HISTOLOGY OF SQUAMOUS CARCINOMA

The histological features of squamous carcinoma are summarized
and compared with other forms of lung cancer in
Table 22.2. Invasion is recognized as angulated nests or individual
tumor cells that have broken away from the surface
epithelium and become embedded in the stromal tissues as
shown in Figure 22.6. The invasive cells may form keratin
pearls (KP) and intercellular bridges 60 and may develop irregular
areas of central necrosis described as geographic
necrosis. Nuclear features include irregular nuclei and coarse
chromatin. The cytoplasm may show clearing, most clearly
seen in clear cell variant of squamous cell carcinoma, which
can resemble vacuolization as might be seen in adenocarcinomas.
Squamous tumors also elicit a variable stromal response
consisting of loose fibroblastic tissue with an inflammatory
component that may include plasma cells, macrophages,
and lymphocytes. In more poorly differentiated carcinomas,
there is less keratinization and intercellular bridges may be
difficult to identify. Here, the overall epithelioid architecture
of the tumor cells is important for diagnosis. In these
cases, there may be maturation of cells from a basilar zone
to a central area where there is loss of the typical verticality
of the epithelium.
Several variants of squamous cell carcinoma are described,
which may mimic other tumors either clinically or histologically.
A papillary variant of squamous cell carcinoma, for
example, may present as an exophytic endobronchial mass
and histologically show a prominent in situ pattern. While an
invasive component is usually present, a superficial biopsy may
not be diagnostic of invasive tumor.
A small cell variant of squamous cell carcinoma may
mimic neuroendocrine tumors including small cell carcinoma.
Differences in nuclear pattern and mitotic activity
are helpful in distinguishing these two tumors, as are
neuroendocrine immunohistochemical stains. If sampled
sufficiently, diagnostic squamous features (i.e., keratin,
intercellular bridges) can be identified focally in basaloid
squamous carcinomas.
Another consideration in the differential diagnosis is the
distinction of squamous cell carcinoma from SCLC (discussed
later). The nuclear architecture of squamous carcinoma uniquely
contains large and atypical nucleoli that are not present in small
cell carcinoma. There is also clumping of nuclear chromatin
that permits distinction from the chromatin of SCLC.
Typically, squamous carcinomas spread directly through
and replace tissue at the interface between normal lung and
carcinoma. Squamous carcinomas can frequently be found
spreading through the alveolar septae rather than along the
surfaces of the alveolar walls as is typically observed in bronchioloalveolar
carcinomas described later. However, a minority
of tumors may also spread through the alveoli.