72 year old woman with a mediastinal mass and lymphadenopathy in the neck.  The bone marrow was not involved.

Diagnosis:
Biphenotypic Lymphoblastic Lymphoma

Discussion
Immunohistochemistry and flow cytometry are mainstays in immunophenotyping acute leukemia for purposes of classification by cell lineage.  Detection of multiple antigens associated with different cell lineages on the blasts of an acute leukemia complicates this task.  The current World Health Organization Classification of Hematopoietic Tumours addresses these cases under the general heading of “acute leukemias of ambiguous lineage”.1  It relies on the scoring system originally proposed by the European Group for the Immunological Characterization of Leukemias (EGIL), published in 1995 (see Table).2  Briefly, antigens have been assigned a number (score) of 2, 1 or 0.5.  Higher numbers (scores) are given to antigens that more consistently associate with a particular lineage.  A lineage is represented if the total score for that particular lineage adds up to more than 2 points.  If the score for two lineages adds up to more than two points, the acute leukemia may be considered bi-lineal (two separate cell lines represented) or bi-phenotypic (one cell line represented, expressing multiple antigens).

Scoring system for markers proposed by the EGIL.

The variety of antigens expressed in this case, complicated by subjective differences in interpretation of the immunohistochemistry (IHC) results, make this case and others like it, particularly difficult.  IHC available on the first lymph node biopsy revealed expression of CD79a, CD7 and TdT.  CD20, Pax-5, CD2, CD3, CD5, CD10 and myeloperoxidase appeared negative.  Occasional cells expressed weak CD56.  This totaled 2.5 “B-lymphoid” points and 1 “T-lymphoid” point – overall favoring a diagnosis of precursor B-lymphoblastic lymphoma.  Flow cytometry on a second lymph node biopsy at the treating hospital revealed expression of CD5, CD7, CD10, CD33 and TdT.  CD79a was interpreted as non-specific staining for both the original IHC stain and  the flow assay on the new lymph node biopsy.  Myeloperoxidase was again interpreted as negative.  This totaled 0.5 “B-lymphoid” points and 3 “T-lymphoid points” – overall favoring a precursor T-lymphoblastic lymphoma.

Interpretation of the IHC stain for CD79a was critical in this case.  The absence of convincing expression by flow cytometry in the second biopsy and the absence of staining in a subset of the malignant cells by IHC in the first biopsy led to the interpretation as negative at the treating hospital.

To further compound the diagnostic dilemma, flow cytometric analysis also detected the expression of the myeloid marker CD33 by the blasts.  Although CD79a has been identified as an aberrant marker in acute myeloid leukemia, particular those harboring a t(8;21)3 , aside from CD33 no other myeloid antigens were demonstrable by flow cytometry or IHC.  Therefore, it is very unlikely that this represents a myeloid leukemia.

This case illustrates the difficulty of classifying acute leukemias expressing rare and complicated immunophenotypes and how the subjectivity of interpretation can further confuse matters.  When interpreted as CD79a positive, this represents a blastic hematopoietic tumor expressing B-cell, T-cell and myeloid antigens.  According to the EGIL scoring matrix, this should be diagnosed as a bi-phenotypic precursor lymphoblastic lymphoma with evidence for both B and T-cell differentiation.  The clinical presentation and morphological findings also favor a precursor lymphoblastic lymphoma.    Such biphenotypic lymphomas are rare.  The majority of biphenotypic leukemias express myeloid and either B or T-cell differentiation.4,5 

Submitted by Christopher Felten, D.O.,* Joel Chan, M.D.,* and Jennifer Galloway, M.D.**

*Pathology Inc., Torrance and El Monte Laboratories, CA
**Laboratory Corporation of America, San Diego, CA

References:
1. World Health Organization Classification of Tumours: Pathology and Genetics of Tumours of Haematopoietic and Lymphoid Tissues.  IARC Press 2001.

2. Ludwig WD, Matutes E, Orfao A, van’t Veer MB.  Proposals for the immunological classification of acute leukemias.  Leukemia. 1995;9:1783-1786.

3. Tiacci E, Pileri S, Orleth A, et.al.  PAX5 expression in acute leukemias: Higher B-lineage specificity than CD79a and selective association with t(8;21)-acute myelogenous leukemia.  Cancer Research.  2004;64:7399-7404.

4. Lau LG, Tan LK, Koay ES, Ee MH, Tan SH, Liu TC. Acute lymphoblastic leukemia with the phenotype of a putative B-cell/T-cell bipotential precursor.  Am J Hematol. 2004;77:156-60.

5. Owaidah TM, Beihany A, Iqbal MA, Elkum N, Roberts GT.  Cytogenetics, molecular and ultrastructural characteristics of bi-phenotypic acute leukemia identified by the EGIL scoring system.  Leukemia. 2006;20:620-6.

First Posted 01.29.2007