Hematopathology Diagnoses Too Easy to Miss!
Malignant Lymphomas that Mimic Other Lymphomas of a Different Type with Very Different Clinical Implications (continued)
Marsha Kinney, James Cook and Steven Swerdlow
Primary mediastinal (thymic) large B-cell lymphoma
The patient is a 39 year old woman who was injured in a fall from a ladder. A
subsequent CT scan identified a mediastinal mass that focally invaded into lung parenchyma. A biopsy of
the mediastinal mass and a wedge resection of the lung were performed.
The histologic sections demonstrated a diffuse and focally
nodular proliferation of intermediate to large lymphoid cells with moderate amounts of pale to clear
cytoplasm. There was accompanying fibrosis that focally produced nodules of tumor cells. Scattered
cells were very large with irregular nuclear contours and more prominent nucleoli. Some of these latter
cells resided within lacunar spaces or had a hyperchromatic, "mummified" appearance.
Flow cytometric studies showed a predominance of T-cells,
while the B-cells included a population with polytypic immunoglobulin light chain expression and another
population (about 50% of the B-cells) that lacked surface immunoglobulin entirely. Immunohistochemical
stains demonstrated the vast majority of the proliferation to consist of sheets of CD20 positive cells
that coexpressed CD45 (LCA) and CD30. The largest atypical cells, including the lacunar forms, were
positive for CD15.
Primary Mediastinal Large B-cell Lymphoma:
mediastinal large B-cell lymphoma (PMLBCL) was first acknowledged as a formal entity in the REAL
classification of 1994,  and is recognized in the current WHO classification as a subtype of
diffuse large B-cell lymphoma with distinct clinical and pathologic features.  It has been
proposed that PMLBCL is related to a unique subset of B-cells that normally reside within the
Clinically, these lymphomas will typically present as large anterior mediastinal masses, with symptoms
usually related directly to impingement on mediastinal structures. Some cases may present with
disseminated involvement, including extranodal sites such as kidney and brain. Patients with PMLBCL
typically present in their 3rd to 5th decade of life, younger than patients with
DLBCL overall, and there is a female predominance.
PMLBCL display several characteristic morphologic features which led to the first suggestions that
these cases were a distinct pathologic entity. Histologically, these specimens consist of predominantly
diffuse proliferations of intermediate to large lymphoid cells, often with moderate amounts of clear to
There is frequent sclerosis, which is usually mild to moderate in amount
and is located interstitially throughout the proliferation. In some cases, there may be bands of dense
sclerosis which focally separate the tumor mass into nodules. The cytologic features are somewhat
variable, both between cases and within a case. Some PMLBCL will show scattered large to very large
cells with irregular nuclear contours. These largest cells may be binucleate, and may be seen to reside
within lacunar spaces, creating a Reed-Sternberg-like appearance. In some cases, these
Reed-Sternberg-like cells may be prominent.
Flow cytometric studies show that most cases lack detectable surface immunoglobulin expression,
although rearranged immunoglobulin heavy chain genes may be detected by PCR or Southern blot
Using flow cytometry or immunostains, most cases are clearly positive for B-cell
markers including CD20 and CD79a. The B-cell transcription factors PAX5, PU.1, OCT2, and BOB1 are also
positive.  Approximately one-third of cases have been reported to be positive for CD10 and
47-100% reported to be positive for BCL6 protein. Most cases also appear to be positive for BCL2
Immunohistochemical studies have shown most cases to be positive for CD30,
although the extent of positivity may vary from cases with weak and focal expression to those with
strong, diffuse positivity.
CD45 is typically strongly positive, and CD15 expression is
characteristically absent, although exceptions to this rule clearly occur (see discussion of "grey zone"
Expression of the MAL protein, typically associated with T-thymocytes and rarely seen in other B-cell
lymphomas, has been documented in a majority of cases of PMLBCL.
MAL protein is also
expressed in an unusual subset of B-cells that normally resides within the thymus, leading to speculation
that PMBCL represent a neoplastic counterpart of this differentiated B-cell subset. Unfortunately,
antibodies to MAL protein are not commercially available at the present time.
Little information is available regarding classical cytogenetic studies of PMBCL. Using CGH studies
and other molecular techniques, gains of chromosome 9p appear to be a common, recurring feature in
Amplification of the REL gene, on chromosome 2p,
also has been described in many cases of PMLBCL.  To date, however, neither of these
potential molecular markers has been shown to be clinically useful for the recognition of PMBCL in
PMBCL vs Classical Hodgkin Lymphoma:
As described above, cases of PMBCL
may show clinical, morphologic, and phenotypic features that overlap with classical Hodgkin lymphoma.
Like many cases of classical Hodgkin lymphoma, most PMLBCL will present as a large anterior mediastinal
mass. Histologically, Reed-Sternberg-like cells may be seen in many PMBCL cases, and the characteristic
fibrosis may focally create nodules of tumor that simulate the architectural background of nodular
sclerosis Hodgkin lymphoma. The immunophenotype in PMLBCL, including CD30 expression and lack of surface
immunoglobulin rearrangements, may also create a differential diagnosis with classical Hodgkin lymphoma.
In this regard, it is very interesting that recent expression microarray studies have shown that PMLBCL
cases show an expression profile that clearly distinguishes PMLBCL from the other forms of DLBCL, and
that shows overlap with the expression profile of classical Hodgkin lymphoma.
studies seem to support the long-standing suspicion that PMLBCL may be pathogenetically related to
classical Hodgkin lymphoma.
Fortunately, in routine clinical practice, many cases of PMBCL can be separated from other cases of
DLBCL and from classical Hodgkin lymphoma on the basis of the clinical, morphologic and phenotypic
findings. In excisional specimens, cases of PMLBCL will display areas containing confluent sheets of
intermediate to large B-cells, leading to an appropriate diagnosis even if Reed-Sternberg-like cells are
present. Most problematic are needle biopsies, which may be partially crushed, and may reveal sclerosis
and Reed-Sternberg-like cells that may lead to a misdiagnosis of classical Hodgkin lymphoma. Because
classical Hodgkin lymphoma and PMLBCL are generally treated with different forms of therapy, misdiagnosis
will have clinical implications.
A battery of immunohistochemical stains can be extremely useful to distinguish between PMBCL and
classical Hodgkin lymphoma. CD20 and CD45 expression may be seen in cases of classical Hodgkin lymphoma,
but such staining is typically weak and variable.
In contrast, PMBCL is typically
strongly and diffusely positive for both CD20 and CD45.
Analysis of the B-cell
transcription factors OCT2 and BOB1 is also very valuable. Almost all cases of Hodgkin lymphoma will be
negative for either OCT2, BOB1, or, in more than two thirds of cases, both of these markers.
PMBCL, on the other hand, should be positive for both of these markers.
Mediastinal "Grey Zone" Lymphomas:
While most cases of PMLBCL are clearly
distinguishable from classical Hodgkin lymphoma, there are also clearly cases that span the findings in
each of these categories. Such cases have been termed "grey zone" lymphomas, and they have been proposed
to represent a "missing link" in the continuum between classical Hodgkin lymphoma and PMLBCL.
These cases include those with morphologic features most suggestive of PMLBCL, but a phenotypic
profile most consistent with classical Hodgkin lymphoma, and vice versa. In the 2008 revised WHO
classification, these cases are recognized in a new category termed "B-cell lymphoma, unclassifiable,
with features intermediate between DLBCL and classical Hodgkin lymphoma." 
The findings in case 10 include focal areas with sclerotic tumor nodules, a background that focally
includes small lymphocytes and eosinophils, and Reed-Sternberg-like cells that express CD30 and CD15 and
appear to lack CD45. This case may therefore represent part of the spectrum that some would term a "grey
These uncommon cases represent a true diagnostic challenge for pathologists. In all such cases, an
extensive battery of immunophenotypic studies should be performed. If, after extensive phenotyping, a
clear diagnosis is not apparent, it may be appropriate to issue a diagnosis of "malignant lymphoma" or
"grey zone lymphoma" with a comment describing the differential diagnosis. If only a needle biopsy is
initially available, a larger excisional specimen should be requested, as a larger, more representative
specimen may lead to a clear diagnosis. Whenever possible, however, the pathologist should attempt to
favor either PMBCL or classical Hodgkin lymphoma based upon the preponderance of the available evidence –
providing the clinician with as much guidance as possible in the subsequent choice of therapy (i.e.
Hodgkin vs non-Hodgkin directed therapy).
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