DIAGNOSTIC IMMUNOHISTOCHEMISTRY OF INVASIVE SQUAMOUS CARCINOMA

Immunohistochemical markers may be used for diagnosis, prognosis,
or prediction of response to treatment with targeted agents.
In this section, the focus is mainly on the diagnosis whereas in the
next, molecular lesions that are important in squamous carcinogenesis
are discussed. Immunohistochemical predictors of response
to targeted therapy are discussed elsewhere in this volume.
Cytokeratin and Its Isotypes: Pan-CK, PCK5/6, CK7,
and CK20 Immunohistochemical markers have been used
in lung cancer largely to distinguish poorly differentiated
metastatic tumors that may mimic the histological appearance
of squamous carcinoma. Such tumors include large cell lymphoma,
melanoma, germ cell malignancies, and sarcoma. The
most helpful markers in this context are the cytokeratins (CKs).
CK intermediate filaments are expressed in several different
isotypic forms. Pan-CK antibodies recognize epitopes that are
common to most of the CK isotypes. A pan-CK stain such as
AE1/AE3 cocktail usually suffices to distinguish poorly differentiated
squamous carcinomas from other poorly differentiated
tumors. Occasionally, however, poorly differentiated tumors
are pan-CK negative and these cases specific anti-CK isotypes
such as CK5/6 may be positive and clarify the diagnosis.
A more complicated problem is the use of isotype-specific
antikeratin antibodies to distinguish squamous carcinoma from
other non–small cell lung carcinoma (NSCLC) types. With the
emerging importance of squamous histology in predicting response
to targeted agents, the question of whether immunohistochemical
markers could be help to distinguish squamous from
other non–small cell histologies has become more important.
Several of the immunohistochemical markers useful for making
this distinction are CKs (Table 22.3). The CKs form a large family
of related proteins that associate to form mature filaments in
epithelial cells and tumors. 66–68 Central airway squamous tumors
express different CKs than tumors originating from peripheral
airways. Squamous carcinomas express CK5/6 at a frequency of
80%, 69–71 but adenocarcinomas express this protein at a lesser
frequency. 70–73 However, there is so much overlap in the expression
of CK5/6 among the tumor types, staining for CK5/6 by
itself is not a reliable marker for squamous carcinoma. 74
A second CK protein that has been suggested as a useful
diagnostic aid is CK7. In squamous carcinoma, CK7 is notable
by its absence with three quarters of squamous carcinomas
negative for this marker. 70,75–77 Here again, however, the
number of positive cases found among squamous carcinoma
is sufficient to limit the utility of the protein as a diagnostic
discriminant of squamous carcinoma.
P63 P63 is a transcription factor and homologue of p53 that
is important in epithelial cell differentiation. It is expressed by
myoepithelial and reserve support cells and has been proposed
as a possible marker of squamous phenotype. The marker is
frequently expressed at high level in the nuclei of squamous
carcinoma cells and overexpression is associated with p63 gene
amplification. 78 Several studies have shown that the marker has
high sensitivity for squamous carcinoma with 95% of tumors
immunoreactive with anti-p63 antibody. 69,79–81 However,
specificity is variable, with several studies reporting that 0% to
30% of adenocarcinomas express p63 . 79–82
Antibodies against p63 or CK5/6 or both have been
paired with anti–thyroid transcription factor-1 (TTF-1) (discussed
later) in a single immunohistochemical panel to distinguish
squamous carcinoma from adenocarcinoma. Sensitivities
and specificities for the double antibody test are high (80% to
100%) but the numbers of cases reported is small. 82,83 Double
antibody testing may ultimately prove useful for poorly differentiated
tumors where little tissue is available but this test will
require further validation in specific clinical contexts.