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Difficulty: MediumPathology and Biopsy Interpretation

A 58-year-old man is referred to a hematologist for evaluation of progressive fatigue, easy bruising, and early satiety over the past six months. Physical examination is notable for massive splenomegaly extending 10 cm below the left costal margin; there is no palpable cervical, axillary, or inguinal lymphadenopathy. Laboratory studies show a hemoglobin of 8.28.2 g/dL, leukocyte count of 2,100/μL2,100/\mu L, and platelet count of 65,000/μL65,000/\mu L. A bone marrow aspiration is attempted but yields no fluid (a "dry tap"). The subsequent bone marrow core biopsy demonstrates a hypercellular marrow with extensive infiltration by mononuclear cells possessing abundant, pale cytoplasm and fine, irregular cytoplasmic projections. A reticulin stain of the marrow is markedly positive. Flow cytometry and immunohistochemistry of the biopsy specimen reveal that the neoplastic cells strongly express CD20, CD11c, CD25, and CD103.

Based on the histopathologic and immunophenotypic findings, which of the following is the most likely diagnosis?

  1. A
    Primary myelofibrosis
  2. B
    Splenic marginal zone lymphoma
  3. Hairy cell leukemiaAnswer
  4. D
    Chronic lymphocytic leukemia
  5. E
    Mantle cell lymphoma

Answer

Hairy cell leukemia
The correct answer is hairy cell leukemia. This patient's presentation of pancytopenia, massive splenomegaly (without prominent lymphadenopathy), and a "dry tap" on aspiration is the classic clinical triad. The diagnosis is definitively established by the bone marrow biopsy, which demonstrates mononuclear cells with fine cytoplasmic projections ("hairy" cells) alongside prominent reticulin fibrosis. Furthermore, the characteristic immunohistochemical profile (positive for CD20, CD11c, CD25, and CD103) confirms the diagnosis and differentiates it from other B-cell neoplasms.

Step-by-Step Solution

1
Evaluate the clinical presentation and initial laboratory findings.
The patient has pancytopenia, massive splenomegaly without lymphadenopathy, and a "dry tap" on bone marrow aspiration.
This triad strongly suggests a marrow-infiltrative process causing fibrosis and subsequent extramedullary hematopoiesis or splenic pooling.
2
Analyze the morphological findings on the bone marrow core biopsy.
The biopsy shows an infiltrate of mononuclear cells with abundant, pale cytoplasm and "hair-like" cytoplasmic projections, along with marked reticulin fibrosis.
These cytological features are classic for hairy cells, and the associated fibrosis explains the inability to aspirate marrow.
3
Correlate the morphology with the immunophenotypic profile.
The neoplastic cells express mature B-cell markers (CD20) alongside highly specific markers including CD11c, CD25, and CD103.
This distinct combination of surface markers definitively confirms the diagnosis of hairy cell leukemia and distinguishes it from other B-cell lymphoproliferative disorders.

Key Concept

Hairy cell leukemia is diagnosed by the combination of a "dry tap" marrow with reticulin fibrosis, characteristic "hairy" mononuclear cells, and a specific CD11c+, CD25+, CD103+ immunophenotype.
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Topics

Pathology and Biopsy Interpretation

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