Staging refers to the process of determining how severe the cancer and the stage helps to determine the prognosis (i.e. the chance that a patient will recover or die of a disease). There are many staging systems for predicting the outcome of thyroid cancer. These staging systems look at various characteristics of the cancer as well as the patient. Fortunately, most types of thyroid cancer have an excellent prognosis. Unlike other types of cancer, the prognosis of differentiated thyroid cancer largely depends on the age of the patient at the time it is diagnosed. Patients who are 45 years of age or younger rarely die of their disease regardless of whether the surrounding lymph nodes are affected, tumor is left behind, the tumor extends into the veins or surrounding structures, or even if there is metastatic disease. However, for older patients all of these factors play a significant role in long term disease-free survival. Similarly, unlike other cancers where involved lymph nodes usually means a worse prognosis, positive lymph nodes in the neck for thyroid cancer do NOT decrease survival. Positive nodes do increase the chance that the cancer may come back, but usually these recurrences are treatable with additional surgery or another dose of RAI ablation.
The TNM method is the most universally used staging method and applies to both papillary and follicular thyroid cancers.
It was introduced in 1987 by the International Union Against Cancer and adopted by the American Joint Commission on Cancer.
TNM stands for Tumor/Node Metastasis/Distant Metastasis:
T: Tumor size (in cm)
- T1: < 2cm
- T2: 2-4 cm
- T3: > 4 cm
- T4: tumor grows outside of the thyroid
- T4a: grows into nearby structures
- T4b: grows in spine or nearby large blood vessels
N: Lymph Nodes
- NX: regional lymph nodes can't be assessed
- N0: no involved regional lymph nodes
- N1: involved regional lymph nodes
- N1a: involved central neck lymph nodes
- N1b: involved lateral neck or mediastinal (chest) lymph nodes
M: Distant Metastases
(i.e. cancer has spread to other areas of the body like the lung or bone)
- M0: no distant metastases
- M1: distant metastes
Based on these three categories, the cancer is assigned a Stage of 1, 2, 3 or 4. Stage 1 is the least advanced form of cancer with the best prognosis, and Stage 4 is the most advanced category. The table below shows the likelihood of a local recurrence (i.e. recurrence of thyroid cancer in the neck region), distant recurrence (i.e. recurrence of cancer in other areas of the body), and mortality (i.e. death) based on the stage of a given tumor for well-differentiated thyroid cancers in general.
Again, for patients with well-differentiated thyroid cancer (i.e. papillary, follicular, and Hurthle cell cancer), age is the most important prognostic factor. If a patient is younger than 45, even if there are distant metastases, they are considered a Stage II and have an excellent prognosis.
For patients with medullary thyroid cancer, age is not as important a prognostic factor and for all patients with medullary thyroid cancer, the "Age>45" column should be used for determining their prognosis. In general, patients with medullary thyroid cancer only in the thyroid (T1-3), have a 95% 10-year survival. Patients with medullary cancer that has spread to the neck lymph nodes (N1) have a 10-year survival of 75%. For those patients with metastatic medullary thyroid cancer, the 10-year survival is 20 to 40%
It is important to note that classifications made by a pathologist can be very subjective, particularly for tumors that have few distinctive characteristics, which is often the case with follicular cancer. For this reason, reliably staging a follicular thyroid cancer is more difficult because it is influenced by the subjective classification made by the pathologist. For example, some pathologists define a microscopic capsular invasion and a microscopic vascular invasion (minute extensions of the tumor which can only be seen under a microscope) as cancer while others may not.
The MACIS method was developed by the Mayo Clinic based on careful evaluation of a large group of patients and was developed to determine the prognosis of patients with papillary thyroid cancer. MACIS is an abbreviation for the factors taken into account to predict survival: (distant) Metastasis or spread of the cancer to areas outside the neck, Age of the patient at the time the tumor was discovered, Invasion into surrounding areas of the neck as seen by the naked eye, Completeness of surgical resection (or removal) of the tumor, and Size of the tumor. Each of these factors is mathematically scored as shown in the table below:
Once the score for each factor is calculated, they are added up to get a total MACIS score and this total predicts the likelihood that the patient will live 20 years from the time the tumor was discovered. Fortunately most patients fall into the low risk category (MACIS score less than 6.0) and are cured of the cancer at the time of surgery.
These tables demonstrate the 5-year survival for the different types of thyroid cancer:
* These tables are adapted from the American Cancer Society