Only a very small proportion of thyroid nodules can be described as cancer, and most of them are benign. According to cancer incidence data from national estimates in 2012, the incidence of thyroid cancer is approximately three times higher in females than males (Cancer Information Service, National Cancer Center, Japan). Compared to other cancers, thyroid cancer progresses more slowly and is easier to cure.
Thyroid cancers include papillary carcinoma, follicular carcinoma, poorly differentiated carcinoma, medullary carcinoma, and undifferentiated carcinoma. According to national aggregate statistics (Japanese Society of Thyroid Surgery 2014), papillary carcinoma is by far the most common form of thyroid cancer, comprising 92.5% of all cases. Follicular carcinoma, medullary carcinoma, and undifferentiated carcinoma account for 4.8%, 1.3%, and 1.4%, respectively. Papillary carcinoma and follicular carcinomas are also called differentiated carcinomas, owing to the fact that the cells are already mature and grow slowly.
The human body consists of cells. In general, cells with more complex and specialized functions are more differentiated and mature. Given their maturity, highly differentiated cancer cells do not easily metastasize. On the other hand, less differentiated and immature cancer cells tend to metastasize.
- (1) Papillary carcinoma
- Papillary carcinoma is progresses gradually and accounts for more than 90% of thyroid cancer cases. In the early stages, only nodules are present and the carcinoma develops very slowly. When papillary carcinoma advances, its symptoms include difficulty in breathing, hoarseness, and trouble swallowing. However, in recent years patients with these symptoms are rare due to early detection. Patients often seek testing when they become aware of nodules or ultrasonography detects thyroid nodules during regular health check-ups.
Papillary carcinoma rarely metastasizes to distant organs, but frequently spreads to the lymph nodes around the thyroid during early stages. Therefore, the first clue for some patients that something is abnormal is swelling of the lymph nodes on the side of the neck. However, even if papillary carcinoma metastasizes to the lymph nodes, it still grows slowly and treatment at this stage also often successful. At our hospital, the 20-year survival rate of for papillary carcinoma patients who have undergone surgery is higher than 90%. In short, our experience shows that, unlike other cancers, papillary carcinoma is extremely curable.
- (2) Follicular carcinoma
- Follicular carcinoma accounts for approximately 5% of thyroid cancer. Similar to papillary carcinoma, the presence of nodules is the only abnormality in most cases. This carcinoma rarely invades the lymph nodes, but may metastasize to distant locations, such as the lungs and bone. However, follicular carcinoma also progresses slowly. If treated early, it can be cured at a considerably high rate. In our hospital, the 10-year survival rate is 89.9%.
- (3) Poorly differentiated carcinoma
- Among papillary carcinomas and follicular carcinomas, cancers with low levels of differentiation on the cellular level are called poorly differentiated carcinomas. Since poorly differentiated carcinomas progress somewhat faster than the more common papillary and follicular carcinomas, they have a higher degree of malignancy and require appropriate treatment.
- (4) Medullary carcinoma
- Medullary carcinoma is a special carcinoma, accounting for approximately 1-2% of all thyroid cancer cases. Unlike papillary carcinoma and follicular carcinoma, which develop from follicle cells producing thyroid hormones, medullary carcinoma develops from parafollicular cells. These cells produce calcitonin, a hormone that lowers blood calcium levels. Some cases of medullary carcinoma occur in families and have a genetic basis. Genetic testing can diagnose whether an individual bears genes that can cause hereditary medullary carcinoma. Some hereditary medullary carcinomas are associated with diseases of other endocrine glands, such as adrenal pheochromocytoma and hyperparathyroidism. These carcinomas are called multiple endocrine neoplasia (MEN).
- (5) Undifferentiated carcinoma
- Since undifferentiated carcinoma cells are very immature, this type of carcinoma grows rapidly and is highly malignant. Undifferentiated carcinoma is common in the elderly, with a male-to-female ratio of 1:2. It is found in approximately 1-2% of thyroid cancer cases.
The figure below shows the age distribution of thyroid cancers treated at our hospital. Papillary carcinoma and follicular carcinoma are found in relatively young young and middle-aged people. However, undifferentiated carcinoma primarily occurs in people older than 50, and in particular elderly individuals over 60. In this regard, the distribution of undifferentiated carcinoma resembles stomach cancer and lung cancer.
In general, cancers that afflict young people tend to progress rapidly and are difficult to treat. However, thyroid cancer is an exceptional. Since thyroid cancer is easily curable, patients should undergo medical examinations as soon as possible if they notice nodules.
There are other varieties of malignant thyroid tumors. One type is malignant lymphoma, which occurs rarely.
■Age and number of patients with papillary carcinoma, follicular carcinoma, and other cancers