Your pathology report for oncocytic carcinoma of the thyroid gland

By Jason Wasserman MD PhD FRCPC
February 4, 2026

Oncocytic carcinoma (previously called Hurthle cell carcinoma) is a rare type of thyroid cancer that arises from follicular cells, which produce thyroid hormone. This tumour is made up mostly of oncocytic cells, a specialized type of thyroid cell that appears larger and more pink under the microscope because it contains many mitochondria (the parts of the cell that produce energy).

This article explains how oncocytic carcinoma is diagnosed, classified, and assessed, what features pathologists look for in the tumour, and how these findings relate to prognosis and follow-up.

Where does oncocytic carcinoma arise?

Most oncocytic carcinomas arise within the thyroid gland. Still, rare tumours can develop in ectopic thyroid tissue, such as the tongue (lingual thyroid) or chest (mediastinum), where thyroid tissue formed outside its usual location during development.

What are the symptoms of oncocytic carcinoma?

Many patients notice a slowly enlarging, painless thyroid nodule. Tumours with limited invasion are often found incidentally on imaging or routine examination.

Larger or more invasive tumours may cause:

  • A visible or palpable neck mass.

  • Pressure or tightness in the neck.

  • Difficulty swallowing or breathing.

  • Hoarseness.

Most patients have normal thyroid hormone levels, so symptoms of hyperthyroidism or hypothyroidism are uncommon.

How is this diagnosis made?

Diagnosing oncocytic carcinoma requires several steps, because this tumour cannot be definitively diagnosed by imaging or needle biopsy alone.

The diagnostic process may include:

Each step provides important information, but the final diagnosis is almost always made after surgery.

Imaging

Ultrasound is typically the first imaging test. Oncocytic carcinoma cannot be reliably distinguished from benign oncocytic adenoma by ultrasound alone. Many tumours appear as solid nodules with a surrounding capsule-induced halo.

Tumours that have grown beyond the capsule may show irregular margins or extension into surrounding tissues. Most oncocytic carcinomas show low uptake on radioactive iodine scans, although rare functioning tumours exist. Many are FDG-PET–avid, meaning they take up glucose on PET imaging.

Fine-needle aspiration (FNA)

FNA can identify an oncocytic follicular tumour, but it cannot determine whether the tumour is benign or malignant. This is because invasion cannot be assessed on small biopsy samples.

As a result, FNA reports often use terms such as oncocytic neoplasm or suspicious for oncocytic neoplasm, and surgical removal is required to establish the diagnosis.

Microscopic features

Under the microscope, oncocytic carcinoma is typically a well-circumscribed, thickly encapsulated tumour composed of at least 75% oncocytic cells.

The tumour cells have abundant pink, granular cytoplasm and round nuclei with prominent nucleoli. Growth patterns are often solid or trabecular, with fewer follicles than seen in benign tumours. The capsule is usually thicker than that of an oncocytic adenoma and may show calcifications.

The most critical microscopic findings are capsular invasion and vascular invasion. Capsular invasion means tumour cells have entirely grown through the capsule. Vascular invasion means tumour cells are found inside blood vessels, often attached to the vessel wall or mixed with a blood clot. These findings confirm that the tumour is malignant and help determine its aggressiveness.

Tumour classification (subtypes)

Oncocytic carcinoma is further subdivided into different types based on how the tumour grows and spreads. These include minimally invasive, encapsulated angioinvasive, and widely invasive oncocytic carcinoma. This classification is important because it is based on whether the tumour has grown through its capsule and whether it has entered blood vessels, features that strongly influence how the cancer behaves and the risk of spread.

Minimally invasive oncocytic carcinoma

This subtype shows tumour invasion through the capsule but no invasion into blood vessels. The tumour remains otherwise well contained.

Minimally invasive oncocytic carcinoma usually behaves indolently and has an excellent prognosis when completely removed. Additional treatment beyond surgery is often not required.

Encapsulated angioinvasive oncocytic carcinoma

These tumours are fully encapsulated but show invasion into blood vessels. Pathologists carefully count the number of involved vessels and describe the invasion as:

  • Limited vascular invasion (fewer than 4 vessels), or

  • Extensive vascular invasion (4 or more vessels)

Tumours with limited vascular invasion have an intermediate risk of spread, while those with extensive vascular invasion behave more aggressively and require closer follow-up.

Widely invasive oncocytic carcinoma

Widely invasive tumours show extensive infiltration into the surrounding thyroid tissue or soft tissues, often with multiple areas of vascular invasion.

This subtype has the highest risk of recurrence and distant metastasis, commonly to the lungs, bones, or liver, and requires more intensive management and long-term surveillance.

Extrathyroidal extension

Extrathyroidal extension means tumour cells have grown beyond the thyroid gland into surrounding tissues. A thin layer of connective tissue normally surrounds the thyroid, and most oncocytic carcinomas remain confined to the gland. When tumour cells extend beyond this boundary, the finding is called extrathyroidal extension.

Pathologists describe extrathyroidal extension as either microscopic or macroscopic (gross).

  • Microscopic extrathyroidal extension means that tumour cells extend just beyond the thyroid capsule and can only be seen under the microscope. This type of extension is common and, by itself, does not change the tumour stage because it has not been shown to affect prognosis significantly.

  • Macroscopic (gross) extrathyroidal extension means that the tumour has grown visibly into nearby structures, such as neck muscles, the trachea (windpipe), esophagus, or major blood vessels. This type of extension can be seen during surgery or on imaging studies.

Only macroscopic extrathyroidal extension is used to increase the tumour stage. This is important because visible invasion into surrounding organs indicates a more advanced tumour, a higher risk of recurrence, and may influence treatment decisions such as the need for additional surgery, radioactive iodine, or other therapies.

High-grade transformation

In rare cases, oncocytic carcinoma can undergo high-grade transformation, meaning the tumour changes into a more aggressive form of thyroid cancer, such as poorly differentiated thyroid carcinoma or high-grade differentiated thyroid carcinoma, oncocytic type.

When this happens, the tumour may show:

  • Tumour necrosis (areas of dead tumour cells).

  • Increased numbers of dividing cells.

  • Abnormal mitotic figures.

  • Loss of typical oncocytic features.

Tumours with high-grade transformation are more aggressive, are often resistant to radioactive iodine, and have a worse prognosis. Identifying these features is critical because they significantly affect treatment decisions and follow-up.

Lymph nodes

Lymph nodes are small immune organs that filter lymphatic fluid. Cancer cells can spread from the thyroid to nearby lymph nodes through lymphatic vessels.

Unlike follicular thyroid carcinoma, oncocytic carcinoma can involve lymph nodes, although this is still less common than spread through the bloodstream. Any lymph nodes removed during surgery are examined under the microscope, and the findings are reported as positive or negative for tumour cells.

The presence of tumour cells in lymph nodes may increase the stage of the cancer and can influence recommendations for additional treatment and surveillance.

Pathologic stage (pTNM)

​​The pathologic stage for oncocytic carcinoma is based on the TNM staging system, an internationally recognized system created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M)  to determine the complete pathologic stage (pTNM). Your pathologist will examine the submitted tissue and assign a number to each part. In general, a higher number means a more advanced disease and a worse prognosis.

Tumour stage (pT)

Oncocytic carcinoma is assigned a tumour stage of 1-4 based on tumour size and the presence of tumour cells outside the thyroid.

  • T1 – The tumour is less than or equal to 2 cm, and the cancer cells do not extend beyond the thyroid gland.
  • T2 – The tumour is greater than 2 cm but less than or equal to 4 cm, and the cancer cells do not extend beyond the thyroid gland.
  • T3 – The tumour is greater than 4 cm OR the cancer cells extend into the muscles outside of the thyroid gland.
  • T4 – The cancer cells extend to structures or organs outside of the thyroid gland, including the trachea, larynx, or esophagus.

Nodal stage (pN)

Oncocytic carcinoma is given a nodal stage of 0 or 1 based on the presence or absence of tumour cells in a lymph node and the location of the involved lymph nodes.

  • N0 – No tumour cells were found in any of the lymph nodes examined.
  • N1a – Tumour cells were found in one or more lymph nodes from levels 6 or 7.
  • N1b – Tumour cells were found in one or more lymph nodes from levels 1 through 5.
  • NX – No lymph nodes were sent to pathology for examination.

Prognosis and prediction

The prognosis for oncocytic carcinoma depends primarily on the extent of invasion, especially vascular invasion.

  • Tumours with capsular invasion only have an excellent outcome.

  • Tumours with limited vascular invasion have an intermediate risk.

  • Tumours with extensive vascular invasion or wide invasion have a significantly worse prognosis.

A small percentage of oncocytic carcinomas can later transform into anaplastic thyroid carcinoma, either at recurrence or at initial diagnosis, which carries a very poor prognosis.

Questions to ask your doctor

  • Was my tumour minimally invasive or widely invasive?

  • Was vascular invasion present, and how extensive was it?

  • Do I need additional treatment such as radioactive iodine?

  • What is my risk of recurrence or spread?

  • How often will I need follow-up imaging or blood tests?

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