Follicular neoplasm of the thyroid gland

What does follicular neoplasm mean?

Follicular neoplasm is a term pathologists use to describe a tumour in the thyroid gland. This diagnosis is usually made after a procedure called a fine-needle aspiration (FNA). Follicular neoplasm is a preliminary diagnosis that includes both non-cancerous and cancerous conditions.

These conditions include:

• Adenomatoid nodule – Adenomatoid nodule is a non-cancerous type of growth in the thyroid gland. Most adenomatoid nodules develop as part of a condition called nodular thyroid hyperplasia.
• Follicular adenoma – Follicular adenoma is a non-cancerous type of thyroid tumour. The tumour cells are separated from the normal thyroid gland by a thin tissue barrier called a capsule.
• Follicular carcinoma – Follicular carcinoma is a type of thyroid cancer. Most tumours are at least partially separated from the normal thyroid gland by a thin barrier called a capsule. The tumour cells in follicular carcinoma look similar to the cells in a follicular adenoma. The most important difference is that in follicular carcinoma, the tumour cells cross the capsule and spread into the surrounding thyroid gland.
• Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) – NIFTP is a non-cancerous type of thyroid tumour. The tumour cells are separated from the normal thyroid gland by a thin tissue barrier called a capsule.
• Follicular variant of papillary thyroid carcinoma – Follicular variant of papillary thyroid carcinoma is a type of thyroid cancer. Most tumours are at least partially separated from the normal thyroid gland by a thin barrier called a capsule. The cells in a follicular variant of papillary thyroid carcinoma look similar to the cells in an NIFTP. The most important difference is that in follicular variant papillary thyroid carcinoma, the tumour cells cross the capsule and spread into the surrounding thyroid gland.

In order to determine which of these conditions you have, the entire tumour needs to be removed and examined under a microscope. In particular, the pathologist needs to be able to determine if the tumour is surrounded by a capsule and if the cells inside the tumour have crossed the capsule into the surrounding thyroid gland. This examination can only be performed after the entire tumour is removed. Look for the final diagnosis in your pathology report after the tumour is removed.

The thyroid gland

The thyroid is a U-shaped gland located in the front of the neck. The normal thyroid gland is divided into right and left lobes that are connected in the middle by the isthmus. Some people also have another small lobe above the isthmus called the pyramidal lobe.

The thyroid gland makes thyroid hormone. Most of the cells in the thyroid gland are called follicular cells. The follicular cells connect together to form small round structures called follicles. Thyroid hormone is stored in a material called colloid which fills the centre of follicles.

Thyroid nodules

A nodule is an abnormal growth in the thyroid gland. Large nodules can be felt or seen as a lump in the front of the neck. Finding a nodule in your thyroid gland does not necessarily mean that you have cancer. A nodule can be caused by non-cancerous growth or by cancer. Additional tests are required to determine the cause of the nodule.

Ultrasound and biopsy

An imaging test called an ultrasound is usually performed to see the nodule inside the thyroid gland. The ultrasound allows your doctor to measure the nodule and to see if it is solid or filled with fluid. During the ultrasound, a small sample of the nodule will be removed in a procedure called fine-needle aspiration. The sample is then put on a slide so it can be examined by a pathologist under the microscope.

How do pathologists make this diagnosis?

The diagnosis of follicular neoplasm is usually made after a small amount of tissue is removed in a procedure called a fine-needle aspiration or FNA. However, this diagnosis is sometimes made after the entire tumour has been removed along with the surrounding thyroid lobe.

When examined under the microscope, the cells in a follicular neoplasm look similar to normal follicular cells. However, the follicles are often smaller than normal. These small follicles are called microfollicles. The follicular cells may also be arranged in small groups or even as single-detached cells. Your pathologist may use the term atypical to describe cells that have an abnormal shape, colour, or size compared to normal, healthy, follicular cells. Cells may become atypical as a result of cancer, infection, radiation, medications, or inflammation.

What happens next?

Most patients who receive a diagnosis of follicular neoplasm after a fine-needle aspiration will be offered surgery to remove half of the thyroid gland with the tumour. Depending on the final diagnosis, additional treatment may be offered.

by Adnan Karavelic MD FRCPC (updated September 19, 2021)