Thyroid Cancer in Ontario- no explanation

Cancer Incidence and Mortality in Ontario, 1964-2002 Data quality Risk
factors Prevention and early detection Trends and regional differences
Graphs References
 What is thyroid cancer? (Canadian Cancer Society) The following
report is one of a series describing the incidence of cancer and
mortality in Ontario over the period 1964 - 2002, highlighting trends
and regional differences over the 1990s and early 2000s.

The thyroid gland ranks 7th among cancer sites in Ontario females.
There is a substantial female excess, with an overall female to male
rate ratio of 3:1. The incidence of thyroid cancer has been increasing
over the 1980s and 1990s, although mortality has been declining. The
age-specific incidence in females rises steeply from teenage years to
about age 30 and remains level thereafter, making it the most common
cancer in young women. In males, the increase with age is more gradual.

There is rather striking international variation, with a five-fold
difference between high incidence populations (e.g. Iceland, Hawaii,
Philippines, and Filipinos in Los Angeles) and low incidence areas
(e.g.
United Kingdom, Netherlands, Denmark and India).Ontario has an
intermediate
incidence.1 Within Canada, rates are higher in Ontario, and lower in
the
West.2

Papillary cancer is the predominant histologic type, accounting for 53%
of cases in Ontario. Follicular cancer comprises 17% of all Ontario
cases, while adenocarcinomas and medullary cancers are 11% and 4%
respectively.
Most of the remainder consists of undifferentiated or anaplastic
thyroid cancer which affects mainly the elderly and has poor survival.

Data Quality

Although a high percentage of thyroid cancers in the Ontario Cancer
Registry are microscopically verified (85%), the verification rate
should be close to
100%.2

Occult papillary cancers have been identified in 5-10% of autopsy
specimens, indicating a high population prevalence of cancers that are
generally of no clinical significance.3 Increased medical vigilance
could therefore result in a spurious increase in reported rates.

Risk Factors

Ionizing radiation is a known cause of thyroid cancer, especially
papillary
carcinoma.4 Particularly well-documented is the excess risk associated
with childhood irradiation of the head and neck for conditions such as
tinea
capitis.4 Dietary iodine is felt to play a role as increased rates of
follicular carcinoma tend to be found in iodine-deficient areas, and
elevated rates of papillary carcinoma are associated with iodine-rich
areas.5 The average Canadian consumption of iodine is about six times
the suggested minimum, which may explain why follicular carcinoma is
rare in Ontario, relative to the papillary form. There is an
association with nodular thyroid disease,3 and a high incidence of
follicular and anaplastic carcinoma in those parts of the world where
goitre is endemic due to iodine
deficiency.3,6 Although other dietary evidence is inconsistent, it does
appear that consumption of seafood increases risk, while cruciferous
vegetables (e.g. broccoli, brussel sprouts) are protective.7,8

Factors increasing the level of thyroid-stimulating hormone, such as
endogenous female sex hormones and use of oral contraceptives and other
estrogen-containing medications, appear to be associated with an
elevated risk of thyroid cancer,9 while factors which may inhibit
hormone production, such as cigarette smoking, appear to be
protective.10

Prevention and Early Detection

Palpation and observation of the thyroid have been used as detection
methods for population-based screening programs;11 however, the utility
of such programs is questionable because of the large number of
indolent thyroid cancers in the population. Screening programs using
additional diagnostic tools such as ultrasound have been carried out on
high-risk persons, such as those who received head and neck radiation
as children. However, there is no evidence that the prognosis of cases
detected in these programs is improved, and false positive rates are
high.4

Trends and Regional Differences

The overall incidence of thyroid cancer has been rising in both sexes
by about 6-9% per year over the 1990s. The increase is evident in all
age groups. The increase is mainly due to papillary carcinoma. This may
reflect an increase in therapeutic radiation exposure or an increase in
the rate of detection of occult papillary carcinomas, which are often
of minimal clinical significance.

Thyroid cancer shows considerable geographic variation within Ontario,
with significantly high rates in Toronto for both sexes and Central
West-Peel and Central East for women. Conversely, the rates are
significantly lower in the Central South, East-Ottawa and Northwest for
both sexes and in East-Kingston, Northeast and South West-Windsor for
females. As with international and interprovincial variation, there is
no clear explanation for patterns within Ontario.

<There are some graphs here that don't copy>

References

Parkin DM, Whelan SL, Ferlay J Teppo L and Thomas DB, eds.. Cancer
Incidence in Five Continents. Vol VIII Lyon: International Agency for
Research on Cancer, 2002. (IARC Scientific Publication No. 155 Chen VW,
Wu XC and Andrews PA, eds. Cancer Incidence in North America,
1996-2000. Volume One: Incidence. Sacramento, CA: North American
Association of Central Cancer Registries, April 2003 Norton JA, Doppman
JL. Jensen RT. Cancer of the endocrine system. In: DeVita VT Jr,
Hellman S, Rosenberg SA, edg. Cancer Principles and Practice of
Oncology, 3rd ed. Philadelphia: J.B.|Lippincott, 1989:1269-1344.
Committee on the Biological Effects of Ionizing Radiation. Health
Effects of Exposure to Low Levels of Ionizing Radiation. BEIR V.
Washington: National Academy Press, 1990 Williams ED, Doniach I,
Bjarnason O, et al. Thyroid cancer in an iodine rich
area: a histopathological study. Cancer. 1977;39:215-22 Ron E. Thyroid
cancer. In: Schottenfeld D, fraumeni JF Jr, eds. Cancer Epidemiology
and Prevention, Second Edition. New York: University Press,
1996:1000-21
Kolonel LN, Hankin JH, Wilkens LR, et al. An epidemiologic study of
thyroid cancer in Hawaii. Cancer Causes and Control. 1990;1:223-34.
Ron E, Kleinerman RA, Boice JD Jr, et al. A population-based
case-control study of thyroid cancer. J Natl Cancer Inst. 1987;79:1-12
McTiernan A.M, Weiss ND, Darling JR. Incidence of thyroid cancer in
women in relation to reproductive and hormonal factors. Am J Epidemiol.
1984;120:423-35
Kreiger N, Parkes R, Hill G, et al. Case control study of Thyroid
Cancer:
Examination of Tobacco Smoking as a Risk Factor. Presentation at OCTRF
14th Clinical Cancer research conference; Lake Couchiching, Ontario,
October 1993

Ishida T, Izuo M, Ogawa T, et al. Evalation of mass screening for
thyroid cancer. Jpn J Clin Oncol. 1988;18:289-95.

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Posted by Fig for J

Not right now. Brain & time (esp. brain) a bit shorttoday.
But, a "huge increase", if they're talking %, on a small base, still may not
be that many numbers. Certainly not when compared to, say, breast cancer --
where even a small % change is a lot of people.
bj