Are all Thyroid Cancers the same?

Throughout all the posts Analisa and I have been doing it has become apparent that when talking about thyroid cancer not all are the same.  There is Papillary carcinoma, Follicular carcinoma, and Sporadic and Familial Medullary Thyroid carcinoma, although some occur more often than others (the three being a rare form of thyroid cancer.) So why do we care?  Before I answer I will explain the differences in all of them as provided by the American Cancer Institute1.  

Papillary carcinoma: Most common type of thyroid cancer, 8 out of 10. It is a growth in one lobe of the thyroid, and can spread to the lymph nodes in the neck.  Often not fatal and easily treated.
Follicular Carcinoma: Next most common, about 1 in 10, usually caused because a patient is not getting enough iodine. Not as common to spread to the lymph nodes.  It is also harder to treat and detect.
Medullary Thyroid carcinoma: Develops in the C cells of the thyroid, the cells responsible for secreting calitonin (hormone responsible for controlling calcium levels in the blood.)  Can spread to the lung, liver, and lymph nodes before detection of the tumor in the thyroid.
      Sporadic: Affects one thyroid gland, and is not familial related.
      Familial: Is inherited in every generation of a family and develops early in childhood.  Tumors                            are often found in both lobes of the thyroid. 
All of these cancers arise from different causes and some are hard to detect making it particularly difficult to treat.  The types of thyroid cancer of particular interest for our project are papillary, follicular, and Medullary; all of which have been discussed in previous blog posts.

First, compare papillary and follicular carcinomas.  One is caused because of a lack of iodine in the diet, follicular, and the other can be caused because of radioactive iodine, papillary, as seen in the Chernobyl accident.  It is interesting to see that thyroid cancer can be caused both by too much iodine and too little iodine.  In this study E.D. Willa and colleagues looked at two different areas, Iceland and Northeast Scotland, in a histopatholigical study.  Iceland is known to have high levels of iodine, and Scotland to have low levels of iodine.  The researchers looked at the incidents of thyroid cancer in relation to where the patients lived.

Table 1. Percentage of thyroid cancers in Iceland and Northeast Scotland.

The table on the left shows the breakdown of each type of cancer from the study.  As you can see the percentage of Papillary carcinomas in Iceland is higher (by 17%), which makes sense because Iceland has a higher level of iodine, which is a risk factor for Papillary carcinoma.  But what this study also does is question whether or not follicular cancer has a risk factor of low iodine levels.  If low levels of iodine were a risk factor for Follicular carcinoma you would expect to see a higher level of this type in Scotland, but you do not.  There is not a large difference in the incidents of Follicular cancer in Iceland as compared to Scotland, only a 1% different.  This suggests that there is no correlation between iodine intake and Follicular carcinoma, as suggested by the American Cancer Institute. 
This study also account for Medullary carcinomas, which only account for 5% of the population in Iceland.  This follows suit with the idea that Medullary carcinomas are a rare form of thyroid cancer.  This study could have differentiated the Medullary carcinomas into sporadic and hereditary.  Splitting it would show if there is also a correlation of iodine and this particular type of cancer.  If all 5% were sporadic there would be a positive correlation, but if all 5% were hereditary it would be a negative correlation.
Overall this study shows that high levels of iodine are positively correlated with cancer, but low levels do not necessarily lead to Follicular carcinoma. Knowing exactly what the risk factors are for Thyroid cancer are important to finding accurate methods to prevent the cancer from forming.

Sources
"Thyroid Cancer." American Cancer Institute. Web. May 22.

Williams, E.D. "Thyroid Cancer in an Iodine Rich Area." Cancer. Wiley Online Library. June 2006. Web.  May 22.