Small cell carcinomas (SmCC) are
malignancies that derive from neuroendocrine cells. The World Health
Organisation (WHO) in 2010 classified SmCC as a subgroup of neuroendocrine
carcinoma (NEC) which accounts for approximately 0.6 percent of all colorectal
cancers. NEC has two histologic subgroups, namely large cell carcinoma (LCC)
and small cell carcinoma (SmCC). LCC can be difficult to distinguish from
poorly differentiated adenocarcinoma.
Despite the fact that the
gastrointestinal tract (GI) has the largest number of neuroendocrine cells in
the body it is unusual for such malignancies to occur here. First described in
1952, nearly 650 cases of gastrointestinal SmCC have been reported in the
literature until 2007. The oesophagus is the commonest
site of incidence of GI SmCC (53%) followed by the colon (13%), stomach
(11%), gallbladder (8.4%) and rectum (7.3%). The aggressive neoplastic nature
of SmCC is characterised by rapid growth, high mitotic proliferation rate
(>20 mitotic figures per 10 high-power fields [HPF] or a Ki-67 index
>20%), early dissemination and poor prognosis.
SmCC of the rectum is an
extrapulmonary small cell carcinoma (EPSmCC) entity, which morphologically
resembles pulmonary small cell carcinoma (PSmCC). SmCC was originally thought
to derive from amine-precursor uptake and decarboxylase cells (APUD). However,
the pluripotent stem cell, with its potential for variable differentiation, is
now the most widely accepted theory [6-9]. Although it is important to rule out
a PSmCC as the primary tumour the treatment paradigm for EPSmCC is extrapolated
form the platinum based therapy used to treat PSmCC .
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