Diffuse large B-cell lymphoma is a cancer of B cells, a type of white blood cell responsible
for producing antibodies.
It is the most common type of non-Hodgkin lymphoma among adults, with an annual incidence
of 7-8 cases per 100,000 people per year.
This cancer occurs primarily in older individuals, with a median age of diagnosis at approximately
70 years of age, though it can also occur in children and young adults in rare cases.
DLBCL is an aggressive tumour which can arise in virtually any part of the body, and the
first sign of this illness is typically the observation of a rapidly growing mass, sometimes
associated with fever, weight loss, and night sweats.
The causes of diffuse large B-cell lymphoma are not well understood.
Usually DLBCL arises from normal B cells, but it can also represent a malignant transformation
of other types of lymphoma or leukemia.
An underlying immunodeficiency, is a significant risk factor.
Infection with Epstein-Barr virus has also been found to contribute to the development
of some subgroups of DLBCL.
Diagnosis of DLBCL is made by removing a portion of the tumour through a biopsy, and then examining
this tissue using a microscope.
Usually an experienced hematopathologist makes this diagnosis.
Several subtypes of DLBCL have been identified, each having a different clinical presentation
and prognosis.
However, the usual treatment for each of these is chemotherapy, often in combination with
an antibody targeted at the tumour cells.
Through these treatments, more than half of patients with DLBCL can be cured, and overall
survival for older adults at five years is around 58%.
Classification Diffuse large B-cell lymphoma encompasses
a biologically and clinically diverse set of diseases, many of which cannot be separated
from one another by well-defined and widely accepted criteria.
The World Health Organization classification system defines more than a dozen subtypes,
each of which can be differentiated based on the location of the tumour, the presence
of other cells within the tumour, and whether the patient has certain other illnesses related
to DLBCL.
One of these well-defined groupings of particular note is "primary mediastinal large B-cell
lymphoma", which arises within the thymus or mediastinal lymph nodes.
In some cases, a tumour may share many features with both DLBCL and Burkitt lymphoma.
In these situations, the tumour is classified as simply “B-cell lymphoma, unclassifiable,
with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma”.
A similar situation can arise between DLBCL and Hodgkin’s lymphoma; the tumour is then
classified as “B-cell lymphoma, unclassifiable, with features intermediate between diffuse
large B-cell lymphoma and Hodgkin’s lymphoma”.
When a case of DLBCL does not conform to any of the well-defined subtypes, and is also
not considered unclassifiable, then it is classified as “diffuse large B-cell lymphoma,
not otherwise specified”.
The majority of DLBCL cases fall into this category.
Much research has been devoted to separating this still-heterogeneous group; such distinctions
are usually made along lines of cellular morphology, gene expression, and immunohistochemical properties.
Morphology Within cellular morphology three variants
are most commonly seen: centroblastic, immunoblastic, and anaplastic.
Most cases of DLBCL are centroblastic, having the appearance of medium-to-large-sized lymphocytes
with scanty cytoplasm.
Oval or round nuclei containing fine chromatin are prominently visible, having two to four
nucleoli within each nucleus.
Sometimes the tumour may be monomorphic, composed almost entirely of centroblasts.
However, most cases are polymorphic, with a mixture of centroblastic and immunoblastic
cells.
Immunoblasts have significant basophilic cytoplasm and a central nucleolus.
A tumour can be classified as immunoblastic if greater than 90% of its cells are immunoblasts.
This distinction can be problematic, however, because hematopathologists reviewing the microscope
slides may often disagree on whether a collection of cells is best characterized as centroblasts
or immunoblasts.
Such disagreement indicates poor inter-rater reliability.
The third morphologic variant, anaplastic, consists of tumour cells which appear very
differently from their normal B cell counterparts.
The cells are generally very large with a round, oval, or polygonal shape and pleomorphic
nuclei, and may resemble Hodgkin cells or Reed-Sternberg cells.
Gene expression Gene expression profiling studies have also
attempted to distinguish heterogeneous groups of DLBCL from each other.
These studies examine thousands of genes simultaneously using a DNA microarray, looking for patterns
which may help in grouping cases of DLBCL.
Many studies now suggest that cases of DLBCL, NOS can be separated into two groups on the
basis of their gene expression profiles; these groups are known as germinal centre B-cell-like
and activated B-cell-like.
Tumour cells in the germinal centre B-cell-like subgroup resemble normal B cells in the germinal
centre closely, and are generally associated with a favourable prognosis.
Activated B-cell-like tumour cells are associated with a poorer prognosis, and derive their
name from studies which show the continuous activation of certain pathways normally activated
when B cells interact with an antigen.
The NF-κB pathway, which is normally involved in transforming B cells into plasma cells,
is an important example of one such pathway.
Another notable finding of recent gene expression studies is the importance of the cells and
microscopic structures interspersed between the malignant B cells within the DLBCL tumor,
an area commonly known as the tumour microenvironment.
The presence of gene expression signatures commonly associated with macrophages, T cells,
and remodelling of the extracellular matrix seems to be associated with an improved prognosis
and better overall survival.
Alternatively, expression of genes coding for pro-angiogenic factors is correlated with
poorer survival.
Immunohistochemistry With the apparent success of gene expression
profiling in separating biologically distinct cases of DLBCL, NOS, some researchers examined
whether a similar distinction could be made using immunohistochemical staining, a widely
used method for characterizing tissue samples.
This technique uses highly specific antibody-based stains to detect proteins on a microscope
slide, and since microarrays are not widely available for routine clinical use, IHC is
a desirable alternative.
Many of these studies focused on stains against the products of prognostically significant
genes which had been implicated in DLBCL gene expression studies.
Examples of such genes include BCL2, BCL6, MUM1, LMO2, MYC, and p21.
Several algorithms for separating DLBCL cases by IHC arose out of this research, categorizing
tissue samples into groups most commonly known as GCB and non-GCB.
The correlation between these GCB/non-GCB immunohistochemical groupings and the GCB/ABC
groupings used in gene expression profiling studies is uncertain, as is their prognostic
value.
This uncertainty may arise in part due to poor inter-rater reliability in performing
common immunohistochemical stains.
Symptoms The most typical symptom at the time of diagnosis
is a mass that is rapidly enlarging and located in a part of the body with multiple lymph
nodes.
Treatment Chemotherapy
Standard treatment is CHOP-R, also referred to as R-CHOP, an improved form of CHOP with
the addition of rituximab, which has increased the rates of complete responses for DLBCL
patients, particularly elderly patients.
R-CHOP is a combination of one monoclonal antibody, 3 chemotherapy drugs, and one steroid:
rituximab, cyclophosphamide doxorubicin, vincristine, and prednisone.
Chemotherapy is administered intravenously and is most effective when it is administered
multiple times over a period of months.
The number of cycles of chemotherapy given depends on the stage of the disease.
patients with limited stage disease receive 3 cycles of therapy, while patients with extensive
disease 6 or 8 cycles of chemotherapy.
In the United States, 6 cycles is the preferred approach rather than 8 cycles.
A new development is obtaining a PET scan after completing two cycles of chemotherapy,
to help make further decisions after chemotherapy.
Older people Older people are not able to tolerate therapy
well.
Muliple lower intensity regimens have been attempted in this age group.
People receiving chemotherapy commonly have a PICC line in their arm near the elbow or
a surgically implanted port.
Radiation therapy Radiation is often added in the treatment.
It is used commonly after completing 3 cycles of treatment in limited stage disease.
In extensive disease, radiation can be used at the end of the treatment, after 6-8 cycles
of chemotherapy, to areas of bulky involvement.
Radiation therapy alone is not an effective treatment for this disease.
Prognosis The germinal-center subtype has the best prognosis,
with 66.6% of treated patients surviving more than five years.
The IPI score is used in prognosis in clinical practice.
Lenalidomide has been recently shown to improve outcomes in the non-germinal center subtype.
For children with diffuse large B-cell lymphomas, most studies have found 5-year survival rates
ranging from about 70% to more than 90%.
Research A second regimen under evaluation is R-EPOCH,
which demonstrated a 5-year progression-free survival of 79% in a phase II trial.
A phase III trial, CALGB 50303, is now comparing R-EPOCH with R-CHOP in patients with newly
diagnosed DLBCL.
One area of active research is on separating patients into groups based on their prognosis
and how likely they are to benefit from different drugs.
Methods like gene expression profiling and next-generation sequencing may result in more
effective and more personalized treatment.
See also Primary mediastinal B-cell lymphoma - a subgroup
of diffuse large B-cell lymphoma arising in the mediastinum of young adults
Germinal center B-cell like diffuse large B-cell lymphoma - a subgroup of diffuse large
B-cell lymphoma which seem to arise from normal germinal center B-cells