Follicular Subtype of Papillary Thyroid Carcinoma: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
October 9, 2026

Follicular variant of papillary thyroid carcinoma is a type of thyroid cancer and a form of papillary thyroid carcinoma, the most common cancer of the thyroid gland. Some reports call it the follicular subtype of papillary thyroid carcinoma, the name used by the World Health Organization since 2022, and both names describe the same tumor.

The thyroid is a small gland at the front of the neck that makes hormones controlling how the body uses energy. Its normal cells grow in small, round sacs called follicles. In this cancer, the tumor cells also grow in follicles. It’s called papillary carcinoma because of the appearance of the nucleus, the part of each cell that holds its DNA.

The name covers more than one kind of tumor. Some are surrounded by a thin capsule, while others spread into the thyroid tissue around them, and these tumors behave differently. For that reason, your pathology report will say exactly which kind you have.

This article explains how doctors diagnose follicular variant of papillary thyroid carcinoma and how they tell its different types apart. It also explains what the invasion, grade, margin, lymph node, biomarker, and stage findings in your report mean.

Who gets follicular variant of papillary thyroid carcinoma?

Follicular variant of papillary thyroid carcinoma is most often found in adults between about 30 and 60 years old, although it can also occur in children and teenagers. Like other thyroid cancers, it is about three times more common in women than in men.

What causes follicular variant of papillary thyroid carcinoma?

In most people, the cause of follicular variant of papillary thyroid carcinoma is not known. Radiation to the head or neck during childhood is the best-known risk factor, but most people with this cancer have never had that kind of radiation.

The cancer develops because of changes in genes inside the tumor cells, and these changes are not inherited. Encapsulated tumors most often have a change in one of the RAS genes, while infiltrative tumors more often have a change in the BRAF gene. These gene changes are described in the biomarker section below. In rare cases, thyroid cancer runs in families.

What are the symptoms?

Most people with follicular variant of papillary thyroid carcinoma have no symptoms when the tumor is found. Doctors often discover it as a lump in the thyroid, called a thyroid nodule, during a physical exam or a scan done for another reason.

A larger tumor can cause a visible or palpable lump in the neck, trouble swallowing, or a hoarse voice. Thyroid hormone levels in the blood are usually normal, so blood tests rarely indicate cancer.

How is the diagnosis made?

Follicular variant of papillary thyroid carcinoma usually starts as a nodule seen on a neck ultrasound. A doctor then uses a thin needle to remove cells from the nodule in a test called fine-needle aspiration (FNA), and a pathologist examines the cells under a microscope.

For this type of tumor, the needle sample often cannot give a final answer. The cells can look very similar to those in noncancerous thyroid nodules, and a few cells may not show whether the tumor has grown through its capsule. Results such as follicular neoplasm or suspicious for follicular neoplasm are therefore common. Molecular tests on the needle sample can help your doctors decide whether surgery is needed.

The final diagnosis is made after surgery, when the surgeon removes either half of the thyroid, called a lobectomy, or the whole gland, called a total thyroidectomy. A pathologist then examines the entire edge of the tumor under the microscope, looking for tumor cells that have grown through the capsule or into blood vessels. If cancer is confirmed, an ultrasound of the neck is often used to look for spread to lymph nodes.

What are the different types?

Follicular variant of papillary thyroid carcinoma looks very similar to two other thyroid tumors under the microscope, so pathologists divide these tumors into three groups. The World Health Organization (WHO) classification, 5th edition, published in 2022, changed how these groups are named, so your report may use either the newer or the older name.

Infiltrative follicular variant of papillary thyroid carcinoma. This tumor has no capsule and grows directly into the surrounding thyroid tissue. It behaves much like the common, or classic, type of papillary thyroid carcinoma. When it spreads, it usually travels first to neck lymph nodes rather than distant organs.
Invasive encapsulated follicular variant papillary thyroid carcinoma. This tumor is surrounded by a thin capsule, and tumor cells have grown through that capsule or into blood vessels. It behaves more like follicular thyroid carcinoma than like classic papillary carcinoma, rarely spreading to lymph nodes and more often traveling through the blood to the lungs or bones. Since 2022, the WHO has listed it as its own type of thyroid cancer, separate from papillary thyroid carcinoma, although it keeps its familiar name.
NIFTP. A capsule also surrounds this tumor, but no tumor cells have grown through the capsule or into blood vessels. It was considered a cancer until 2016, when long-term studies showed that it rarely comes back once it has been removed. Our article on NIFTP explains this diagnosis.

We can only distinguish these tumors after the entire tumor, including its capsule, has been removed and examined. This is why we determine the type after surgery, not from the needle sample. Invasive encapsulated tumors are further divided into six categories, which are described in the next section.

Capsular invasion and invasive encapsulated categories

Capsular invasion is the key finding in encapsulated follicular-pattern papillary thyroid carcinoma. It means tumor cells have grown through the capsule, the thin layer of tissue around the tumor. This finding is what separates invasive encapsulated follicular variant papillary thyroid carcinoma from NIFTP, and a tumor with capsular invasion is a cancer even if nothing else is found.

When the diagnosis is invasive encapsulated follicular variant papillary carcinoma, your report will name one of six categories. The category tells you how far the tumor has invaded. It also says whether the tumor cells are oncocytic, meaning they look large and pink because they are packed with mitochondria, the parts of a cell that make energy.

Minimally invasive follicular variant papillary carcinoma. Tumor cells have grown through the capsule but have not entered any blood vessels. This category has the best outlook, and spread to other parts of the body is very rare. For many people, surgery alone is enough treatment.
Minimally invasive oncocytic follicular variant papillary carcinoma. This is the same as the minimally invasive category, except most tumor cells are oncocytic. The outlook is similar to that of the minimally invasive category.
Encapsulated angioinvasive follicular variant papillary carcinoma. Tumor cells are seen inside blood vessels in or just outside the capsule, although the tumor is still mostly surrounded by its capsule. Even one invaded blood vessel puts a tumor in this group. The chance of spread is higher than in a minimally invasive tumor and increases with the number of vessels involved.
Encapsulated angioinvasive oncocytic follicular variant papillary carcinoma. This is the same as the encapsulated angioinvasive category, except most tumor cells are oncocytic. Your report will also give the number of invaded blood vessels.
Widely invasive follicular variant papillary carcinoma. The tumor has grown widely through its capsule into the surrounding thyroid, or beyond the thyroid altogether. This category has the highest chance of spreading, usually through the blood to the lungs or bones.
Widely invasive oncocytic follicular variant papillary carcinoma. This is the same as the widely invasive category, except most tumor cells are oncocytic. Assess and treat it the same way.

For invasive encapsulated tumors, how far the tumor has invaded matters much more than whether the cells are oncocytic. These six categories do not apply to the infiltrative variant, which has no capsule.

Histologic grade

Most cancers are graded by how abnormal the cells look, but follicular variant of papillary thyroid carcinoma is not graded that way. Instead, the pathologist checks for two features that would show the tumor is growing faster than usual. Every report should include the mitotic count, and many reports also comment on necrosis.

Mitotic count. A mitotic figure is a cell that is in the process of dividing. The pathologist counts these cells in a set area of the tumor, and in most tumors the count is low.
Necrosis. Necrosis means areas of dead tumor cells. It is uncommon in this cancer, so many reports only mention it when it is seen. It may also be mentioned when the mitotic count is high, and the pathologist is checking for a faster-growing type of thyroid cancer.

If the pathologist finds 5 or more mitotic figures in a set area, or any necrosis, the tumor is reclassified as a differentiated high-grade thyroid carcinoma. Less often, the tumor is reclassified as a poorly differentiated thyroid carcinoma. Both diagnoses are more likely to recur or spread, so doctors usually treat and follow them more closely.

Tumor size and extrathyroidal extension

Your report will give the size of the follicular variant of papillary thyroid carcinoma, measured in centimeters (cm) at its widest point. Size is one of the main factors used to stage the tumor, and tumors larger than 4 cm are assigned a higher stage.

Your report will also say whether the tumor has grown outside the thyroid gland. This finding is called extrathyroidal extension, and your report may describe it in one of these ways:

No extrathyroidal extension. The tumor is confined to the thyroid gland. This is the most common result for this cancer.
Microscopic extrathyroidal extension. A small amount of tumor has grown into the tissue just outside the thyroid, and it can only be seen with a microscope. Under the current staging system, this finding no longer changes the stage and has little effect on the outlook.
Gross extrathyroidal extension. The tumor has grown far enough outside the thyroid for the surgeon to see it during the operation. This raises the stage. Growth into the thin muscles at the front of the neck, called the strap muscles, is stage pT3b, while growth into the windpipe, voice box, or esophagus is stage pT4a.

Vascular invasion

Vascular invasion, also called angioinvasion, means tumor cells from the follicular variant of papillary thyroid carcinoma are seen inside a blood vessel. It is reported in every case as either present or not identified. It is one of the most important findings in your report because cancer cells in blood vessels can travel to distant organs, such as the lungs or bones.

When vascular invasion is present, many reports give the number of blood vessels involved. Older reports may use the word “focal” for fewer than four vessels and “extensive” for four or more. A larger number of invaded vessels means a higher chance of the cancer spreading, and your team uses this number when planning treatment after surgery.

Lymphatic invasion

Lymphatic invasion means cells from the follicular variant of papillary thyroid carcinoma are seen inside a lymphatic channel, a thin vessel that carries fluid to the lymph nodes. It is reported separately from vascular invasion, as either present or not identified. Lymphatic invasion is more common in the infiltrative variant, which tends to spread to the neck. lymph nodes

Margins

A margin is the edge of tissue the surgeon cuts when removing the follicular variant of papillary thyroid carcinoma. The pathologist examines the margins to see whether any tumor cells are present at the cut edge.

Negative margin. No tumor cells are seen at the cut edge, suggesting the surgeon removed the tumor completely. Some reports also give the distance from the tumor to the closest margin.
Positive margin. Tumor cells are seen at the cut edge, which suggests some tumor may remain in the neck. Your team may consider radioactive iodine treatment or, less often, another operation.

Lymph nodes

Follicular variant of papillary thyroid carcinoma can spread to lymph nodes, small immune organs that filter fluid from tissues. The lymph nodes closest to the thyroid sit in the center of the neck, an area called level 6. The lymph nodes along the sides of the neck are called levels 2 to 5.

Lymph nodes are not removed in every operation for this cancer. They are usually removed when an ultrasound or the surgeon finds nodes that look abnormal, sometimes in an operation called a neck dissection. If lymph nodes were removed, your report will say how many were examined and how many contain cancer.

Many reports also give the size of the largest cancer deposit in a lymph node. Very small deposits, measuring 2 mm or less, add little to the chance of the cancer coming back. Deposits larger than 3 cm, or cancer in many lymph nodes, are linked to a higher chance of the cancer coming back in the neck.

Your report may also mention extranodal extension. This means cancer cells have broken through a lymph node’s outer capsule into the surrounding tissue. This finding is linked to a higher chance of the cancer coming back. Spread to lymph nodes is common in the infiltrative variant and uncommon in encapsulated tumors.

Biomarker and molecular testing

Biomarker testing for follicular variant of papillary thyroid carcinoma looks for changes in genes inside the tumor cells. It is not needed for every patient, and many reports do not include it. When done, it helps predict how the cancer is likely to behave, and for advanced cancer, it can show whether targeted drugs might be an option.

Testing usually uses next-generation sequencing (NGS), a test that checks many genes at once. It may be done on the needle sample before surgery, to help decide whether surgery is needed, or on the tumor after it has been removed. Results are usually reported as “detected” or “not detected” for each gene.

RAS

The RAS genes, called NRAS, HRAS, and KRAS, make proteins that help control when cells grow and divide. A change in one of these genes is the most common finding in encapsulated tumors. RAS changes are also found in noncancerous thyroid nodules, so a RAS change on its own does not mean a nodule is a cancer.

If your report says a RAS change was detected, this fits with an encapsulated, follicular-pattern tumor and does not by itself change treatment. If no RAS change was detected, that is also a common result and does not change the diagnosis. Our article on RAS mutations in thyroid cancer explains these results in more detail.

BRAF V600E

BRAF is another gene that helps control cell growth, and V600E is the most common change. This change is most common in the infiltrative variant and classic papillary carcinoma, and it is uncommon in encapsulated tumors.

If the BRAF V600E change is detected and the cancer later spreads and stops responding to standard treatment, it can make a person eligible for BRAF-targeted drugs. The combination of dabrafenib and trametinib received accelerated approval from the FDA in June 2022 for advanced solid tumors with this change that have grown after earlier treatment. “Not detected” is the usual result in encapsulated tumors. Our article on BRAF mutations in thyroid cancer explains more.

TERT promoter

TERT is a gene that allows cells to keep dividing, and its promoter is the stretch of DNA that switches it on. A change in the TERT promoter is linked to a higher chance of the cancer coming back or spreading. The risk is highest when a TERT change occurs alongside a RAS or BRAF change. Most tumors do not have a TERT change, so a “not detected” result is reassuring.

RET and NTRK gene fusions

A fusion means two genes have broken and joined, creating an abnormal gene that drives tumor growth. In thyroid cancer, fusions involving RET or NTRK matter most because targeted drugs exist for both. Fusions are uncommon in this cancer, and a result of “not detected” is the usual finding.

For advanced cancer with a RET fusion, the FDA fully approved selpercatinib in July 2026. The approval covers solid tumors that have grown after earlier treatment, in people 2 years and older. For advanced cancer with an NTRK fusion, larotrectinib was approved by the FDA in 2018 as the first drug for solid tumors with this change. Our articles on RET in thyroid cancer and NTRK fusions in cancer explain these results.

The gene changes described in this section are found only in the tumor cells, so they are not inherited and cannot be passed on to children. Drug approval and access differ by country, and your care team can explain what applies where you live. For more information on tumor testing, visit our Biomarkers and Genetic Testing section.

Pathologic stage (pTNM)

The pathologic stage for follicular variant of papillary thyroid carcinoma is assigned after surgery, using the TNM system from the AJCC Cancer Staging Manual, 8th edition. The tumor stage (pT) and nodal stage (pN) come from the pathology report, while the metastasis stage (M) is usually determined by imaging.

Tumor stage (pT)

pT1. The tumor is 2 cm or smaller and confined to the thyroid. It is called pT1a if it is 1 cm or smaller and pT1b if it is larger than 1 cm.
pT2. The tumor is larger than 2 cm but not larger than 4 cm, and it is confined to the thyroid.
pT3a. The tumor is larger than 4 cm and confined to the thyroid.
pT3b. The tumor has grown into the strap muscles at the front of the neck, whatever its size.
pT4a. The tumor has grown into the windpipe, voice box, esophagus, the nerve that controls the voice box, or the tissue under the skin.
pT4b. The tumor has grown into the tissue in front of the spine or surrounds a major blood vessel in the neck or chest.

Nodal stage (pN)

pN0. No cancer was found in the examined lymph nodes.
pN1a. Cancer was found in lymph nodes in the center of the neck (level 6) or the upper chest (level 7).
pN1b. Cancer was found in lymph nodes along the sides of the neck or behind the throat.

If no lymph nodes were removed, your report may say that the nodal stage was not assigned. For thyroid cancer, the overall stage also depends on age. In people younger than 55, the cancer is stage I unless it has spread to distant organs, which makes it stage II. In people 55 and older, the stage rises with tumor size, growth outside the thyroid, and spread to lymph nodes.

What is the prognosis?

The prognosis for follicular variant of papillary thyroid carcinoma is very good, and most people are cured with surgery, with or without radioactive iodine. The figures below come from large groups of people with thyroid cancer of all types, most of whom had papillary carcinoma.

Cancer confined to the thyroid. Nearly everyone, more than 99 in 100 people, is alive five years after diagnosis.
Cancer that has spread to nearby lymph nodes. About 98 in 100 people are alive five years after diagnosis.
Cancer that has spread to distant organs. About half of people are alive five years after diagnosis, and the other half are not. Spread to distant organs is uncommon, affecting about 3 in 100 people at diagnosis.

The type of tumor also matters. Minimally invasive encapsulated tumors have an excellent outlook and very rarely spread. The infiltrative variant often spreads to neck lymph nodes but is still usually cured. Widely invasive tumors and tumors with many invaded blood vessels have a higher chance of spreading to distant organs.

Features in your report that are linked to a higher chance of the cancer coming back include:

Vascular invasion. More invaded blood vessels mean a higher chance of spread to the lungs or bones.
Gross extrathyroidal extension. Growth outside the thyroid that the surgeon can see raises the stage and the chance of the cancer coming back.
Lymph node spread. Many involved lymph nodes, large deposits, or extranodal extension raise the chance of the cancer coming back in the neck.
Positive margin. Some tumor cells may have been left behind in the neck.
High-grade features or a TERT change. These findings are linked to a higher chance of the cancer coming back or spreading.

What happens after the diagnosis?

After a diagnosis of follicular variant of papillary thyroid carcinoma, your care team uses the findings in your report to decide how much treatment you need. The team often includes an endocrinologist, a surgeon, and a nuclear medicine specialist. For many people with a small tumor that has no high-risk features, surgery alone is enough.

Surgery. A lobectomy may be enough for a small tumor confined to the thyroid. If your report shows higher-risk features, such as extensive vascular invasion or gross extrathyroidal extension, your team may suggest a second operation. This is called a completion thyroidectomy, and it removes the rest of the thyroid.
Radioactive iodine. You take this treatment as a pill; thyroid cells, including cancer cells, absorb the iodine and are destroyed by its radiation. The American Thyroid Association guidelines, updated in 2025, reserve it mainly for people whose report shows higher-risk features.
Thyroid hormone pills. After the whole thyroid is removed, a daily pill replaces the hormone the gland used to make.
Follow-up. Follow-up includes neck ultrasounds and a blood test for thyroglobulin, a protein thyroid cells make. A rising thyroglobulin level can be the first sign of cancer recurrence.
Treatment for advanced cancer. If the cancer spreads and no longer responds to radioactive iodine, doctors may consider other drugs. The FDA approved lenvatinib in 2015 for this situation, and the targeted drugs described in the biomarker section may be options for tumors with specific gene changes.

Questions to ask your doctor

Is my tumor the infiltrative variant or an encapsulated tumor?
If it is encapsulated, which of the six categories does it fall into?
Was vascular invasion found, and how many blood vessels were involved?
What was the mitotic count, and did the tumor show any high-grade features?
How large was the tumor, and had it grown outside the thyroid?
Were the margins negative?
Were any lymph nodes removed, and how large was the largest deposit of cancer?
What is the stage of my cancer?
Was biomarker testing done, and were any changes such as BRAF, RAS, or TERT found?
Do I need the rest of my thyroid removed?
Do I need radioactive iodine?
How often will I need blood tests and ultrasounds, and for how many years?