THYROID CANCER – TYPES OF THYROID CANCER
Normally the thyroid gland contains two types of cells:
- follicular cells, which produce and stores the hormone (thyroxine) and the protein thyroglobulin
- C-cells (Para follicular cells) produce calcitonin, a hormone that regulates body levels of calcium and phosphate.
Cancer with follicular cell constitute more than 95% of cases and the remainder mainly involve C-cells. Therefore, thyroid cancer can be classified according to the cell type of origin into 5 types:
- Papillary carcinoma
- Follicular carcinoma
- Medullary carcinoma
- Anaplastic carcinoma
- Other Thyroid malignancies
1. Papillary carcinoma
This is the most common of thyroid cancers and account for 60% of all thyroid malignancies (cancer). The tumor develops from follicular cells of the thyroid and consists of columnar cells in folds (papillae).
- It accounts for most of the irradiation induced thyroid cancers.
- It affects the younger age group usually between 30-50 years of age.
- It is the least aggressive thyroid cancer which tends to grow slowly and remains localized to thyroid for years.
- Spread usually take place through the lymph channels to other parts of thyroid gland and the regional lymph nodes.
- Recurrence (return or shows up again) may occur in a few cases especially in young people with small primary tumor. The recurrence may occur in 3 ways: local recurrence ((tumor occurring within the thyroid bed or remnants or adjacent tissues of neck, excluding lymph nodes), postoperative metastasis(spread of cancer) to the lymph nodes, post-operative metastasis to distant areas.
- Radiological thyroid scanning shows papillary carcinoma as relatively “cold” nodules (i.e. does not concentrate iodine as thyroid is not functioning). However some of the papillary carcinoma may absorb iodine making it possible for diagnostic scanning and treatment with iodine after total thyroidectomy.
- The 10 year survival rate is 96%.
2. Follicular carcinoma
This accounts for 16% of all thyroid cancers. Microscopically, it may resemble normal thyroid tissue. It is also believed to originate from the follicular cells of the thyroid. The tumor usually presents as a single encapsulated lesion.
- Tends to occur in the older age group (mean age being >50 years)
- Follicular carcinoma often functions like normal thyroid tissue. The tumor avidly absorbs iodine making it possible for diagnostic scanning and treatment with iodine after total thyroidectomy.
- In rare cases because of tumor load the follicular carcinoma may secrete increased thyroxine causing hyperthyroidism (increased thyroid function)
- It is more malignant than papillary carcinoma
- Follicular cancer tends to spread through the bloodstream, spreading to various parts of the body, often to the bone, lungs and the liver.
- Bone metastasis may be the first manifestation of the disease. The bones commonly involved are the long bones (like femur-bone in the thigh region) or flat bones like skull, sternum (breastbone) or pelvis. Occasionally a person may present with persistent neck or back pain due to metastasis of follicular cancer to the vertebrae.
- Spread to cervical (neck region) lymph nodes is rare.
3. Medullary carcinoma
This accounts for 5% of all thyroid cancers and arises from C-cells(Para follicular cells) of the thyroid.
- Tends to occur usually after the age of 40 years
- It first appears as a hard nodule or firm mass in the thyroid gland or as an enlargement of regional lymph nodes. It does not concentrate (accumulate) iodine.
- The tumor secretes excessive amounts of Calcitonin and occasionally produces other hormones (like prostaglandins, serotonin, ACTH, corticotropin releasing hormone and other peptides). Serum calcitonin is useful in monitoring response to treatment.
- About 20% of these carcinoma are familial (occurring among members of family). Therefore, close blood relatives of a person with this type of cancer should be screened for a genetic abnormality. The familial variety usually appears at a younger age, is more often bilateral (involving both the thyroid lobes), is less likely associated with cervical (neck) metastasis and has a good prognosis (more chances of recovery).
- About one third of the cancer are sporadic (appearing singly) and about one third of them are associated with other types of endocrine cancers (multiple endocrine neoplasia syndrome –MEN type2). Sporadic variety is more aggressive as compared to familial variety.
- It is more malignant as compared to follicular carcinoma.
- Early metastasis (spread) of the cancer take place through local lymph channels (lymphatics) to adjacent muscles, trachea, lymph nodes in the neck (cervical) and chest (mediastinal) region. Late metastasis takes place through blood stream to the bones, lungs, adrenal glands or liver.
4. Anaplastic carcinoma (Undifferentiated carcinoma)
This accounts for 5-10% of all thyroid cancers
- Usually affects patients over 50 years of age.
- It usually presents as a rapidly enlarging, painful mass in a multinodular goiter (enlargement of thyroid gland). The tumor does not concentrate iodine.
- The tumor is non-capsulated (i.e. without a covering) and extends widely distorting the shape of the thyroid gland. It is stony hard and at some places it may be soft or friable.
- It is the most aggressive thyroid cancer and metastasizes(spreads) early to the surrounding lymph nodes and distant areas.
- The tumor invades and presses the surrounding structures causing hoarseness, difficulty in swallowing, inspiratory stridor and vocal cord paralysis.
- On physical examination of the tumor the overlying skin is warm and discolored. The mass is fixed to the adjacent structures, therefore moves poorly on swallowing.
- Prospects of disease is usually not good but patients with fully localized tumor (seen in MRI) have a better prognosis.
5. Other Thyroid malignancies
These account for about 3% of thyroid cancers.
- more common in women
- Usually presents as a rapidly enlarging, painful mass originating from a multinodular or diffuse goiter (enlargement of thyroid gland) affected by autoimmune thyroiditis.
- 20% of cases are associated with hypothyroidism (under functioning of thyroid).
- It may be related with systemic lymphomas (lymphomas in other parts of the body)
- Metastatic tumors: Sometimes the thyroid may be involved due to metastasis (spread) of cancer from other areas especially cancer of breast, bronchi, kidney or malignant melanomas.