world cancer report - iarc

Fig. 5.137 Age-specific incidence of thyroid cancer in men and in women in the Ukraine, Iceland and
Germany. Incidence is higher in women and shows a marked peak.
are high in mountainous areas, such as
the Alps, Andes, and Himalayas, where
severe iodine deficiency was or still is
common [4]. However, several high-risk
populations live on islands (such as
Hawaii and Iceland), where iodine intake
is generally high. The relationship
between iodine intake and risk of thyroid
cancer appears to be complex, since both
deficiency and excess may inhibit the synthesis
of thyroid hormones and cause
goitre [5]. The two main types of thyroid
carcinoma (papillary and follicular) may be
linked to iodine-rich and iodine-deficient
diets, respectively [6]. Other dietary factors,
including cruciferous and goitrogenic
vegetables [7], may play a role in thyroid
carcinogenesis.
Thyroid cancer occurs approximately
three times more frequently in women
than in men, reaching a maximum at
about age 45. Hormonal factors may play
a role in etiology. Results from epidemiological
studies, however, have been inconsistent:
some have found an association
between parity and risk of thyroid cancer
while others did not. The most current
data suggest that menstrual and reproductive
factors are weakly related to thyroid
cancer risk [8]. Apart from irradiation
in childhood, goitre and benign nodules
258 Human cancers by organ site
are the strongest risk factors with a relative
risk of approximately 3 and 30,
respectively [9]. The role of hypothyroidism
and hyperthyroidism is less clear.
Detection
Thyroid cancer commonly causes no obvious
symptoms in its early stages. The vast
majority of cancers become clinically evident
as thyroid nodules. However, only a
minority of all thyroid nodules is malignant.
Many nodules are found in asymptomatic
patients on physical examination of
the neck. Some cases have a history of
rapid increase in size and/or pain in the
region of the nodule. Hoarseness, dyspnoea
and dysphagia reflect local invasion
of the recurrent laryngeal nerve, trachea
and oesophagus, respectively. A small
subset of patients presents with palpable
cervical lymphadenopathy without an
identifiable thyroid primary. High-resolution
ultrasonography is useful for size
assessment of nodules and for detection
of unpalpable nodules. Differences in
echogenicity, vascularity or tests of thyroid
function cannot distinguish benign
from malignant nodules. The single most
important diagnostic procedure is the fine
needle aspiration biopsy, performed under
ultrasound guidance.
Pathology and genetics
Thyroid follicular cells give rise to both
well-differentiated cancers and also to
poorly differentiated and undifferentiated
(anaplastic) cancers. Well differentiated
cancers are further classified into papillary
and follicular carcinomas and other rare
types. Stromal and immune cells of the
thyroid are responsible for sarcoma and
lymphoma, respectively. Approximately
90% of malignant thyroid nodules are welldifferentiated
cancers.
Papillary and follicular cancers have the
lowest degree of clinical malignancy.
Papillary carcinoma has a propensity to
invade lymphatic spaces and leads to
microscopic multifocal lesions in the
gland and a high incidence of regional
lymph node metastases. Follicular carcinoma
is unifocal and thickly encapsulated.
It has a propensity to invade veins and not
lymphatics.
Thyroid parafollicular cells (C cells) give
rise to medullary carcinomas which usually
produce calcitonin.
Insular (poorly differentiated) carcinomas
are considered to be of intermediate differentiation
and consequently to exhibit
Fig. 5.138 Clinical examination of the thyroid
gland of a child at risk following radioactive exposure
as a result of the Chernobyl accident.
Fig. 5.139 Histopathological features of a papillary
thyroid carcinoma.