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An Unusual Clonal Disorder of Bone Marrow–Derived Hematopoietic Progenitor Cells

Hans-Peter Horny MD
DOI: http://dx.doi.org/10.1309/AJCPPXHMN5CJOXHZ 438-447 First published online: 1 September 2009


Mastocytosis, an unusual disorder of bone marrow–derived, clonally transformed hematopoietic progenitor cells, exhibits a broad spectrum of clinical and morphologic features ranging from a self-limiting benign disorder (ie, juvenile cutaneous mastocytosis) to highly aggressive neoplasms like mast cell leukemia. Principally, mastocytosis should be divided in 2 main subentities: cutaneous mastocytosis and systemic mastocytosis mainly involving the bone marrow. Mastocytosis is a morphologic diagnosis and should not be diagnosed on the basis of clinical findings alone. Pathologists need to be aware of the disease and its mimickers. Application of the defined diagnostic criteria can confirm or exclude mastocytosis in most cases. Use of antibodies against tryptase, CD117 (KIT), and CD25 is recommended in every suspected case. Because most cases of systemic mastocytosis show a very low degree of infiltration of the bone marrow, antitryptase and anti-CD117 are of major importance for screening and quantification of mast cells, in particular to detect even small compact infiltrates as the only major diagnostic criterion for mastocytosis. Expression of CD25 on mast cells is defined as a minor diagnostic criterion and is usually seen only in mastocytosis but not in reactive states of mast cell hyperplasia.

Key Words:
  • Mastocytosis
  • Mast cell
  • Bone marrow
  • KITD816V
  • Systemic mastocytosis with associated hematologic non–mast cell clonal disease
  • Systemic mastocytosis
  • Mast cell leukemia

Introduction to Mastocytosis

Mastocytosis is a morphologic diagnosis. By definition, the disease cannot be diagnosed on the basis of clinical findings alone. Therefore, particularly in view of its rarity and often unusual appearances and the mostly low or minimal degree of tissue infiltration, pathologists should be familiar with the diagnostic criteria defined for mastocytosis not only to be able to assess or exclude true mastocytosis but also to recognize its mimickers.1,2 Mimickers of mastocytosis are reactive states of mast cell hyperplasia on the one hand and rare neoplastic hematologic disorders such as tryptase-positive acute myeloid leukemia (AML) or myelomastocytic leukemia on the other hand.


Mastocytosis is a clonal disease derived from hematopoietic bone marrow progenitor cells that manifests with an unusually broad spectrum of clinical and morphologic appearances.

Diagnostic Criteria

The major diagnostic criterion for mastocytosis is focal compact tissue infiltrate predominantly composed of mast cells.

The minor diagnostic criteria are as follows: (1) prominent spindling of mast cells (>25%), (2) atypical immunophenotype of mast cells with expression of CD25, (3) activating point mutation of c-kit in codon 816 (usually KITD816V), and (4) chronically elevated serum tryptase level (>20 ng/mL).

It is important to note that the only major and 2 of 4 minor diagnostic criteria (1 and 2) are morphologic criteria. Diagnosis of mastocytosis can be established if the major and at least 1 minor criterion or at least 3 minor criteria (when the major criterion is missed) are present.1,2 From a practical viewpoint, most cases of mastocytosis can be easily diagnosed when focal compact infiltrates with a significant proportion of spindle-shaped mast cells are present. Cases of mastocytosis with exclusive occurrence of round mast cells can be diagnosed after demonstration of CD25 expression by the mast cells.3 If compact mast cell infiltrates are totally absent, the presence of diffusely scattered, spindle-shaped mast cells with CD25 expression alone does not allow a diagnosis of mastocytosis to be established. However, demonstration of KITD816V and/or a chronically elevated serum tryptase level enables diagnosis of mastocytosis in these comparatively rare cases because 3 or 4 minor criteria are fulfilled.46

Although bone marrow is the main tissue for diagnosis of systemic mastocytosis (SM), the demonstration of compact mast cell infiltrates in extramedullary tissues like lymph node, spleen, mucosa, etc, should be regarded as indicative of involvement by the disease.7,8 Rarely, the diagnosis of mastocytosis is first established in the mucosa of the gastrointestinal tract or in the spleen, and involvement of the bone marrow is recognized in a second step when an adequate tissue sample is analyzed. It is unlikely that pure intestinal or splenic mastocytosis exists. In all such cases, the meticulous study of the bone marrow reveals systemic disease, although the degree of tissue infiltration may vary widely. To be able to separate the different forms of SM, pathologists must be aware of clinical symptoms, especially the so-called B findings, referring to an organomegaly (hepatosplenomegaly and/or lymphadenopathy), and even C findings, indicating organ dysfunction due to widespread mast cell infiltration (eg, cytopenia and/or ascites in aggressive SM with strong infiltration of bone marrow and liver). The approach to the diagnosis of mastocytosis is complex and, regarding the relative rarity of the disease, usually more difficult than in other hematologic malignancies. The World Health Organization2 classification of mastocytosis is given in Table 1.

Frequency of Mastocytosis

It is not possible to estimate the true frequency of mastocytosis for several reasons: (1) Indolent SM is the most frequent diagnosis. When the bone marrow of patients with long-standing adult-type cutaneous mastocytosis (mostly urticaria pigmentosa) is examined, it is found to be positive in more than 80% of cases.9 (2) SM with associated hematologic non–mast cell clonal disease (SM-AHNMD) is the most frequent diagnosis in patients without cutaneous disease but in whom there is a clinical suspicion of a hematologic neoplasm.1012 When all cases of myeloid neoplasm, irrespective of the subtype (myelodysplastic syndrome [MDS], myelodysplastic/myeloproliferative neoplasm [MD/MPN], MPN, AML), are studied with antibodies against tryptase, CD25, and CD117, in up to 10%, the presence of SM (SM-AHNMD) is revealed. Almost 100% of patients with SM-AHNMD have the KITD816V mutation; however, the presence of this mutation is not limited to the SM compartment but can be detected in a varying but significant proportion of AHNMDs when microdissectional analyses are performed.13 Myeloid neoplasms may even obscure SM, which is detected only after chemotherapy and remission of the basic disease (“occult mastocytosis”).14 (3) Juvenile patients almost exclusively have cutaneous mastocytosis without clinical signs of systemic involvement and a good chance of spontaneous regression at puberty.15 Adults with cutaneous mastocytosis should be monitored for systemic spread (indolent SM) of the disease when the serum tryptase level is chronically elevated and/or the cutaneous mast cells exhibit an atypical immunophenotype with expression of CD25. Detection of mutated Kit (D816V) in the skin should prompt the suspicion of SM, and examination of a bone marrow biopsy specimen is strongly recommended in every such case.

View this table:
Table 1

To give a rough idea of the frequencies of the various subtypes of mastocytosis in the adult, the following scheme may be of help: cutaneous mastocytosis > indolent SM >> SM-AHNMD >> aggressive SM > mast cell leukemia (MCL) >> mast cell sarcoma >> extracutaneous mastocytoma (which is extremely rare).

Typical Morphologic Appearances of Various Defined Subtypes in the Bone Marrow and Provisional Entities of SM

Indolent SM

Indolent SM has mild multifocal involvement of the bone marrow (generally up to 5% of the section area). Hematopoiesis is intact. In cases with a diffuse increase of spindle-shaped CD25-expressing mast cells, another minor criterion (KITD816V or elevated serum tryptase level) is necessary to definitively establish the diagnosis.

Systemic Mastocytosis With AHNMD

Here, a multifocal involvement of the bone marrow (between 1% and 20% of the section area) also is usually present, but in up to 10% of cases, SM is revealed only by using immunohistochemical analysis to detect even small or minute compact infiltrates obscured by the dominating hematologic neoplasms. Both compartments of the disease should be subcategorized. AHNMD in about 30% to 40% of the cases manifests as chronic myelomonocytic leukemia or other disorders of the MD/MPN family. If possible, SM should also be further subtyped but can best be termed an isolated bone marrow mastocytosis (as a subtype of indolent SM) in most cases. SM-AHNMD in itself is very heterogeneous. On the one hand, SM-chronic myelomonocytic leukemia shows KITD816V in almost all cases in the SM and the AHNMD compartment of the disease. On the other hand, in SM-myeloproliferative neoplasm with eosinophilia (MPNEo), KITD816V is missed not only in the MPNEo but, very surprisingly, also in the SM, although compact infiltrates of CD25+ mast cells are present. Regarding other types of SM-AHNMD (eg, SM-AML, SM-MPN, and SM–plasma cell myeloma), the incidence of KITD816V in AHNMD varies greatly, assuming 20% to 60% of diagnoses.

Aggressive SM

In aggressive SM,16 the bone marrow is markedly infiltrated by often confluent, dense sheets of mast cells (≥30% of the section). In cases with diffuse, compact bone marrow infiltration, MCL can only be ruled by the study of bone marrow and blood smears (aggressive SM, bone marrow smear shows far less than 20% mast cells and blood is without circulating mast cells; aleukemic MCL, bone marrow smear shows more than 20% mast cells and blood is without mast cells; MCL, bone marrow smear with more than 20% mast cells and blood contains circulating mast cells). To establish a diagnosis of aggressive SM, signs of organ dysfunction (C findings) should be present. Aggressive SM always is accompanied by cytopenias. Accordingly, the hematopoiesis is often markedly reduced or even exhibits signs of dysplasia. An SM-AHNMD (SM-MDS) in such cases is difficult to assess or exclude.

Mast Cell Leukemia

In MCL,17 the bone marrow usually is diffusely infiltrated by sheets of mast cells (≥70% of section area). Diagnosis of MCL, however, can only be established when more than 20% atypical mast cells are found in bone marrow smears. Further subtyping (aleukemic MCL) depends on the presence of circulating mast cells (usually >10% of leukocytes).

Mast Cell Sarcoma

Although mast cell sarcoma1820 is very rare and almost always primarily involves extramedullary tissues, the terminal phase of the disease is that of an MCL.

Smoldering SM

Smoldering SM21,22 assumes an intermediate position between indolent and aggressive SM, showing more pronounced bone marrow involvement (≥20%) and signs of organomegaly (B findings), whereas C findings are missing. Smoldering SM usually exhibits expansion of KITD816V to other non–mast cell lines, usually neutrophils, and may also show dysplastic changes of blood cell precursors, making the differential diagnosis with overt myelodysplasia (SM-MDS) very difficult in some cases.

Isolated Bone Marrow Mastocytosis

Isolated bone marrow mastocytosis morphologically manifests like indolent SM, but the clinical findings of cutaneous involvement are missing. Isolated bone marrow mastocytosis usually is found in patients with hymenoptera allergy and in SM-AHNMD.

Well-Differentiated SM

Well-differentiated SM23 manifests as an exclusively round-cell type of the disease with compact multifocal infiltrates lacking CD25. Because the demonstration of a point mutation outside codon 816 of c-kit (ie, KITF522P) does not meet the defined minor criterion, it is necessary to consider the serum tryptase level of the patient. If the serum tryptase level is persistently elevated, a diagnosis of SM should be established.

“Occult” Mastocytosis

Occult mastocytosis is used as a preliminary and descriptive term for rare cases of mastocytosis that were initially obscured by a malignant hematologic disorder in the setting of SM-AHNMD. After chemotherapy and disappearance of the associated neoplasm, typical compact mast cell infiltrates were disclosed.14 As a rule, appropriate examination of the primary trephine biopsy specimen enables diagnosis of mastocytosis retrospectively, usually based on 3 minor criteria (spindling, CD25 expression, and KITD816V). The term occult mastocytosis can also be used in rare cases of mastocytosis in which previously examined tissue is available for reevaluation. For example, KITD816V+ SM with multifocal bone marrow infiltration is diagnosed and lymph nodes removed at the staging of prostatic cancer many years ago were still available. Despite a lack of morphologic evidence of infiltrates of mastocytosis, it is possible in a few cases to demonstrate the presence of KITD816V in such lymph nodes.24

Differential Diagnostic Aspects

Mastocytosis has to be separated from reactive states, in particular mast cell hyperplasia on the one hand and neoplastic diseases on the other. The finding of a monoclonal mast cell activation syndrome with disseminated scattered mast cells carrying KITD816V but not fulfilling the criteria for mastocytosis, in particular, compact infiltrates are missing, is a preliminary description of a still ill-defined status (analogous to monoclonal gammopathy of undetermined significance). The following neoplastic disorders need to be included in the differential diagnosis of mastocytosis:

Myelomastocytic leukemia25,26 represents an extremely rare type of myelogenous leukemia with prominent signs of mast cell differentiation but does not fulfill the criteria for mastocytosis; in particular, compact mast cell infiltrates and KITD816V are missing. The histologic picture is dominated by blast cells expressing myeloid and mast cell–associated antigens (such as tryptase and/or chymase). A few cases exhibiting marked dysplastic features (similar to the MDS type, refractory anemia with excess of blasts-2) have been observed.

Tryptase-positive AML27 is also a rare, relatively ill-defined subtype of AML, usually within the French-American-British M0 or M1 subgroup with strong expression of tryptase but lacking other features of mastocytosis or myelomastocytic leukemia. Because the serum tryptase level is usually markedly elevated, it is possible to monitor the disease serologically. Like myelomastocytic leukemia, tryptase-positive AML is recognized only when tryptase immunohistochemical analysis is routinely applied.

Chronic basophilic leukemia is a very rare myeloproliferative neoplasm usually manifesting as secondary “basophilic crisis” in preexisting chronic myeloid leukemia. De novo (primary) chronic basophilic leukemia is exceedingly rare, but it exists. Because neoplastic basophils express tryptase in immunohistochemically detectable amounts, it might be challenging to separate basophils from mast cells. Cytomorphologically, basophils are small to medium-sized round cells, whereas mast cells are usually larger, and often spindle-shaped. To confirm the mast cell nature of a tryptase-positive round cell, immunostaining with anti-KIT (CD117) is strongly recommended in all questionable cases because basophils are always CD117–. Specific antibodies against basophilic antigens such as CD123, 2D7, and BB1 (the latter 2 are not commercially available) should be used in such cases.

MPNEo28 was formerly termed chronic eosinophilic leukemia/hypereosinophilic syndrome and is associated with the FIP1L1-PDGFR-α fusion gene. Morphologically, more than 50% of all cases show a significant increase in mast cells, which are often spindle-shaped and exhibit an atypical immunophenotype with expression of CD25. However, criteria for the diagnosis of SM, in particular SM-AHNMD, are usually not fulfilled because compact mast cell infiltrates are almost always missed and the KITD816V mutation is not detected.

It is important to be aware that the different forms of mastocytosis include various differential diagnoses. The differential diagnoses of cutaneous mastocytosis include mast cell hyperplasia and indolent SM. The differential diagnoses of indolent SM include well-differentiated SM, isolated bone marrow mastocytosis, smoldering SM, mast cell hyperplasia, monoclonal mast cell activation syndrome (which is an ill-defined state not fulfilling the criteria for SM), lymphoplasmacytic lymphoma (especially in cases with pronounced bone marrow lymphocytosis), and the so-called fibromastocytic lesion (which is also ill-defined, showing localized bone marrow fibrosis with increased numbers of spindle-shaped mast cells, but CD25 and KITD816V are missing).

The differential diagnoses of SM-AHNMD include occult mastocytosis, tryptase-positive AML, myelomastocytic leukemia, and myeloproliferative neoplasm with atypical mast cells. The differential diagnoses of aggressive SM include smoldering SM, SM-AHNMD, aleukemic MCL, myelomastocytic leukemia, and tryptase-positive AML. The differential diagnoses of MCL include myelomastocytic leukemia, tryptase-positive AML, basophilic leukemia, monocytic leukemia, SM-AHNMD, hairy cell leukemia, and aggressive SM (in cases of aleukemic MCL).

Tryptase-Positive Round Cell Infiltrate of the Bone Marrow

Tryptase-positive round cell infiltrate of the bone marrow29 (TROCI-bm) is a recently described, immunohistochemically defined finding exhibiting focal or diffuse but always compact tissue infiltrates consisting exclusively of tryptase-positive round cells. TROCI-bm is seen only in rare hematologic neoplasms. Diffuse TROCI-bm is encountered in myelomastocytic leukemia, MCL, or chronic basophilic leukemia, and focal TROCI-bm is seen in the common type of SM, well-differentiated mastocytosis, and chronic basophilic leukemia, especially in accelerated phase chronic myeloid leukemia.

Routine Workup of Cases of Suspected SM

The routine workup in suspected cases of SM includes the following: (1) bone marrow trephine biopsy specimen (>2 cm), (2) bone marrow smears, and (3) blood smears. The biopsy specimen should be fixed in 5% buffered formalin and acid-decalcified in EDTA overnight. Antibodies against CD25, CD117, and tryptase should be applied.3033 In cases of suspected SM-AHNMD, further immunohistochemical staining with appropriate antibodies according to the subtype of the AHNMD should also be applied. The tissue can also be used for demonstration of the KITD816V mutation. Blood and bone marrow smears are crucial to be able to separate aggressive SM from aleukemic MCL and aleukemic from leukemic MCL, respectively.

Review of Workshop Cases

A total of 46 cases with a diagnosis of mastocytosis were submitted. Five (~10%) of these cases were moved to another disease category. In 31 of 46 cases, there was agreement between the diagnosis made by the submitter and by the panel. The subcategory of mastocytosis was changed in 8 cases. Two cases were difficult to interpret owing to insufficient material and/or clinical information. Regarding the final diagnosis, SM-AHNMD was by far the most frequent, followed by MCL. This reflects the hematopathologic practice of the presenters. Cases of indolent SM and cutaneous mastocytosis were relatively rare, not reflecting the true incidence of these disease subcategories. The spectrum of AHNMD showed a broad range, including AML, MDS/MPN, myeloproliferative neoplasm, and multiple myeloma.

Five cases were selected to demonstrate the broad spectrum of mastocytosis and possible difficulties in achieving a correct diagnosis. They are discussed in the following sections.

Case 55

The case involved a 67-year-old woman with colonic adenocarcinoma and a persistently elevated serum tryptase above 20 ng/mL. CBC results were unremarkable.

A right hemicolectomy specimen contained a 5.5-cm mucinous adenocarcinoma at the cecum infiltrating the lamina muscularis propria (pT2). Lamina propria mucosae was densely infiltrated by tryptase-positive and CD117+ round mast cells exhibiting an atypical immunophenotype with coexpression of CD25 Image 1. Mast cell infiltrates were found adjacent to and distant from the carcinoma. The bone marrow biopsy specimen was widely normocellular with trilineage hematopoiesis and multifocal compact mast cell infiltrates. Mast cells were also mostly round and hypogranulated. The activating point mutation KITD816V of c-kit was detected in the colonic mucosa.

The diagnosis was SM with mild infiltration of bone marrow and marked involvement of the mucosa of the large bowel (and concurrent adenocarcinoma).

This is a good example demonstrating that pure “intestinal” mastocytosis does not exist. If adequate histologic analysis of bone marrow is performed in cases with prominent mucosal infiltration, all turn out to belong to the SM subcategory. Several patterns of mucosal involvement in patients with SM and proven bone marrow involvement have been detected34: (1) disseminated atypical mast cells expressing CD25 and carrying the KITD816V without forming compact infiltrates; (2) multifocal compact mast infiltrates; (3) band-like, superficial, dense mast cell infiltrates resembling in some respect collagenous colitis; (4) diffuse, dense mast cell infiltrates mimicking chronic inflammatory bowel disease (as found in this case); and (5) mast cell sarcoma (1 reported case).

Case 83

A 66-year-old man was admitted to the emergency department with major left flank pain. He had no visible skin lesions. Computed tomography revealed marked splenomegaly. A CBC showed leukocytosis (WBC count, ~20,000/μL [20 × 109/L]) with mild eosinophilia (10% [0.10]), normochromic anemia (RBC count, 3 × 106/μL [3 × 1012/L]), and marked thrombocytopenia (platelet count, 42 × 103/μL [42 × 109/L]). The patient was scheduled for splenectomy.

Image 1

(Case 55) A, Dense mast cell infiltrate in colonic lamina propria (H&E). B, Immunohistochemical stain for CD117 highlights mast cell infiltrate. Contributed by D.A. Hall and coworkers.

The unfixed splenectomy specimen weighed 1,736 g and measured 26 × 17.5 × 10 cm. Cut sections revealed a mottled pink-red parenchyma. Histologic studies showed disseminated infiltrates consisting of spindle-shaped, hypogranulated mast cells coexpressing CD25, CD68, and CD117 Image 2. The microarchitecture of the spleen was widely preserved. A malignant lymphoma was ruled out. The activating point mutation KITD816V was detected subsequently.

The diagnosis was splenic mastocytosis.

Splenic mastocytosis is rare.35,36 Similar to cases exhibiting “intestinal” mastocytosis, meticulous analysis of bone marrow biopsy specimens should reveal some compact infiltrates, thus enabling establishment of a diagnosis of SM with prominent infiltration of the spleen. Moreover, it is possible to exclude or assess an associated non–mast cell hematologic neoplasm that might be obscured in the spleen by mast cell infiltrates.

Case 43

The case involved a 42 year-old woman with dyspnea and a skin rash.

There were significant abnormalities in the CBC, including mild leukocytosis (WBC count, 13,000/μL [13.3 × 109/L]) and moderate normochromic-normocytic anemia (hemoglobin level, 9.3 g/dL [93 g/L]); the platelet count was almost normal (144 × 103/μL [144 × 109/L]). Of the circulating leukocytes, 22% were identified as atypical mast cells. Bone marrow histologic examination revealed diffuse and focal infiltration by atypical mast cells coexpressing tryptase and CD117, whereas myeloperoxidase was negative Image 3. The serum tryptase level was markedly elevated (>200 ng/μL). No KIT mutation at exon 17 was detected.

The diagnosis was MCL.

During a period of about 2 months, multiple physicians had seen the patient before a correct diagnosis was established. It is crucial to examine blood and bone marrow smears to be able to diagnose MCL because the histologic picture in aggressive SM is indistinguishable from MCL in many cases. Only in cases in which more than 20% of all nucleated bone marrow cells are identified as mast cells should a diagnosis of MCL be established.37 MCL is the only subcategory of SM in which smears are necessary for diagnosis. A considerable number (up to 50%) of the aggressive and leukemic subtypes of SM do not carry the activating point mutations of c-kit that are seen in the overwhelming majority of patients with indolent SM or SM-AHNMD.

Image 2

(Case 83) A, Numerous spindle mast cells admixed with eosinophils and lymphocytes forming discrete aggregates in splenic parenchyma (H&E). B, CD117 immunostain highlights sizeable aggregates of atypical mast cells. C, Coexpression of CD25 in mast cell aggregates highlighting an atypical immunophenotype. Contributed by A.M. Medina and coworkers.

Case 167

The case involved a 47-year-old man with episodic diarrhea and hepatosplenomegaly. The CBC revealed mild leukocytosis (WBC count, 13,900/μL [13.9 × 109/L]) with marked eosinophilia (eosinophil count, 1,800/μL [1.8 × 109/L]) and monocytosis (monocyte count, 1,200/μL [1.2 × 109/L]). There was also a mild normochromic anemia (hemoglobin level, 13.9 g/dL [139 g/L]) and thrombocytosis (platelet count, 605 × 103/μL [605 × 109/L]). The serum tryptase level was strongly elevated (334 ng/μL). There were no skin lesions and no signs of organ dysfunction, and, in particular, no hypersplenism.

Blood and bone marrow smears yielded no significant dysplastic changes of hematopoietic cells and no increase in blast cells. Circulating mast cells were absent. Bone marrow histologic examination revealed multifocal, dense, compact infiltrates of partially spindle-shaped mast cells, some adjacent lymphoid nodules, and an interstitial increase in eosinophils Image 4. Although histologically mast cell infiltration amounted to 20% to 30% of the section area, the number of atypical mast cells in the smear preparation was low and did not exceed 4% of all nucleated bone marrow cells. Molecular analysis revealed the activating point mutation KITD816V in bone marrow and blood, whereas the FIP1L1-PDGRF-α fusion gene was not detected.

Image 3

(Case 43) A, Circulating hypogranulated mast cells in the peripheral blood smear (Wright-Giemsa). B, Aggregates of atypical, hypogranulated mast cells in the bone marrow aspirate (Wright-Giemsa). C, CD117+ mast cell aggregates. Contributed by C.R. Kjeldsberg and coworkers.

Image 4

(Case 167) A, Bone marrow biopsy specimen showing dense aggregates of partially spindle-shaped mast cells surrounded by a strongly hypercellular marrow with marked increase in eosinophils (H&E). B, Tryptase immunostain highlighting mast cell aggregates. Contributed by I. Maric and coworkers.

The diagnosis was SM-AHNMD. SM was subtyped as smoldering SM with eosinophilia, and the AHNMD was subtyped as chronic myelomonocytic leukemia.

The degree of bone marrow infiltration in this case amounts to up to 30% of the section area and, therefore, exceeds that of typical indolent SM or isolated bone marrow mastocytosis by far. Regarding the presence of KITD816V in blood leukocytes, this finding indicates that KITD816V is not restricted to mast cells (as in indolent SM) but can be found also in non–mast cell hematopoietic cells, allowing the diagnosis of smoldering SM manifesting with B findings (organomegaly) but lacking C findings. Because the FIP1L1-PDGFR-α fusion gene is missing, a myeloproliferative neoplasm with eosinophilia (MPNEo; formerly chronic eosinophilic leukemia) can be ruled out (compare with case 199).

Case 199

The case involved a 51-year-old man with “dermatitis,” mucosal ulcerations, and splenomegaly. The CBC revealed slight leukocytosis (WBC count, 9,100/μL [9.1 × 109/L]) with marked eosinophilia (eosinophil proportion, 40% [0.40]). The serum tryptase level was elevated (27.8 ng/μL).

Blood smears showed marked eosinophilia with a preponderance of atypical hypogranulated variants. Bone marrow histologic examination revealed slight hypercellularity with a moderate diffuse increase in eosinophils and mild reticulin fibrosis. Immunohistochemical staining with antitryptase enabled detection of a mild increase in diffusely distributed spindle-shaped mast cells without a tendency to aggregate or form compact clusters but with coexpression of CD25 by flow cytometry Image 5. Bone marrow smears demonstrated an eosinophil count of 30% of all nucleated cells but no increase in blast cells or mast cells. Molecular studies revealed the FIP1L1-PDGFR-α fusion gene to be present but the absence of KITD816V.

The diagnosis was MPNEo with increase in atypical (CD25+) mast cells.

This is a very typical case of MPNEo exhibiting an increase in atypical mast cells.28,38 However, the criteria for diagnosis of SM (or better, SM-AHNMD) were not fulfilled. First, an elevated serum tryptase level does not count as a minor diagnostic criterion in cases with myeloid (non–mast cell) neoplasms, and, second, spindle-shaped morphologic features can only be used as a minor criterion in compact mast cell infiltrates (when the original description is strictly applied).

Image 5

(Case 199) A, Bone marrow biopsy specimen showing significant eosinophilia with mature eosinophils and their precursors. Scattered spindle-shaped mast cells are hardly detectable (H&E). B, Bone marrow biopsy specimen with numerous scattered spindle-shaped mast cells without forming compact infiltrates highlighted by tryptase immunostain. Contributed by I. Maric and coworkers.


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