochemistry](https://onco.cc/technologies/histopathology-ihc/), [Multiparameter flow cytometry MRD](https://onco.cc/technologies/flow-cytometry-mrd/), [KIT](https://onco.cc/targets/kit/), [Driver mutation](https://onco.cc/terms/driver-mutation/))
- Symptom control: H1 and H2 antihistamines, cromoglicate, leukotriene antagonists, proton-pump inhibitors; omalizumab for recurrent anaphylaxis; adrenaline autoinjectors for all; venom immunotherapy after sting anaphylaxis; bisphosphonates for osteoporosis. ([Electronic patient-reported outcome (ePRO) symptom monitoring](https://onco.cc/technologies/epro-symptom-monitoring/))
- Moderate to severe symptoms despite supportive care: Avapritinib 25 mg daily (PIONEER, approved 2023), avoided when platelets are below 50 x 10^9/L; cladribine or interferon alfa as older cytoreductive options. ([Avapritinib](https://onco.cc/drugs/avapritinib/), [(PIONEER) Study to Evaluate Efficacy and Safety of Avapritinib (BLU-285), A Selective KIT Mutation-targeted Tyrosine Kinase Inhibitor, Versus Placebo ](https://onco.cc/trials/nct03731260/), [Cladribine](https://onco.cc/drugs/cladribine/), [Interferon alfa-2a/2b](https://onco.cc/drugs/interferon-alfa/), [Small-molecule kinase inhibitors](https://onco.cc/technologies/kinase-inhibitors/))
- Trials: Elenestinib (HARBOR) and bezuclastinib (Summit) as more selective KIT D816V inhibitors. ([Elenestinib](https://onco.cc/drugs/elenestinib/), [(HARBOR) Study to Evaluate Efficacy and Safety of BLU-263 Versus Placebo in Patients With Indolent Systemic Mastocytosis](https://onco.cc/trials/nct04910685/), [Bezuclastinib](https://onco.cc/drugs/bezuclastinib/), [(Summit) A Study to Evaluate the Efficacy and Safety of CGT9486 Versus Placebo in Patients With Indolent or Smoldering Systemic Mastocytosis](https://onco.cc/trials/nct05186753/))
- Monitoring: Tryptase, KIT allele burden and blood count yearly; bone density; marrow re-examination only if progression is suspected. ([Liquid biopsy (ctDNA)](https://onco.cc/technologies/liquid-biopsy/), [Molecular response (MMR, MR4, treatment-free remission)](https://onco.cc/terms/molecular-response/))

## State of the art

- Avapritinib is the first disease-modifying drug approved for indolent systemic mastocytosis.
- Peripheral blood KIT D816V testing has replaced marrow for diagnosis and monitoring in many centres.
- Hereditary alpha-tryptasaemia testing explains discordant tryptase levels.

## Open problems

- Whether avapritinib alters the long-term course or only symptoms is unknown.
- Anaphylaxis remains life-threatening and unpredictable.
- Symptoms correlate poorly with mast cell burden.
- Many patients wait years for a diagnosis.

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Mastocytosis
- PIONEER (NEJM Evidence 2023): https://doi.org/10.1056/EVIDoa2200339
- NCCN Systemic Mastocytosis: https://www.nccn.org/guidelines/guidelines-detail?category=1&id=1478
- Wikipedia: https://en.wikipedia.org/wiki/Mastocytosis

## Connected records

- cancers: [Advanced systemic mastocytosis (aggressive SM, SM with an associated haematological neoplasm, mast cell leukaemia)](https://onco.cc/cancers/advanced-systemic-mastocytosis/), [Essential thrombocythaemia (ET)](https://onco.cc/cancers/essential-thrombocythaemia/), [Systemic mastocytosis](https://onco.cc/cancers/systemic-mastocytosis/)
- technologies: [Electronic patient-reported outcome (ePRO) symptom monitoring](https://onco.cc/technologies/epro-symptom-monitoring/), [Histopathology & immunohistochemistry](https://onco.cc/technologies/histopathology-ihc/), [Liquid biopsy (ctDNA)](https://onco.cc/technologies/liquid-biopsy/), [Multiparameter flow cytometry MRD](https://onco.cc/technologies/flow-cytometry-mrd/), [Small-molecule kinase inhibitors](https://onco.cc/technologies/kinase-inhibitors/)
- targets: [KIT](https://onco.cc/targets/kit/)
- drugs: [Avapritinib](https://onco.cc/drugs/avapritinib/), [Bezuclastinib](https://onco.cc/drugs/bezuclastinib/), [Cladribine](https://onco.cc/drugs/cladribine/), [Elenestinib](https://onco.cc/drugs/elenestinib/), [Interferon alfa-2a/2b](https://onco.cc/drugs/interferon-alfa/)
- terms: [Driver mutation](https://onco.cc/terms/driver-mutation/), [Molecular response (MMR, MR4, treatment-free remission)](https://onco.cc/terms/molecular-response/)
- trials: [(HARBOR) Study to Evaluate Efficacy and Safety of BLU-263 Versus Placebo in Patients With Indolent Systemic Mastocytosis](https://onco.cc/trials/nct04910685/), [(PIONEER) Study to Evaluate Efficacy and Safety of Avapritinib (BLU-285), A Selective KIT Mutation-targeted Tyrosine Kinase Inhibitor, Versus Placebo ](https://onco.cc/trials/nct03731260/), [(Summit) A Study to Evaluate the Efficacy and Safety of CGT9486 Versus Placebo in Patients With Indolent or Smoldering Systemic Mastocytosis](https://onco.cc/trials/nct05186753/)
- key papers: [PIONEER: avapritinib versus placebo in indolent systemic mastocytosis](https://onco.cc/key-papers/paper-pioneer-avapritinib-indolent-systemic-mastocytosis-nejm-evid-2023/), [WHO classification of haematolymphoid tumours, fifth edition: myeloid and histiocytic neoplasms](https://onco.cc/key-papers/paper-who-2022-myeloid-khoury-leukemia-2022/)

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