Many data registries have shown an
increasing trend in thyroid cancer incidence, however it is questionable as to
whether the numbers are due to an actual increase in incidence or due to and
increase in detection of pre-existing cancers because of an improvement and
increase in cancer screening techniques. Dr. Louise Davies and Dr. H. Gilbert
Welch have investigated the cause of this increasing trend through data
analysis and have published their findings1. This pattern is
further investigated by a separate Q and A article2 published by
Yasuhiro Ito, Yuri E. Nikiforov, Martin Schlumberger, and Riccardo Vigneri.
Both works do not completely rule out the idea that incidence of thyroid
cancer is increasing, however they both support the claim that the trend is
most likely due to an increase in detection of the cancer through improved
diagnostic procedures.
While
the data on thyroid cancer overall seems to point to an increase in incidence,
there are many other patterns within the data that do not correspond with this
assumption. For instance, if it were true that more and more people are
acquiring thyroid cancer, there would be an increase in incidences across all
stages of the cancer, this however is not the case. Looking at Figure 1. 1, Davies and Welch observe that the increase
is mostly due to the increase in smaller sizes and lower stages of cancer1. Furthermore, if there were a true increase,
there would be an increase in suspected risk factors of the cancer, in this
case radiation, which is not the case since radiation exposure is less common
today, as therapeutic radiation for common diseases has not been used since the
1950s1.
Even looking back 50 years ago,
pathologists recorded that thyroid cancers were commonly found during autopsies
even when the deceased had never reported any of the symptoms during their life1. This indicates that there were many more
incidences of thyroid cancer in the past that have gone unreported. The reason
that they are showing up now is due to the improvement in the sensitivity of
diagnostic techniques. These techniques include ultrasound and fine needle
aspiration (biopsy) which are able to detect tumors as small as 0.2cm (symptoms
generally occur for tumors larger than 5cm)1.
The increase is observed in
countries with improved medical care, while in countries of Africa, where the
medical care is not so available, this increase has not been seen. Furthermore
there is a positive correlation between high socioeconomic status and thyroid
cancer incidence. Countries with good healthcare and households with high
socioeconomic status have the resources to make these new diagnostic techniques
available to them and therefore they detect more incidences of thyroid cancers2. 
A true increase would also show an
increase in mortality rates due to thyroid cancer which is not the case as seen
in Figure 21. The stable mortality and
increasing incidence is observed because the slope is mainly due to the
increased detection of small asymptomatic tumors by the sensitive
diagnostic techniques. This is not to
say that all small thyroid tumors are
asymptomatic, more aggressive types of
thyroid cancers (follicular and poorly differentiated) can be symptomatic even
at small sizes, however an increase in incidences among these types is not observed
(Figure 1.)1.
All these factors indicate that the
increase in incidence is due to the increase in cancer detection and not so
much an increase in accumulation of the cancer. The controversy surrounding
this viewed increase in incidence is the problem of overdiagnosis. As
exemplified from the findings of cancers in the autopsies, many of the small
thyroid cancers will never become symptomatic in a persons lifetime, however
today, these asymptomatic cancers are being
treated as they are detected. The main treatment for thyroid cancer is a
thyroidectomy, which is the surgical removal of all or parts of the thyroid
gland3. The risks for this procedure include hypoparathyroidism
and compromised voice quality1, a high
expense for a treatment of an asymptomatic nodule. To decrease overdiagnosis,
the American Thyroid Association recommends that fine needle aspiration
biopsies only be performed on nodules larger than 5mm2. They also recommend that, instead of treatment of smaller
thyroid cancers, yearly or biannual follow ups should be performed, with
surgery only suggested if the cancer shows signs that is it progressing (i.e. size
increase)2.
[1]Davies, Loiuse MD,MS; Welch, Gilbert H. "Increasing Incidence of Thyroid Cancer in the United States, 1973-2002. JAMA, May 10, 2006-Vol 295, No. 18 (Reprinted).
[2]Ito, Y. et al. "Increasing incidence of thyroid cancer: controversies explored" Nat. Rev. Endocrinol. 9, 178-184 (2013); published online 29 Jan 2013.
[3] N.A. "Q and A: Thyroidectomy". American Thyroid Association. August 2008. web. May 11 2014.
