22 December 2021

Surge in thyroid cancer cases may not only be due to overdiagnosis

Written by Dawn Teh

Thyroid cancer sparked significant concern over the past decade as one of the fastest rising cancers in the US and multiple other developed nations like South Korea and the UK.¹

The small, butterfly-shaped gland that sits near the base of the neck releases hormones that affect multiple bodily functions like metabolism, heart rate, breathing.² Thyroid cancer symptoms include a lump that can be felt on the neck, vocal changes, difficulty swallowing, pain in the neck area, and swollen lymph nodes.³

From 1975 to 2009, there was a three-fold increase in thyroid cancer in the US, with the sharpest increase being in the early 2000s.⁴ An average of 4.6 per 100,000 people in the US were diagnosed with thyroid cancer annually between 1974 to 1977. This sharply increased to 14.4 cases per 100,000 people a year from 2010 to 2013.⁵

Narrowing down the cause of this dramatic rise in numbers has been the interest of many experts. And because of better medical technology and increased frequency of cancer checks, some doctors had initially pinpointed overdiagnosis as the main cause.

While that explanation has held for several years, other experts have suggested that isn't the end of the story. In fact, there may be other environmental causes that need to be further investigated as well.  

Inflated thyroid cancer numbers: The overdiagnosis argument

The red flag for overdiagnosis was signalled when experts looked at incident rates alongside mortality rates. They realized that while more cases of thyroid cancer were being identified, the number of people dying from it each year was remaining relatively stable.

This didn't make sense to medical professionals because increased detection should correlate with reduced deaths. Especially given that the survival rate for thyroid cancer is pretty high — 98% of those in the early stages are expected to live past 5 years after treatment according to the American Cancer Society.⁶ ⁷

This led to the conclusion that doctors may be overdiagnosing patients — this is when a condition is diagnosed when it might not have even caused any symptoms or issues in the first place.

Why were so many cases suddenly being picked up? In the case of South Korea where the most dramatic increase was seen, this was due to population-wide health screenings. Over the course of 2 decades, thyroid cancer rates had increased by fifteen times.

In the US, a similar increase was seen. But unlike South Korea, this was most likely due to improvements in medical technology (like diagnostic ultrasound to fine-needle biopsies) that are better able to pick up tumours. In some cases, it might even be detected accidentally during a test for an unrelated condition.⁷ ⁸ ⁹

This meant that smaller thyroid tumours (less than half an inch) could now be discovered and treated.

Had these cases been left alone, many of them may have never been noticed by patients and would have largely stayed the same size. It's estimated that a third of people can have thyroid cancers that do not cause harm.² ⁷

But it doesn't just stop there. When there is overdiagnosis, overtreatment usually follows. In this case of thyroid cancer, the default was for doctors to treat all thyroid cancers, no matter how small it was. So people were undergoing invasive procedures that could potentially cause even more problems.

With thyroid removal, there is the risk of vocal cord damage. While hormone therapy can lead to osteoporosis and a host of other short-term side effects. The latest research also shows that radioactive iodine treatment can lead to increased long-term risk of other solid cancers.⁷ ¹⁰ ¹¹

For an asymptomatic thyroid cancer patient evaluating these costs of treatment, adopting an approach of regularly monitoring their condition may not sound so crazy. Especially when the mortality rate has remained relatively constant despite people aggressively treating all thyroid cancers for the past decade. But in the past, many might not have even been given the chance to do "watchful waiting" as most physicians would have pushed for treating all detected cancers.²

The wider problem of overdiagnosing cancer

Unfortunately, overdiagnosis is not just isolated to thyroid cancer. Experts are increasingly concerned about how it's impacting the way we identify and treat other cancers as well.

Some experts estimate that about 25% of breast cancers detected through mammograms, 50% of lung cancers detected by chest x-rays and/or mucus testing, and 60% of prostate cancers discovered through the prostate-specific antigen blood test are overdiagnosed.¹²

This has sparked many changes in diagnosis and treatment advice. For example, prostate cancer screening has been scaled back in places like the US and UK now that most have realised most men will die with it and not from it.¹³ ⁷ ¹⁴

In 2015, new recommendations were released from the American Thyroid Association (ATA) suggesting that early-stage thyroid tumours be monitored closely of their progression first instead of removed immediately.¹⁵

With a change in screening policy, we’ve seen a stabilization in thyroid cancer numbers over the past few years. From 2013 to 2017, the prevalence rate for men has remained relatively stable. And for women, it has declined by 2% each year.⁶

The American Cancer Society's most recent estimates in 2021 state that the incidence rate in the US is at about 44,280 new cases, and 2,200 deaths.¹⁶

While the plateau in detected cases may signal that we've overcome the overdiagnosis issue, some experts aren't totally convinced. This is because several studies have shown that about half of the changes that we see in thyroid cancer rates in the US may not be attributed to overdiagnosis.¹⁷ Therefore, there have been calls to further investigate other factors behind the true rise in thyroid cancer incidence.

As David Goldenberg, MD, FACS, professor and chair of the Department of Otolaryngology — Head and Neck Surgery at Penn State College of Medicine, has written in an editorial writeup for Cancer: “Although overdiagnosis certainly occurs, this does not absolve us of the duty to seek the cause of the real rise in thyroid cancer. In this light, we must closely examine the factors that may be causing a true increase in thyroid cancer, as well as differentiating which thyroid cancers will behave in an indolent fashion vs. an aggressive fashion.”¹⁸ 

Overdiagnosis may not be the end of the story

So if overdiagnosis is only part of the problem, then what else is contributing to the rise in thyroid cancer numbers? Some have cited environmental pollutants and iodine intake levels as possible factors.¹⁹ ²⁰

But stronger evidence is suggesting that obesity might have something to do with rising cases — a risk factor that already has a well-established link between obesity and other cancers like breast cancer, prostate cancer, colon cancer, and more.²¹

Obesity

Similar to the trend that we have seen in thyroid cancer, one study found that the rates of obesity in the US also had a 3-fold increase between 1960-2012 with the sharpest increase occurring between 1980-2010.²²

Furthermore, a 2019 study that tracked more than 11 million Americans over the course of 4 years found that thyroid cases increased significantly in participants with higher waist circumference (WC) and body mass index (BMI).

The incidence rate also significantly decreased for those that were obese but then lost weight. And conversely, the thyroid cancer incidence rate increased among those who were lean and then became obese over the course of the study.²¹

Another 2012 study published in the journal JAMA Surgery, has shown that as patient body mass index (BMI) increases, so do individuals' risks of developing more aggressive forms of thyroid cancer. As a result, these researchers have even suggested that thyroid cancer screening may be warranted for obese individuals.²³

Globally, the proportion of thyroid disability-adjusted life-years (DALYs), or the number of years lost due to disease, that is attributable to high BMI is highest in developed regions.²⁴

Radiation

In addition to obesity, the increase in radiation exposure from medical imaging has been cited as another cause of the rise in thyroid cancer cases. And most of this increase comes from the increased use of medical imaging.²⁵ Since the 1980s, exposure to radiation from medical imaging in the US has increased by almost 600% from a mean of 0.54 mSv per capita dose to 3.0 mSv in 2006.²⁶

One study found that exposure to CT scans was related to an increased risk of developing thyroid microcarcinomas (tumours less than or equal to 1cm). And people who had dental x-rays more than once a year were at an increased risk of thyroid cancer compared to those who had it every 5 years.¹⁷

However, interpreting this information must also be done with caution as this is only 1 study — and there are others demonstrating contradicting evidence.²⁷ It's also important to bear in mind that when doctors request for medical imaging to be done, there is usually some benefit that counters that risk — like diagnosing a potentially life-threatening illness. So declining a scan may result in much more detrimental effects than going through with it.

Additionally, the radiation risks of these scans can vary from person to person. For example, a child will be much more sensitive to such risks compared to an adult as their cells are still dividing more quickly.²⁷ ²⁸ Therefore, it's always important for patients to be properly educated of the costs and benefits of undergoing any medical imaging based on their individual situation. 

To screen or not to screen for thyroid cancer?

Taken together, the latest investigations about rising thyroid cancer cases suggests that it's most likely due to a mixture of overdiagnosis and several environmental factors.

In practical terms, should asymptomatic people be regularly screened for thyroid cancer? Probably not. But refraining from population-wide screening means that the "early detection or treatment always equates to better outcomes" perception will need to change. And this comes with its own set of problems.

For many, this idea can be difficult to grasp as discovered in one study that showed how patients who choose not to investigate or treat small thyroid tumours can face negative responses from loved ones. And unfortunately, this can lead to increased anxiety, isolation, and a reluctance to monitor their condition closely.²⁹ For some, they might even receive opposition from their own doctors as some maintain a more conservative stance towards any cancer.

As Dr. Raymon Grogan, a specialist in thyroid gland diseases at the University of Chicago Medicine has said in an interview with Medscape, "We don't know who will have a bad cancer. If papillary thyroid cancer is diagnosed, we can't just let it sit there." He further explained that this is because we still don't have the right prognostic tools to determine when cancer will turn harmful or not.³⁰

Therefore, we should not assume that most thyroid tumours are small and aren't harmful. Certain people may be at higher risk of more aggressive cancers — like those battling obesity or are frequently exposed to radiation from medical imaging and other causes. So these are the groups that might benefit from thyroid screening.

While navigating conflicting views from experts can be confusing for patients, the main takeaway appears to be that a one-size-fits-all approach to diagnosing and treating thyroid cancer isn't the most effective strategy. There needs to be transparent communication between doctor and patient about their individual risks and the different options (along with their associated costs and benefits) relating to screening and treatment of thyroid cancer. This will hopefully help to minimize both overdiagnosing and failing to diagnose this highly misunderstood condition.

Sources:
  1. Overdiagnosis and overtreatment of thyroid cancer: A population-based temporal trend study (2017)

  2. Thyroid cancer: Epidemic or overdiagnosis? | Endocrine Web

  3. Thyroid cancer rates tripled since 1975: What's driving the diagnoses? | CBS News

  4. Current thyroid cancer trends in the united states (2014)

  5. Thyroid cancer rates triple, and scientists look for cause | Live Science

  6. Cancer facts & figures 2021 | American Cancer Society

  7. A new study shows why cancer screening can be bad for your health | Vox

  8. Overdiagnosis raises concerns as the incidence of thyroid cancer cases rises globally | Targeted Oncology

  9. Thyroid cancer rates triple, and scientists look for cause | Live Science

  10. Hormonal therapy for thyroid cancer | Canadian Cancer Society

  11. Second cancers after thyroid cancer | American Cancer Society

  12. Overdiagnosis in cancer (2010)

  13. Screening | Cancer Research UK

  14. American cancer society recommendations for prostate cancer early detection | American Cancer Society

  15. 2015 American thyroid association management guidelines for adult patients with thyroid nodules and differentiated thyroid cancer (2015)

  16. Key statistics for thyroid cancer | American Cancer Society

  17. Diagnostic x-ray exposure increases the risk of thyroid microcarcinoma: a population-based case-control study (2016)

  18. We cannot ignore the real component of the rise in thyroid cancer incidence | Research Gate

  19. Do polybrominated diphenyl ethers (PBDEs) increase the risk of thyroid cancer? (2009)

  20. Iodine intake as a risk factor for thyroid cancer: a comprehensive review of animal and human studies (2015)

  21. Weight change is significantly associated with risk of thyroid cancer: A nationwide population-based cohort study (2019)

  22. Trends in Thyroid Cancer Incidence and Mortality in the United States, 1974-2013 (2017)

  23. Risk of advanced papillary thyroid cancer in obese patients (2012)

  24. Global burden of thyroid cancer and its attributable risk factors in 204 countries and territories from 1990 to 2019 (2021)

  25. Radiation risk from medical imaging | Harvard Health Publishing

  26. Medical radiation exposure in the U.S. in 2006: Preliminary results | Research Gate

  27. Radiation risks of medical imaging: Separating fact from fantasy | Radiological Society of North America

  28. Radiation risk of medical imaging for adults and children | Inside Radiology

  29. Patients who choose no intervention for small thyroid cancers report lack of support | NIH: National Cancer Institute

  30. Rise of thyroid cancer is oversimplified, say critics | Medscape

The author, Dawn Teh, is a health writer and former psychologist who enjoys exploring topics about the mind, body, and what helps humans thrive.

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