Lung Cancer.pdf

Pathologic Evaluation of Lung Cancer 69
noma and NSCLC for classification of epithelial tumors diagnosed on the
basis of small tissue samples.
Squamous Cell Carcinoma
Squamous cell carcinoma is the classic smoking-related lung carcinoma. It
usually arises from the bronchi, but rarely tumors originate in the peripheral
lung parenchyma. Squamous cell carcinoma used to be the most common
type of lung cancer, accounting for 25% to 40% of all lung tumors, but
has now been overtaken by adenocarcinoma, which is the most common
lung cancer in many countries around the world.
Squamous cell carcinoma by definition shows polygonal tumor cells
with intercellular bridges with or without keratinization. There are 4 histologic
variants of this cancer: papillary, clear cell, small cell, and basaloid.
It is important to recognize these variants as squamous cell carcinoma and
to avoid confusion between these variants and the other subtypes of lung
cancer. The small cell and basaloid variants may be misdiagnosed as small
cell carcinoma in small biopsy samples. These histologic variants have no
prognostic significance, except possibly for basaloid squamous carcinoma,
which has been reported to behave more aggressively than the
other variants.
Neuroendocrine Tumors of the Lung
There are 4 well-recognized neuroendocrine tumors of the lung: typical
carcinoid, atypical carcinoid, small cell carcinoma, and large cell neuroendocrine
carcinoma (Travis et al, 1998a,b). Until recently, a slew of names
had been used for these tumors, including neuroendocrine carcinoma,
malignant carcinoid, intermediate cell neuroendocrine carcinoma, and peripheral
small cell carcinoma resembling carcinoid. With the publication
of the latest WHO classification system, it is hoped that the confusing
older terminology will become a thing of the past.
Neuroendocrine tumors of the lung form a morphologic spectrum,
with typical carcinoid at one end and small cell carcinoma and large cell
neuroendocrine carcinoma at the other end. The 4 types of neuroendocrine
carcinoma have in common certain characteristic features that are
apparent on light microscopy, electron microscopy, and immunohistochemical
staining and with molecular biology techniques. However, the 4
tumor types differ in terms of incidence, epidemiology, clinical features,
response to therapy, and survival. Of course it needs to be stressed that all
these tumors, including typical carcinoid, are malignant tumors, albeit
with differing biological potential.
Small cell carcinoma is the most common neuroendocrine tumor, comprising
20% to 25% of all lung cancers; the other 3 tumor types are relatively
uncommon, together comprising only 2% to 3% of all malignant
lung tumors. Typical and atypical carcinoids usually affect younger patients
than do small cell carcinoma and large cell neuroendocrine carci-