Thyroid cancer starts in the cells of the thyroid gland. It occurs due to uncontrolled growth of the cells that results in the formation of masses of cells called tumours. Thyroid cancers with growth can invade and spread to the tissues of the neck, surrounding lymph nodes or to distant parts of the body. There are many types of thyroid cancers, but the two most common types of cancers of the thyroid gland are papillary adenocarcinoma of the thyroid (78%) and follicular adenocarcinoma of the thyroid (17%).
Primary thyroid cancer: If thyroid carcinoma is not treated, it will continue to grow. The five-year relative survival rate of thyroid cancer depends on the type of cancer. The 5 year survival rate of papillary thyroid cancer is nearly 100% for stage I cancer, which decreases to 50% in cases with stage IV cancer. Similarly, the 5 year survival rate of follicular thyroid cancer is nearly 100% for stage I cancer, which decreases to 50% in cases with stage IV cancer. The outcome, however, of anaplastic (undifferentiated) carcinoma is not as good. The 5-year relative survival rate for anaplastic (undifferentiated) carcinomas is around 7% (stage IV). All cases of anaplastic (undifferentiated) carcinomas are considered to be stage IV.
Recurrent thyroid cancer: When the cancer recurs (comes back) after it has been treated, it is known as recurrent cancer. Studies suggest that most cases of papillary adenocarcinoma and follicular adenocarcinoma of the thyroid recur in the neck; either in the thyroid bed itself or in cervical lymph nodes. Although, most cases of thyroid cancer are easily treatable and recurrence is common. It affects about 20% to 35% of patients with the disease. Studies have shown that recurrence can occur even decades after the initial therapy. In most cases, recurrence occurs in the microscopic cancer, which was not treated with initial therapy. Early identification of recurrent cancer can improve prognosis and response to treatment. The risk of mortality increases with delay in diagnosis of recurrent cancer.