SMALL CELL LUNG CARCINOMA

The separate and unique histological features of SCLC were first
recognized in 1926 by Bernard who introduced the tumor as oat
cell sarcoma . 132 This subcategory took on greater importance when
it was shown that the response to chemotherapy in these tumors
differed substantially from the response of other tumor types with
the SCLC being particularly sensitive to mitotic inhibitors. 133 It
is for this reason that the main dichotomy of clinical relevance in lung cancer pathological diagnosis remains the distinction between
SCLC and all other forms of lung carcinoma (NSCLC).
Histology of Small Cell Lung Carcinoma SCLC
is an invasive carcinoma composed of small ( 21 microns
or 3 lymphocyte diameters) cells with scanty cytoplasm
(Fig. 22.8). Histologically, these tumors are distinguished by
their finely granular nuclei ( salt and pepper chromatin), by
their small and relatively inconspicuous nucleoli, and by the
tendency for the nuclei to become easily deformed by contact
with other cells and other structures ( nuclear molding ).
These tumors are highly proliferative and rarely is the mitotic
rate less than 10 mitoses per 10 high-power fields (hpf )
so that virtually every hpf contains one or more mitoses.
The neoplastic cells in this tumor type are fragile, and crush
artifact is common particularly in small biopsy samples.
Lymphocytes and other inflammatory cells can also undergo
crush artifact and it is therefore of some importance that definitive
diagnosis be based on examination of well-preserved
cells where nuclei are clearly visible. A unique feature of
SCLC is the Azzopardi effect consisting of an accumulation
of chromatin in the vascular walls supplying the tumor. In
a great majority of cases, the diagnosis of SCLC is obvious
by histological examination alone. However, in some cases,
immunohistochemical studies can serve to buttress the diagnosis
and provide confirmation of diagnosis.
Within the category of SCLC, there is considerable variation
in cell size and cellular configuration. For several years, subsets
of SCLC were included in the SCLC category. The 1967 WHO
classification included oat cell carcinoma and an intermediate
cell variant that differed from classical oat cell carcinoma in cell
size and amount of cytoplasm. These categories proved to be
of no prognostic significance and the two categories were combined
in the latest classification into a single small cell category.
This tumor type comprises 15% to 20% of all lung cancers but
its frequency may be decreasing. 59,134 Typically, small cell carcinomas
present as an endophytic lesion in a central bronchus
with mediastinal lymph node (LN) metastases.
Cytology Success in identifying SCLC by cytological examination
is partially dependent on the type of specimen available
for examination. SCLC may be readily identified bronchial
washings and brushings where the cells are well preserved
and in these cases, cytology may be crucial in arriving at a
definitive diagnosis. However, SCLC has been infrequently
identified in screening trials 135,136 in part because SCLC is not
well preserved in expectorated sputum and also perhaps because
the disaggregation process may damage the fragile cell of
SCLC. The cytological features of SCLC correspond to what is
seen in well-preserved tissue sections. Single cells or small clusters
of cells less than 21 microns in diameter have a high N/C
ratio with only a thin sometimes barely visible rim of cytoplasm
around and enlarged nucleus (Fig. 22.8). Nuclear fragmentation
is often present in association with better-preserved
cells. The nucleus again has finely granular (salt and pepper)
chromatin and small, inconspicuous nuclei. In small clusters of
cells, nuclei are often molded against one another. Nuclear features
are crucial and excellent preservation of cells is required
for diagnosis. Cells that may resemble SCLC are basilar cells
of the respiratory mucosa that usually are smaller and more
uniform that the cells of SCLC. Lymphocytes may also cause
confusion but usually only in poorly preserved specimens.
Diagnostic Immunohistochemisty of Small Cell
Lung Carcinoma Immunohistochemical stains can be
used to verify the neuroendocrine nature of the tumor or help
distinguish SCLC from other NSCLC (Fig. 22.8). SCLC almost
always expresses CK but the amount of this intermediate
filament may be quite small, particularly in less than that optimally
preserved specimens. CK expression serves to distinguish
SCLC from other small blue cell tumors such as lymphoma,
which occasionally can mimic SCLC. SCLC, like adenocarcinomas
of the lung (see succeeding discussion), is usually positive
for the primitive lung differentiation gene TTF-1 with
several studies documenting expression rate in at least 85%
of these tumors. 71,137–144 In specific contexts, TTF-1 may
be useful in distinguishing SCLC from small cell tumors of
other sites. For example, while this marker is expressed in a
large percentage of SCLC, histologically similar Merkel cell
tumor of dermal origin and small cell carcinoma of ovary are
virtually always negative for this marker. 140–142,144–146 Small
cell cervical and colon carcinomas are occasionally positive
(10% to 15% 140,146,147 ) but up to 50% of transitional carcinomas
of bladder 140,147–149 and over 70% of esophageal small cell tumors are reported positive. 150 Reported results for small
cell prostate carcinomas have been variable with some studies
indicating high frequency of expression 147 and other low
frequency of expression. 140,151 The utility of TTF-1 in distinguishing
site of origin of unknown primary is therefore highly
context dependent.
A distinguishing feature of SCLC is its expression of neuroendocrine
markers including neuron specific enolase, synaptophysin,
neural cell adhesion molecule (NCAM) (CD56),
and Leu-7 (CD57). 152–154 Approximately 50% of SCLCs also
express chromogranin A in sufficient quantity to be detectable
in conventional immunohistochemical tests so that in doubtful
cases a negative chromogranin is uninformative. The quantity of
chromogranin in SCLC is usually less than that present in carcinoid,
so that in cases with strongly positive chromogranin stains
carcinoid tumor should be excluded. A salient feature of SCLC
is its high growth fraction; as determined by Ki-67 labeling procedures,
the growth fraction usually exceeds 50%. Ki-67 may
be helpful in distinguishing SCLC from other neuroendocrine
tumors of the lung. A negative differentiating marker is EGFR,
which is expressed in NSCLC but not in SCLC. 154 Finally, it
has been reported that c-kit receptor (CD117) is nearly always
expressed by high-grade neuroendocrine carcinomas including
SCLC and LCNEC, 155–158 an observation that has been fully
exploited neither diagnostically nor therapeutically. The expression
of CD117 is apparently not associated with mutation of
the c-kit receptor gene 159 and no relationship to survival or response
to chemotherapy has been shown to date. 160
The application of immunohistochemical staining procedures
to lung tumors has revealed considerable overlap in staining
properties. It is known for example that about 20% of adenocarcinomas
are focally positive for NCAM, a molecule that
otherwise is almost universally expressed by SCLC. This suggests
a degree of phenotypic plasticity in lung tumors that is confirmed
in gene profiling studies (see discussion that follows).
Molecular Pathology in Small Cell Lung Carcinoma
The unique histological and immunohistochemical features of
this tumor are a reflection of underlying genetic changes that
are increasingly better defined. SCLC was among the first of the
lung tumor types in which genetic changes were defined. These
changes included structural chromosomal abnormalities (deletion
[3p(14–23) 161 ] and the mutations described and listed in
Table 22.4. The pathways most frequently affected by mutation
involve cell regulation and tyrosine kinase signaling genes including
KRAS 84,162 and EGFR 84,163 are rarely mutated. In the succeeding
discussion, genetic lesions are grouped by pathway as in
the previous discussion of squamous carcinoma.
Cell Cycle Genes A number of the mutations and chromosomal
rearrangements that affect cell cycle genes are present
in SCLC.
RB1/CDKN2A (p16) RB1 was first identified as a tumor
suppressor gene in retinoblastoma. Knudson’s “two-hit” model
of carcinogenesis, whereby a dominant tumor suppressor gene
is inactivated by two mutational events eliciting tumor formation,
was formulated in 1971 to explain retinoblastoma
tumorigenesis. 164 Fifteen years later, the retinoblastoma gene,
RB1 , was cloned 165 and the relevance of RB1 to lung cancer
was demonstrated.
RB1 encodes a phosphoprotein that binds to the transcription
factor E2F. Phosphorylation of RB1 releases E2F and
transition from the G1 to S phase of the mitotic cycle occurs.
RB1 is therefore an important cell cycle regulator and tumor
suppressor gene. Loss of RB1 results in an increase in cell proliferation
and more rapid tumor growth.
Structural changes in the RB1 gene in SCLC lines and tumors
have been reported and complete absence of RB1 protein
has been reported to affect all SCLC. 166–168 The mechanism
of RB1 inactivation in human tumors is still not completely
understood. Although RB protein is absent in SCLC, DNA
sequencing has identified mutations in only a minority of tumors
and these consist primarily of deletions resulting in frame
shifts or stop codons. 169
The mutational pattern evident in cell cycle genes in SCLC
is distinct from that in NSCLC with the latter frequently affected
by loss of p16 with and overexpression of cyclin D1 rather
than the near universal loss of RB1 168 observed in SCLC.
TP53 TP53 is more frequently mutated in SCLC than in
the other non–small cell forms of lung cancer. Loss of TP53
function with associated loss of cell cycle checkpoint function
and impairment of apoptotic pathways undoubtedly contributes
to the aggressive growth and hypermutability that are features
of this tumor.
Gene Expression Profiles In virtually all the nucleotide
microarray analyses, small cell tumors emerge as a separate
phenotypic category with numerous gene expression difference
with the non–small cell tumors. 170 Available microarray data
for SCLC has considerably expanded the number of potential
biomarkers that may be useful in the differential diagnosis
and detection of SCLC. Moreover, these studies have indicated
that many of the genes expressed in SCLC are also found during
lung differentiation (see succeeding discussions).
To date, however, the molecular features of SCLC have
not been sufficiently definitive nor have they added sufficient
independent information to histological diagnosis to justify
the general application of diagnostic molecular testing in this
tumor. The diagnosis of this neoplasm therefore continues to
rest on histological examination supplemented by immunohistochemical
studies in difficult cases.