Risk groups for papillary or follicular thyroid cancer

Doctors usually classify papillary or follicular thyroid cancer into risk groups to help them plan treatment and follow-up. They often assign risk groups when they stage the cancer.

Doctors can use different systems to determine the risk group for a papillary or follicular thyroid cancer. These systems are based on factors such as age, tumour size, if the tumour has grown into surrounding tissues (called invasion) and if the cancer has spread to other parts of the body (called metastasis).

Many doctors use the American Thyroid Association (ATA) Risk Stratification System. It classifies papillary or follicular thyroid cancer as low, intermediate or high risk. The ATA system allows the doctor to estimate the risk that the cancer will come back, or recur, and help decide what type of treatment, if any, is needed after surgery. Other systems may classify papillary or follicular thyroid cancer into slightly different risk groups or use slightly different factors for each risk group.

Papillary or follicular thyroid cancer is put in the low-risk group when it:

• is only in the thyroid
• has not grown through the thyroid
• has not spread to nearby tissues or other parts of the body
• is 4 cm or smaller
• is not an aggressive variant, or subtype

When there is a very small amount of cancer (called micrometastasis) in 1–5 nearby lymph nodes, it is considered low-risk papillary or follicular thyroid cancer.

Papillary or follicular thyroid cancer is put in the intermediate-risk group when it:

• is an aggressive variant
• has grown through the thyroid and into tissues around the thyroid
• has spread to more than 5 lymph nodes in the neck
• has invaded blood vessels (called vascular invasion)

Papillary or follicular thyroid cancer is put in the high-risk group when:

• the cancer has spread to other parts of the body (called distant metastases)
• the cancer has grown through the thyroid and into many tissues in the neck (called gross extension)
• the cancer has spread to lymph nodes, and any lymph node with cancer in it is 3 cm or larger in diameter
• the cancerous tumour was not completely removed with surgery (called an incomplete resection)

Age is not a factor used in the ATA system, but differentiated thyroid cancer in people 40 years of age or older is considered a slightly higher risk.