Aggressive systemic mastocytosis mimicking lymphoma: a case report


 Background

Aggressive systemic mastocytosis (ASM) is a very rare form of systemic mastocytosis (SM). The diagnosis of ASM requires the presence of SM criteria with C finding and does not meet the criteria for mast cell leukaemia. Herein, we report an ASM case that initially mimicked lymphoma based on clinical and radiographic analyses.
Case presentation:

A 87-year-old woman was admitted to our hospital due to the enlargement of cervical lymph nodes and weight loss. The lymph node biopsy is infiltrated by neoplastic mast cells with pale, faintly granular cytoplasm, and with reactive eosinophils in the lesions. The infiltrate is often parafollicular in distribution and the remnant of normal follicles can be seen. The neoplastic cell population was subsequently revealed to exhibit differentiation towards the mast cell lineage by expressing CD117 and CD25. Mutation analysis of c-KIT identified D816V mutation in exon 17.
Conclusion

ASM diagnosis can be challenging due to its rarity. This diagnosis can be confirmed or disregarded using immunohistochemical markers, such as CD117, CD2 and CD25, in combination with molecular analysis (KITD816V).


Abstract Background
Aggressive systemic mastocytosis (ASM) is a very rare form of systemic mastocytosis (SM). The diagnosis of ASM requires the presence of SM criteria with C nding and does not meet the criteria for mast cell leukaemia. Herein, we report an ASM case that initially mimicked lymphoma based on clinical and radiographic analyses.

Case presentation:
A 87-year-old woman was admitted to our hospital due to the enlargement of cervical lymph nodes and weight loss. The lymph node biopsy is in ltrated by neoplastic mast cells with pale, faintly granular cytoplasm, and with reactive eosinophils in the lesions. The in ltrate is often parafollicular in distribution and the remnant of normal follicles can be seen. The neoplastic cell population was subsequently revealed to exhibit differentiation towards the mast cell lineage by expressing CD117 and CD25. Mutation analysis of c-KIT identi ed D816V mutation in exon 17. Conclusion ASM diagnosis can be challenging due to its rarity. This diagnosis can be con rmed or disregarded using immunohistochemical markers, such as CD117, CD2 and CD25, in combination with molecular analysis (KITD816V).

Background
Systemic mastocytosis (SM) is a heterogeneous disease characterized by the accumulation of neoplastic mast cells in one or more organ systems. In the revised 2016 WHO classi cation, mastocytosis was classi ed into cutaneous mastocytosis, SM and mast cell sarcoma, which was de ned as a separate disease category [1]. Aggressive Systemic Mastocytosis (ASM), a rare subtype of SM, results from clonal proliferation and invasion of multiple organs by neoplastic mast cells. The clinical presentation varies, dependent on which organ systems are involved, and may take an indolent or rapidly fatal course. Here, we report a case of ASM with lymph nodes involvement as the rst manifestation which mimicking lymphoma.

Case Presentation
An 87-year-old female presented to our hospital in January 2020 because of the enlargement of cervical lymph nodes, and weight loss of 10 kg over the previous 6 months. Physical examination showed the bilateral neck was obviously enlarged with a multinodular appearance, mild splenomegaly and ascites. There were no skin lesions or signs of chronic liver disease. Laboratory studies revealed white blood cell count of 4.5 × 10 9 /L with 40.5% eosinophils (with a normal differential count), red blood cell count of 3.25 × 1012/L, a hemoglobin concentration of 97 g/L, and a platelet count of 48 × 10 9 /L (normal range, 100-300 × 10 9 /L). The serum concentration of C-reactive protein was 12.52 mg/L (normally below 5 mg/L), and the serum tryptase level could not be assessed due to technical limitations. The tumor marker CA125 was present at a level of 54. A core biopsy on the left cervical lymph node was nondiagnostic and complete lymphadenectomy revealed the lymph node was diffusely in ltrated by neoplastic mast cells with abundant, lightly staining cytoplasm, and with reactive eosinophils in the lesions (Fig. 1a-1c). The in ltrate was often parafollicular in distribution and the remnant of normal follicles could be seen. A Giemsa stain revealed presence of increased mast cells in the received tissue. On immunohistochemistry, these neoplastic cells were positive for CD117 (Fig. 1d) and CD25 (Fig. 1e), and negative for CD2 (Fig. 1f), CD34, CD4, CD20, CD3, PAX5, CD68(KP1 and PG-M1), MPO, CD35, CD21, CD23, CD163 and S-100. Gastrointestinal (GI) endoscopic biopsy showed mild chronic gastritis and the gastric biopsies were retrospectively stained for c-kit (Fig. 1g) and CD25 (Fig. 1h), revealing only a few scattered mast cells in the super cial lamina propria, con rming SM in the stomach. In situ hybridization for Epstein-Barr virus (EBV) encoding RNA (EBER) was negative. Polymerase chain reaction (PCR) plus sanger sequencing on lymph node tissue was positive for the KIT D816V mutation (Fig. 1i) and negative for PDGFRA mutation. Thus, our patient met the major criteria and three minor criteria. Based on the clinical, histologic, immunohistochemistry, laboratory ndings and molecular features, a diagnosis of ASM was established. Using a next-generation sequencing (NGS)assay, we documented genomic alterations in KIT, and addition genetic abnormalities were ARID1A, BARD1, MYD88, TET2, KMT2C, POLE and RNF43 mutations, microsatellite instability high (MSI-H) and low tumor mutation burden. The details of genetic mutations in Table 1. to the presence of C-ndings (hypohemoglobinemia, thrombocytopenia, splenomegaly, and gastrointestinal dysfunction), a diagnosis of ASM was made. ASM is a hematopoietic neoplasm characterized by in ltration of visceral organs by neoplastic mast cells with consecutive organopathy and respective clinical and laboratory ndings (so called C-Findings). In ASM the in ltration of mast cells causes impairment function of involved organs and the clinical picture is charaterized by loss of function of involved organs. In our case, the patient has mild ascites, malabsorption, osteolysis and two cytopenia. The patient also presented with constitutional symptoms (weight loss), extensive lymphadenopathy, eosinophilia and splenomegaly, which mimicking a lymphoma. Lymph nodes involvement in ASM is common. Evidence of lymph nodes involvement includes lymphadenopathy. The main sign of lymph nodes involvement in ASM is the pathological accumulation of mast cells in the lymph nodes.
The histological pattern of the mast cell in ltrate can vary depending on the tissue sampled. On lymph node sample, mast cells aggregate predominantly in paracortical areas. In our case, histology of a cervical lymph node revealed follicular lymphoid hyperplasia with interfollicular in ltration by spindle-shaped mast cells with abundant, lightly staining cytoplasm. The presence of multifocal compact mast cell in ltrates or a diffuse compact mast cell in ltration pattern is highly compatible with the diagnosis of mastocytosis. However, in patients with the diffuse in ltration pattern, it is therefore impossible to establish the diagnosis of mastocytosis without additional studies, including the demonstration of an aberrant immunophenotype and/or detection of an activating point mutation in KIT. In the present case, a core biopsy on the cervical lymph node was nondiagnostic,due to prominent eosinophils, which obscure the mast cells in ltrate. Subsequently, lymphadenectomy showed densely packed, spindled mast cells aggregate predominantly in the parafollicular area, which con rmed the diagnosis of mastocytosis. Four patients with ASM with a presentation similar to the one described in this article were previously reported in the literature. In one patient KIT mutation status was not determined [2], and the other three patients were positive for KITD816V [3][4][5].
Previous studies have reported GI tract involvement by mastocytosis in small numbers of patients [6][7][8]. Histologically, involved GI endoscopic biopsy contained scattered single mast cells with expression of both CD117 and CD25, determined to represent minimal involvement by mastocytosis. Due to involvement was very focal and subtle in our case, the diagnosis of mastocytosis in mucosal biopsies was very di cult. The present case was initially diagnosed as chronic gastritis on the basis of the low dense of mast cells, and did not have endoscopic abnormalities, which added to the di culty of diagnosis. Following recognition of mastocytosis in the lymphadenectomy, described above, the gastric biopsies were retrospectively stained for c-kit, revealing only a few scattered mast cells in the lamina propria, con rming SM in the stomach. The histologic differential diagnosis of mastocytosis in the GI tract is with reactive mast cell hyperplasia. Normal/reactive mast cells are usually loosely scattered throughout the sample and display round to oval nuclei with clumped chromatin, a low nuclear: cytoplasmic ratio, and absent or indistinct nucleoli. They are often appeared in normal mucosa or slightly increased in parasitic infection but do not show coexpression of CD117 and CD25, which can help in the differential diagnosis with mastocytosis.
Immunohistochemically, the CD117 positive mast cells with co-expression of CD2 and / or CD 25 is a sensitive and speci c marker in the diagnosis of SM. Compared with normal mast cells, neoplastic mast cells abnormally express CD2 and CD25, but did not express most T or B antigens. Other markers including CD30, CD45, and CD68 are variably positive. In our case, the neoplastic mast cells mainly express CD117 and CD25, aberrant expression of CD2 is not seen in neoplastic mast cells, which is mainly expressed normally on some T lymphocytes. Prior studies have shown an association between CD30/123 expression and advanced disease [9][10][11],but data on this association is not particularly robust. We did not nd CD30 or CD123 expression by neoplastic mast cells in our case.
Most cases of ASM harbor the KIT D816V mutation. The KIT gene, also known as CD117, encodes the KIT proto-oncogene receptor tyrosine kinase (c-KIT), a member of the PDGF receptor type III receptor tyrosine kinase family, which includes PDGFRA, PDGFRB, CSF1R, FLT1, FLT3, FLT4 and KDR [12,13]. KIT is a receptor for stem cell factor, important in regulating growth and development of hematopoietic cells [14]. The KIT gene is anked by the PDGFRA and KDR genes on chromosome 4q12. Ligand binding to KIT results in kinase activation and stimulation of downstream pathways including the RAS/RAF/MEK/ERK and PI3K/AKT/MTOR pathways promoting cell proliferation and survival [15]. A common kinase domain mutation that causes ligand-independent constitutive activation, D816V, occurs in 80-93% of aggressive forms of mastocytosis [16,17]. In our case, some other genetic abnormalities were detected by NGS technology in addition to the KIT mutation, including ARID1A, BARD1, MYD88, TET2, KMT2C, POLE and RNF43 mutations, MSI-H and low tumor mutation burden. TET2 mutation is more frequently identi ed in SM with an associated haematological neoplasm. To the best of our knowledge, this is the rst case of ASM reporting these genes abnormality. The relationship between ASM and these genetic abnormalities has not been documented thus far, so a large number of samples are needed to verify the relationship between ASM and these gene abnormalities.
Rapidly, the patient suffered from tumor dissemination, and nally died from respiratory failure almost 2 months after initial presentation. Several parameters have been described to be associated with an unfavorable prognosis in SM, including an absence of skin lesions, huge osteolyses, weight loss, malabsorption, enlarged liver with portal hypertension, and splenomegaly with hypersplenism [18][19][20][21]. In the present patient, absence of skin lesions, multiple bone lesions, weight loss, malabsorption and splenomegaly were observed. In a previous literature reported that lymphadenopathic mastocytosis with eosinophilia" was described as a separate, clinically aggressive variant of ASM [22]. There are also other cases of SM presenting with lymphadenopathy and eosinophilia exhibited a less aggressive course [3,23]. Lymph node involvement with signi cant lymphadenopathy is extremely rare; when involved, it can mimic malignant lymphoma, creating a diagnostic challenge.

Conclusion
In conclusion, the diagnosis of ASM is frequently delayed due to the non-speci c nature of symptoms and clinical presentations, as exempli ed by our case with a lymphoma-like presentation. Diagnosis is di cult even if a patient shows symptoms of aggressive disease, lymphadenopathy, anemia, splenomegaly, and mild ascites. Herein, we present an unusual case of ASM with CD25 and CD117 expression and KIT mutations, with an aggressive clinical course and poor prognosis. Anyway, additional immunohistochemical and molecular studies are strongly recommended even in all suspected cases.

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