Thyroid Cancer

Introduction

Thyroid cancer is three times more common in women than in men.

It is the 5th most common cancer amongst women in the United States, 9th most common in women worldwide, and the 10th most common cancer amongst Singaporean women. The age standardized rate (ASR) for thyroid cancer in Singapore is 5.8 per 100 000, and about 650 new cancers are diagnosed here every year.

Thyroid cancer does not rank within the top 10 cancers in men.

Although thyroid cancer may not be one of the more common cancers in terms of incidence, it is important for two reasons.

First, thyroid nodules, the most frequent presentation of a thyroid cancer, are very common, and a growing number of subclinical nodules are being serendipitously unearthed by the increasing use of medical imaging.

Second, a little known fact, the incidence of thyroid cancer is increasing at about 7% per year, faster than any other solid organ cancer. The cause for this is not known, but this increase spans across thyroid cancers of all sizes, and therefore cannot be attributed to increased diagnostic scrutiny alone.

Alarming as this may sound, three essential and heartening facts should be borne in mind.

1. Despite the increasing number of thyroid nodules being uncovered, the vast majority of nodules, between 90 -
    95% are benign (not cancerous)
.

2. Thyroid cancer is very eminently treatable. Early diagnosis and treatment of thyroid cancers in good risk
    patients results in cure rates of 98 - 99%, a 20-year prognosis that far surpasses any other solid organ cancer.

3. Most patients with thyroid cancer return to completely normal lives, daily routine and physical activity,
    without any long-term cosmetic or functional impairment after treatment. This is in sharp contradistinction to
    many other cancers, certainly other head and neck cancers, where the effects of treatment carry with it
    significant morbidity, and where disease recurrence and mortality are significant problems.


Risk Factors

           RADIATION:

          The most significant risk factor for the development of thyroid cancer is a history of prior radiation to the
            neck. Studies from countries where segments of the population have been exposed to nuclear fallout (like
           Chernobyl) have found that 30% will develop clinically palpable thyroid nodules, and of this group 30 -
           50% of the nodules will be cancerous. Fortunately the majority of patients we see for thyroid nodules do not
           fall under this category.


           AGE:

          Thyroid cancers occur in higher frequency in patients younger than 20 years of age, and tend to behave
           more aggressively in patients above the age of 50.


           SEX:

          Gender is an important consideration as the risk of malignancy in a nodule is twice that in men than it is
           for women.


           SIZE:

          There is evidence to suggest that thyroid nodules greater than 4 cm in diameter are at increased risk of
           harboring a thyroid cancer.


           GENES:

          Genetic causes do not feature prominently for the most common varieties of thyroid cancer, papillary and
           follicular thyroid cancer. They are however associated with a rarer form, medullary thyroid cancer, and
           with inheritable syndromes that link thyroid cancer with other benign and malignant tumors. A thorough
           review of the other organ systems as well as a detailed family history are therefore important in the
           assessment of a patient presenting with a thyroid nodule.


Treatment

There are three main treatment modalities for thyroid cancer.



SURGERY

Thyroid Surgery is the primary modality of treatment for thyroid cancer.

Most frequently the entire thyroid gland is removed, a procedure called a TOTAL THYROIDECTOMY. In situations where the lymph nodes in the vicinity of the gland are involved by the spread of cancer, additional procedures termed a central or lateral neck dissection may be required. A convenient way of understanding the role of surgery in thyroid cancer is that surgery removes "all macroscopic or gross disease", that is, any disease that can be seen and felt.

A thyroidectomy is one of the safest operations performed today by a Thyroid Surgeon. Recovery from thyroid surgery is quick; most patients are discharged from hospital after an overnight stay, with a normal voice, normal swallowing, minimal postoperative pain and complete independent mobility.   

RADIOIODINE

Radioiodine treatment is often, but not always required, and administered 4 – 6 weeks after surgery. Just as surgery removes all "macroscopic" disease, radioiodine ablates "microscopic disease", small cancer cells that cannot be seen or felt, but may lie within the vicinity of the gland, in the tissue fluid or in the surrounding lymph nodes.

The effects of surgery and radioiodine are therefore complementary.

Radioiodine treatment makes use of the physiological understanding that the thyroid gland requires iodine to produce thyroid hormone, and avidly absorbs iodine from the blood circulation. Thyroid cancer cells behave in much the same way. By labeling iodine molecules with radioactive particles, a high dose of localized radiation can be given to any remnant thyroid or cancer cells only, sparing all the other normal tissues. Unlike external beam radiation used in the treatment of a wide variety of other cancers, and which carries with it significant morbidity from inevitable damage to normal surrounding tissues, radioiodine is taken orally and is generally very well tolerated.


THYROXINE SUPPRESSION

After surgery and radioiodine treatment, patients are put on thyroid medication. The thyroxine tablets, taken once a day in the morning have the dual function of replacing the required amount of hormone for the body to function normally, as well as suppressing thyroid stimulating hormone (TSH), produced by the brain. In normal circumstances TSH stimulates the thyroid gland to produce thyroid hormone, but in thyroid cancer patients it also serves the unintended effect of stimulating the thyroid cancer cells as well. Giving the thyroxine tablets at the appropriate dose to suppress the brain's release of TSH and helps keep the cancer at bay.

This third modality of treatment is referred to as thyroxine suppression therapy.


Surveillance

After completing this multi-modality treatment strategy, patients are put on a comprehensive surveillance program, particularly for the first two years. This consists of regular clinic visits where clinical assessments, a neck ultrasounds and blood tests are performed to ensure that the patient is healthy, is on the right dose of thyroxine and is disease free. Subtle changes in the neck ultrasound or blood tests can pick up disease recurrence very early, which in turn leads to timely and effective re-treatment.

The vast majority of patients, however, lead completely normal and active lives, and are cancer free for a good many years.


Prognosis

There are five major types of thyroid cancer, but differentiated thyroid cancer, consisting of PAPILLARY and FOLLICULAR cancer, account for approximately 85% of all thyroid cancers.


The prognostic factors associated with differentiated thyroid cancer are:

          AGE of the patient (young patients fare significantly better than older patients)
          SIZE of the tumor (tumors less than 4cm do better)
          GRADE of the tumor (the appearance of the cancer under the microscope)
          EXTRA-THYROIDAL EXTENSION (direct extension of cancer into the surrounding tissues)
          METASTASIS (cancer spread to remote sites of the body like the lung or bones)


Unlike other cancers where survival statistics are reported on a 5-year basis (5 year survival curves or rates) thyroid cancer survival data is often reported over a 10 or 20-year intervals. This is because patients with thyroid cancer fare vary much better than other cancers, and the natural history of the disease plays out over decades.


Patients with differentiated thyroid cancer are stratified into three risk groups.

          GOOD risk patients (patients younger than 45 years with tumors less than 4cm), which comprise the
            majority of patients with thyroid cancer, have 20-year survival of between 98 - 99%

          INTERMEDIATE risk patients (young patients with large tumors, or elderly patients with small tumors)
           have 20-year survival of approximately 80 - 85%.

          POOR risk patients (elderly patients with large or aggressive tumors) have a 20-year survival of 50-60%.

The Thyroid Head & Neck Surgery Centre