Atypical carcinoid tumour of the lung

by Katherina Baranova MD and Matt Cecchini MD FRCPC
May 26, 2022

What is an atypical carcinoid tumour?

An atypical carcinoid tumour is a type of lung cancer made up of neuroendocrine cells. In the lung, this tumour starts from the neuroendocrine cells normally found in the walls of the airways and is often located in the central part of the lung near the heart. Since it is closely associated with airways, it can block and lead to the collapse of the lung. It can also invade the airway which can lead to coughing up blood.

Can an atypical carcinoid tumour spread to other parts of the body?

Compared to other types of lung cancer, atypical carcinoid tumours are slow-growing but they can still spread away from the lungs to other parts of the body. The movement of cancer cells to another part of the body is called metastasis.

What is carcinoid syndrome?

The neuroendocrine cells in an atypical carcinoid tumour can make and release hormones such as serotonin. The extra serotonin can cause symptoms such as flushing and diarrhea. Doctors describe these symptoms as carcinoid syndrome. These symptoms atypically only occur if the tumour has spread to the liver.

How is an atypical carcinoid tumour diagnosed?

The diagnosis of atypical carcinoid is usually made after a small sample of tissue is removed from the lung in a procedure called a biopsy or a fine needle aspiration (FNA). However, imaging can also lead a doctor to suspect carcinoid.

When examined under the microscope, atypical carcinoid tumours are made up of cells that all look very similar. Pathologists often describe the nucleus of the cell as “salt and pepper” because the chromatin or genetic material looks like small dark dots on a white background.

In order to make the diagnosis of an atypical carcinoid tumour, your pathologists will count the number of mitotic figures (tumour cells dividing to create new tumour cells). The diagnosis of an atypical carcinoid tumour in the lung requires between 2 and 10 mitotic figures over an area of 2 square millimetres or 10 high powered fields of magnification. Your pathologist will also look for a type of cell death called necrosis.

These features are important because tumours with less than 2 mitotic figures and without necrosis are classified as typical carcinoid tumours. Your pathologist will make this distinction because atypical carcinoid tumours have a higher risk of spreading to other parts of your body or growing back in the lung after treatment.

What other types of tests may be performed to confirm the diagnosis?

Your pathologist may perform a test called immunohistochemistry to confirm the diagnosis. The results will be described as positive (reactive) or negative (non-reactive).

Atypical carcinoid tumours usually show the following results:

Marker Result
Chromogranin Positive (reactive)
Synaptophysin Positive (reactive)
CD56 Positive (reactive)
TTF-1 Usually positive (reactive) but can be negative (non-reactive)


These tests are used to confirm the diagnosis of an atypical carcinoid tumour. Your report may not include all of the results shown above. In addition, since atypical carcinoid tumours show a slow rate of cell division, the Ki-67 proliferation index (a type of immunohistochemistry that highlights dividing cells) is often reported as low.

What is a margin?

To remove a tumour from the lung, normal lung tissue, blood vessels, and airways all have to be cut. Any tissue that is cut when removing a tumour is called a margin and all margins are examined closely for any microscopic evidence of tumour.

If no tumour cells are seen at any of the cut edges of tissue, the margins are called negative. A margin is considered positive when there are tumour cells at the edge of the cut tissue. A positive margin is associated with a higher risk that tumour will re-grow (local recurrence) in the same site after treatment.



What are lymph nodes?

Lymph nodes are small immune organs located throughout the body. Tumour cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour. The movement of tumour cells from the tumour to a lymph node is called metastasis.

Lymph nodes from the neck, chest, and lungs may be removed at the same time as the tumour. These lymph nodes are divided into areas called stations. There are 14 different stations in the neck, chest, and lungs. Your pathology report will describe the number of lymph nodes examined from each station.

Lymph node stations

Your pathologist will carefully examine each lymph node for tumour cells. Lymph nodes that contain tumour cells are often called positive while those that do not contain any tumour cells are called negative. If tumour cells are found in a lymph node, the station of the positive lymph node will be described in your report.

Finding tumour cells in a lymph node increases the nodal stage (see Pathologic stage below) and is associated with a worse prognosis. The nodal stage selected will depend on where the lymph node with tumour cells was located (the station).

How do pathologists determine the pathologic stage for an atypical carcinoid tumour of the lung?

​Atypical carcinoid tumours are staged using the TNM system. 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 tissue submitted and give each part a number. In general, a higher number means more advanced disease and a worse prognosis.

The pathologic stage will only be described in your report after the entire tumour has been removed. It will not be included after a biopsy.

Tumour stage (pT) for atypical carcinoid tumour

Atypical carcinoid is given a tumour stage between 1 and 4 based on the size of the tumour, the number of tumours found in the tissue examined, and whether the tumour has broken through the pleural or has spread to organs around the lungs.

Lung staging

Nodal stage (pN) for atypical carcinoid tumour

Atypical carcinoid is given a nodal stage between 0 and 3 based on the presence or absence of cancer cells in a lymph node and the location of the lymph nodes that contain cancer cells.

  • NX – No lymph nodes were sent for pathologic examination.
  • N0 – No cancer cells were found in any of the lymph nodes examine
  • N1 – Cancer cells were found in at least one lymph node from inside the lung or around the large airways leading into the lung. This stage includes stations 10 through 14.
  • N2 -Cancer cells were found in at least one lymph node from the tissue in the middle of the chest and around the large airways. This stage includes stations 7 through 9.
  • N3 – Cancer cells were found in the neck or in any lymph nodes on the side of the body opposite (contralateral) to the tumour. This stage includes stations 1 through 6.
Metastatic stage (pM) for atypical carcinoid tumour

Atypical carcinoid is given a metastatic stage of 0 or 1 based on the presence of cancer cells in the lung on the opposite side of the body or at a distant body site (for example the brain). The metastatic stage can only be determined if tissue from the opposite lung or distant site is sent for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as pMX.

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