Molecular Diagnosis in Pathology: The Bridge to the 21st Century
Moderators: Dr. Ricardo Lloyd and Dr. George Kontogeorgos
Section 8 -
Balancing Morphology and Molecular Biology in the 21ST Century
Juan Rosai, M.D.
For the best part of the 20th century, the diagnosis and classification of human tumors has been
largely based on their microscopic (i.e., cytologic and architectural) features as seen in
hematoxylin-eosin-stained sections of formalin-fixed, paraffin-embedded material. This generally highly
efficient standard approach was often supplemented with information provided by "ancillary" techniques
used depending on the circumstances, such as "special stains", electron microscopy and
immunohistochemistry, performed with the aim to provide more precise information on the "cell of origin"
of the tumor and on its expected behavior, including its anticipated response to various forms of
therapy. The best studies that have been carried out along these lines are those that have integrated
the various approaches in a selected and synergistic fashion. The end result of this collective effort
was the realization that the examination of H&E-stained section should remain the first step for the
identification and evaluation of a tumor, and also the guide by which to decide which additional studies
are indicated, if any. Great damage to the credibility of pathologists is bound to occur if there is a
systematic unwillingness on their part to explore the potential utility of any new technology, and this
is particularly true in the current molecular biology-dominated era. At the same time, a distinct
possibility for misinterpretation exists whenever a molecular biologist or other non-pathology-based
investigator decides to tackle human tumors with total disregard for everything that has been learned
over the years from the painstaking evaluation of their morphologic and phenotypic characteristics (or,
worse, if he chooses to become his own pathologist in order to evaluate these features).
Areas in which the integration of the morphologic and molecular biologic diagnostic approaches
islikely to be of great benefit – as already proved in several instances - are the following:
 Tumor diagnosis. The value of molecular biology-based tests can be nearly absolute
(such as the finding of the gene fusion associated with desmoplastic small cell tumor, which in practice
has superseded the morphologic diagnosis) or have a relative or circumstantial value in a morphologically
difficult case. For instance, finding a PAX8/PPARg rearrangement or a Ras mutation in an encapsulated
well-differentiated follicular neoplasm of the thyroid gland suggests that the lesion is not an adenoma
bur either a follicular variant of papillary carcinoma or a follicular carcinoma. Siumilarly, the
finding a K-Ras mutation in a pancreatic brush cytology specimen exhibiting atypia suggests that the
lesion is malignant.
 Molecular classification of tumors.
Much has been written in recent years about a forthcoming molecular classification of human tumors
based on gene expression analysis that will revolutioniza tumor taxonomy. Early courageous proposals
along these lines have been made, particularly in the fields of breast cancer and malignant
Whether such classifications will eventually replace the existing schemes
primarily based on morphology remains to be seen. In any event, the role of the pathologist in the
critical evaluation of these proposals will be crucial by asking the following questions: (1) Is the
proposed classification biologically sound ? (2) Does it really represent a novel scheme ? It has been
noted that so far most of the "molecular class discoveries" have actually represented rediscoveries of entities that had already been well-defined on clinico-pathologic
grounds. (3) Can it be integrated to the existing morphology-based classification or should it totally
independent of the latter? (4) Does it offer prognostic information above and beyond that already
supplied by the existing classification, particularly once the individual tumors have been segregated
according to their clinico-pathologic stage and microscopic grade? (5) Does it contain some specific
information that can be correlated with a morphologic parameter or that can be extracted and adapted to
an immunohistochemical assay or some other morphology-based approach? 
[2 ] Targeted therapeutics. This is one of the most exciting developments in oncology at
present, some of the most highly publicized examples being trastuzumab in breast carcinoma, imatinib
mesylate in gastrointestinal stromal tumors, microsatellite instability in colorectal carcinoma, and
1p/19q loss in oligodendrogliomas.  There is no doubt that pathologists will play a major role
in the development of this field, working in close collaboration with oncologists and molecular
biologists. As Jay Hess  stated, this role will include: (1) Providing the primary
diagnosis; (2) Determining before therapy whether a specific molecular target is present; (3) Evaluating
the efficacy and possible side effects of new therapies; (4) Harvesting and analyzing tissues in
 The unknown primary. The presence of markers that are expressed specifically or
preferentially by certain tissue types is the main tool that pathologists have used for the
identification of the site of the primary tumor in cases initially presenting with metastatic disease.
These markers, originally of a cytoarchitectural nature, included in more recent types those identifiable
at an ultrastructural and immunohistochemical level, and are now being detected at a markedly increasing
pace through gene expression analysis. Markers particularly powerful in this regard because of their
high degree of specificity are the nuclear-based transcription factors, such as myogenin, TTF-1 and
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