I have written about this subject- the potential link between thyroid cancer and breast cancer- before. Lately, I have seen more posts ,on the thyroid cancer websites that I visit, from women who have or had both kinds of cancer. I myself had thyroid cancer in 2010, and then breast cancer in 2015. Since no one else in my family has ever had either thyroid or breast cancer, and I did not have any of the known risk factors for breast cancer, I have been curious as to if there is some connection between the two cancers. I have found some new articles about the possible link between these two cancers.
In an article published by " Cancer Epidemiology, Biomarkers and Prevention- A Linkage Between Thyroid and Breast Cancer: A Common Etiology?", by Eric Bolf, Brian Sprague and Frances Carr, published in April, 2019, the evidence seems to point to a link. In the abstract the authors conclude that " breast and thyroid cancers are two malignancies with the highest incidence in women. Women with thyroid cancer are at risk for subsequent breast cancer, and women with breast cancer have an increased incidence of later development of thyroid cancer, suggesting a common etiology. This bidirectional relationship is reported worldwide; however, the underlying reasons for this co-occurrence are unknown."
The above mentioned article cites breast cancer as the most commonly diagnosed cancer in women, with over 250,000 cases per year. Breast cancer survivors are at increased risk for the development of a second primary cancer- frequently thyroid cancer.
Thyroid cancer, which also predominantly affects women, has more than TRIPLED over the past decades, including the aggressive variants. The risk of a second primary cancer , most often breast cancer, in thyroid cancer patients is increased. In an article published by " Clinical Thyroidology", it is stated that patients with a diagnosis of thyroid cancer have a 33% increase in risk of having a second primary cancer.
In the article published by " Cancer Epidemiology, Biomarkers and Prevention", mentioned above, " women with breast cancer are 2-fold more likely to develop future thyroid cancer and women with thyroid cancer have a 67% greater chance of developing breast cancer than the general population. "
Radiation, which is sometimes a treatment option for breast cancer, is a well documented risk factor for cancer and is a major risk factor for thyroid cancer. Some clinical studies have shown that the radiation used in breast cancer treatment alter the thyroid's ability to produce hormones.
Another concern, and a hot topic on thyroid cancer websites, is the association of the administration of RAI, especially in large doses, and the development of breast cancer. According to the article mentioned above, I-131 treatment for thyroid cancer revealed no association between developing breast cancer. A recent study found no difference in breast cancer risk between patients who where treated with I-131 and those who were not.
This study concluded that there is strong evidence of a relationship between thyroid cancer and breast cancer. The study noted that the etiology of these cancers and possible causative factors are just beginning to be studied. Further investigation is certainly needed.
In another paper by Nielsen, White, et, al, entitled " The Breast-Thyroid Cancer Link: A Systematic Review and Meta-Analysis" there is an interesting discussion of the use of RAI in thyroid cancer and the possible risk of breast cancer. Some people think that thyroid cells ( cancerous or not) are the only cells in the body that could pick up the RAI dose. Actually, other organs that concentrate or eliminate iodine include: salivary gland, stomach, small intestine, bladder, and bone marrow. The mammary gland also may be able to concentrate iodine. This study comments that while the RAI delivered to breast tissue is low, it can not be ruled out as a possible carcinogen for breast tissue. Interestingly enough, the study found that there was no significant dose-effect relationship of RAI on breast cancer risk. In other words, just because a patient had a large dose of the RAI, there was no greater risk of developing breast cancer than someone who had a small dose. Further, " the risk of breast cancer following thyroid cancer was significantly elevated in BOTH RAI and non-RAI patients."
In the Nielsen and White study, a shared genetic susceptibility was postulated as a causative factor in the higher rates of a second primary cancer in both breast and thyroid cancer patients. Perhaps the patients have an unidentified mutation. More studies clearly need to be conducted on the role of genetics and the development/ relationship of these cancers.
My personal conclusions after reading through several papers and studies on the link between thyroid and breast cancer are as follows:
1) Yes, there is definitely a link between thyroid and breast cancer.
2)More studies need to be conducted to determine cause(s) of the link.
3) I am not convinced that RAI does not pose somewhat of a risk of developing breast cancer, HOWEVER, I would still have made the decision to receive the RAI in the treatment of my thyroid cancer. I had an aggressive thyroid cancer, stage three, and I feel like had I not had the RAI, it would have returned. I would have tried to lobby for a somewhat lower dose, despite the study that said no dose-effect was found in the RAI dose and the development of breast cancer. This is primarily because I have had and continue to have salivary gland issues. This link is a proven one.
4) Had I known at the time that I had thyroid cancer, eleven years ago, that this link with developing breast cancer existed, I would have had mammograms every 6 months, not yearly. My breast cancer( invasive lobular) was an aggressive one and to quote my oncologist , " a sneaky one". It does not need lymph node involvement to reoccur. Early detection saves lives. Every woman should get, at the very least, yearly mammograms, especially if one has had thyroid cancer.
5) For those women who have had breast cancer, please check your necks! Follow up with a physician who will monitor your thyroid- blood work, scans, etc. Please tell your physician about any changes in swallowing, fullness in the throat area , or just feeling extremely tired or " off". Early detection in thyroid cancer is important, also. Thyroid cancer is NOT THE GOOD CANCER! Some types are more aggressive than others and the effects of living without a thyroid are challenging and life long.
6) As I mentioned earlier, there are so many women who have written in thyroid cancer websites, that they had both thyroid and breast cancer. From my own experience, I suspected a link between the two. Until more studies are conducted, our best option is to be our own best patient advocate. Early detection is a key factor in our well being.
