Atypical Carcinoid Tumour of the Lung: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC and Matt Cecchini MD PhD FRCPC
April 29, 2026

An atypical carcinoid tumor of the lung is an intermediate-grade cancer that develops from neuroendocrine cells — specialized cells found in the walls of the airways that normally produce hormones and chemical signals. Atypical carcinoid tumors are classified as well-differentiated neuroendocrine tumors, Grade 2, one step more aggressive than typical carcinoid tumors (Grade 1) but significantly less aggressive than the high-grade neuroendocrine carcinomas of the lung. Although atypical carcinoid tumors grow slowly compared to most lung cancers, they spread to lymph nodes and occasionally to distant organs more often than typical carcinoid tumors, and they carry a meaningfully higher risk of recurrence after surgery. Understanding these differences is important for making sense of your diagnosis and treatment plan. This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.

What causes atypical carcinoid tumor of the lung?

Like typical carcinoid tumor, atypical carcinoid tumor is not strongly associated with tobacco smoking. This is an important distinction from most other lung cancers — the majority of people diagnosed with atypical carcinoid do not have a significant smoking history. The tumor develops when neuroendocrine cells in the airway lining acquire genetic changes that cause them to grow abnormally, but what triggers these changes in most patients remains unclear.

A recognized cause in a minority of patients is an inherited condition called Multiple Endocrine Neoplasia Type 1 (MEN1). MEN1 is caused by an inherited mutation in the MEN1 gene and predisposes affected individuals to tumors of several hormone-producing glands, including the parathyroid glands, the pituitary gland, and the pancreas. Lung neuroendocrine tumors — including both typical and atypical carcinoids — occur in approximately 5–10% of people with MEN1. If you have a personal or family history of MEN1-associated tumors, your doctor may recommend referral to a genetic counselor to discuss hereditary risk and genetic testing.

What are the symptoms of atypical carcinoid tumor of the lung?

Symptoms depend on the size and location of the tumor. Atypical carcinoid tumors most often arise in the central airways — the larger bronchi that lead into the lungs — though some develop in the smaller peripheral airways near the outer lung.

Tumors in the central airways may cause:

• A persistent cough.
• Coughing up blood.
• Shortness of breath or wheezing caused by partial blockage of an airway.
• Recurrent or slow-to-resolve pneumonia in the same area of the lung, which can occur when a tumor partially obstructs an airway and prevents normal mucus clearance.
• Chest pain or discomfort.

Smaller tumors located in the peripheral lung may cause no symptoms and are often found incidentally on a chest CT scan performed for an unrelated reason.

Because atypical carcinoid tumors are made of neuroendocrine cells, they can occasionally release hormones — most commonly serotonin — into the bloodstream. If enough serotonin accumulates in the circulation, it can cause a constellation of symptoms called carcinoid syndrome, which includes episodes of skin flushing (sudden redness of the face and neck), diarrhea, and occasionally wheezing. Carcinoid syndrome in lung carcinoid tumors almost always means the tumor has spread to the liver, because the liver normally breaks down serotonin before it can accumulate in the bloodstream. In rare cases, atypical carcinoid tumors can produce other hormones that cause additional systemic effects, such as Cushing syndrome from ectopic ACTH production.

What conditions are associated with atypical carcinoid tumor of the lung?

Atypical carcinoid tumor belongs to the family of lung neuroendocrine tumors, which are classified according to how aggressively they grow and how likely they are to spread. Understanding where atypical carcinoid sits in this spectrum helps explain how it differs from the related tumors a patient may encounter when seeking information.

• Typical carcinoid tumor (Grade 1) — The least aggressive lung neuroendocrine tumor. Defined by fewer than 2 cell divisions per 2 square millimeters and no areas of tumor cell death (necrosis). Cured by surgery in the great majority of cases. Learn more about typical carcinoid tumor of the lung.
• Atypical carcinoid tumor (Grade 2) — This diagnosis. An intermediate-grade neuroendocrine tumor is defined by 2–10 cell divisions per 2 square millimeters or by the presence of necrosis. Grows more aggressively than typical carcinoid and carries a higher risk of spreading to lymph nodes and distant organs.
• Large cell neuroendocrine carcinoma (Grade 3) — A high-grade neuroendocrine carcinoma with more than 10 cell divisions per 2 square millimeters and prominent necrosis. Behaves very aggressively and is treated similarly to small cell carcinoma.
• Small cell carcinoma (Grade 3) — The most aggressive neuroendocrine tumor of the lung, with very rapid growth and early spread. Learn more about small cell carcinoma of the lung.

The pathologist’s careful assessment of mitotic activity and necrosis in the tumor specimen is what places a neuroendocrine tumor in the correct category — and this classification directly determines the prognosis and treatment approach.

How is the diagnosis made?

The diagnosis of atypical carcinoid tumor is usually made from a tissue sample obtained by biopsy. Because atypical carcinoid tumors often arise near the central airways, they are frequently accessible by bronchoscopy — a procedure in which a thin flexible tube is passed through the mouth into the airways to visualize the tumor and collect a tissue sample. For tumors in less accessible locations, a CT-guided needle biopsy or fine needle aspiration (FNA) may be used. In some cases, the diagnosis is confirmed only after the entire tumor is removed surgically. Importantly, a definitive distinction between atypical carcinoid and typical carcinoid often cannot be made on a small biopsy sample alone, because determining the grade requires examining representative tissue from across the full tumor — something that is only possible on the complete surgical specimen.

Under the microscope, a pathologist identifies atypical carcinoid tumor by its neuroendocrine growth pattern — tumor cells arranged in organized nests, cords, or trabeculae — combined with the distinctive “salt and pepper” appearance of the cell nucleus, in which the chromatin (genetic material) is finely and evenly dispersed, creating a granular speckled pattern. This nuclear appearance is shared with typical carcinoid tumors and with all neuroendocrine tumors of the lung.

What distinguishes atypical carcinoid from typical carcinoid are two specific pathologic findings that the pathologist carefully evaluates in every case. First, the mitotic count: the pathologist counts the number of mitotic figures — tumor cells caught in the act of dividing — across a standardized area of 2 square millimeters of tumor tissue. A count of 2 or more mitotic figures per 2 square millimeters meets the threshold for atypical carcinoid. Second, the presence of necrosis — patches of dead tumor cells within the tumor. The presence of necrosis in a carcinoid-appearing tumor, even if the mitotic count is below 2, is sufficient to classify the tumor as atypical carcinoid rather than typical carcinoid. Either finding — elevated mitotic count or the presence of necrosis — is enough to make the atypical carcinoid diagnosis.

To confirm the neuroendocrine nature of the tumor and distinguish it from other lung tumors that can look similar under the microscope, the pathologist performs immunohistochemistry (IHC) — a laboratory technique using antibodies linked to colored dyes to detect specific proteins within the cells. Atypical carcinoid tumors show positive staining for chromogranin, synaptophysin, and CD56 (proteins produced by neuroendocrine cells), as well as TTF-1 (a lung lineage marker). The Ki-67 proliferation index — a measure of the fraction of cells actively dividing — is typically in the range of 5–20% in atypical carcinoid, higher than the usually below-5% seen in typical carcinoid but far lower than the 70–100% characteristic of the high-grade neuroendocrine carcinomas (large cell neuroendocrine carcinoma and small cell carcinoma). This intermediate Ki-67 range reflects and reinforces the Grade 2 classification.

Once the diagnosis is confirmed, imaging — including CT of the chest and abdomen and DOTATATE PET-CT (a specialized scan that detects somatostatin receptor-positive tumors) — is performed to determine the full extent of disease and guide treatment planning.

Histologic grade

Atypical carcinoid tumor of the lung is classified as Grade 2 (intermediate-grade well-differentiated neuroendocrine tumor) in the 2021 WHO Classification of Thoracic Tumors. Grade 2 sits between typical carcinoid (Grade 1, low grade) and the high-grade neuroendocrine carcinomas — large cell neuroendocrine carcinoma and small cell carcinoma (both Grade 3). The Grade 2 designation reflects the intermediate biological behavior of atypical carcinoid: it is more prone to lymph node spread and distant metastasis than typical carcinoid, and carries a meaningfully worse prognosis, but it still grows far more slowly and responds very differently to treatment compared to Grade 3 neuroendocrine carcinomas.

The grade is not an additional assessment applied to the tumor after the diagnosis is made — it is embedded in the diagnostic criteria themselves. A tumor is Grade 2 because its mitotic count is 2–10 per 2 square millimeters or because necrosis is present. The grade is therefore a direct reflection of the features observed by the pathologist in the specimen.

Lymphovascular invasion

Lymphovascular invasion (LVI) means that tumor cells have been found within blood or lymphatic vessels — the small channels that carry lymph — in or near the tumor. These vessels can act as pathways for tumor cells to travel to lymph nodes or distant organs.

• Lymphovascular invasion not identified — No tumor cells are seen within vessels near the tumor. This is a favorable finding.
• Lymphovascular invasion present — Tumor cells are found within vessels. This finding is associated with a higher risk of lymph node involvement and distant spread, and is more common in atypical carcinoid than in typical carcinoid. Its presence may influence decisions about the extent of lymph node sampling at surgery and the need for additional treatment afterward.

Pleural invasion

The pleura is the thin membrane that covers the outer surface of the lungs and lines the chest cavity. Pleural invasion means tumor cells have grown into one or both layers of this membrane.

• Pleural invasion not identified — The tumor has not extended to the pleural surface. This is the expected finding for most atypical carcinoid tumors.
• Visceral pleural invasion — Tumor cells have grown through the elastic layer of the pleura covering the lung surface. This increases the T stage and is associated with a higher risk of recurrence and spread.
• Parietal pleural invasion — Tumor cells have grown into the outer pleural layer lining the chest cavity, indicating more advanced local disease.

Surgical margins

Surgical margins are the cut edges of the tissue removed during the operation. The pathologist examines all margins to determine whether the tumor was completely removed.

• Negative margin — No tumor cells are seen at the cut edge of the tissue. This is the most favorable result and indicates the tumor was fully excised.
• Close margin — Tumor cells are present very near the cut edge but do not reach it. Depending on the location and distance, further treatment may be recommended.
• Positive margin — Tumor cells are present at the cut edge. This raises concern that some tumor remains and may lead to a recommendation for additional surgery or radiation therapy to the area.

Lymph nodes

Lymph nodes are small immune organs distributed throughout the chest. During surgery, the surgeon removes lymph nodes from specific locations within the lung and central chest — called lymph node stations — and sends them to the pathologist for examination under the microscope.

The pathology report will describe the total number of lymph nodes examined, their station locations, whether any contain tumor cells, and the size of any deposits found. Lymph node involvement is significantly more common in atypical carcinoid than in typical carcinoid, occurring in approximately 40–50% of cases. When cancer cells are found in a lymph node, this increases the nodal stage (N stage), influences treatment decisions, and is one of the most important predictors of recurrence risk and long-term outcome. The location of involved nodes — within the lung (N1), in the central chest on the same side (N2), or in the opposite side or neck (N3) — is also recorded, as this affects the overall stage and the feasibility of surgical resection.

Biomarker and molecular testing

Biomarker testing in atypical carcinoid tumor closely parallels that of typical carcinoid tumors, as both are well-differentiated neuroendocrine tumors of the lung. The most clinically relevant biomarkers relate to somatostatin receptor expression — which guides both staging imaging and treatment eligibility — and molecular features that identify hereditary risk. Atypical carcinoid does not harbor the targetable driver mutations relevant to non-small cell lung cancer, such as EGFR mutations or ALK rearrangements, and there is no established role for PD-L1 testing or immunotherapy in standard management.

Somatostatin receptor expression

Somatostatin receptors are proteins on the surface of neuroendocrine tumor cells that bind a hormone called somatostatin. Most well-differentiated neuroendocrine tumors of the lung express these receptors, though the proportion of positive tumors is somewhat lower in atypical carcinoid than in typical carcinoid. Somatostatin receptor expression has two important clinical consequences. First, it enables DOTATATE PET-CT imaging — a highly sensitive nuclear scan that detects receptor-positive tumor deposits throughout the body — which is the preferred staging and surveillance imaging tool for lung neuroendocrine tumors. Second, it determines eligibility for treatment with somatostatin analogs (octreotide LAR, lanreotide), which control hormone-related symptoms and have been shown to slow tumor growth in advanced somatostatin receptor-positive neuroendocrine tumors. Receptor expression can be assessed by immunohistochemistry on the tumor specimen or confirmed by DOTATATE PET-CT.

Everolimus

Everolimus (Afinitor) is an oral targeted drug that blocks a growth signaling pathway called mTOR, which is abnormally active in many neuroendocrine tumors. Everolimus is approved for the treatment of progressive, well-differentiated neuroendocrine tumors of lung origin based on the RADIANT-4 trial, which showed significantly longer progression-free survival compared to placebo. Because atypical carcinoid tumors are more likely to progress or recur than typical carcinoid tumors, everolimus is a more commonly needed option in this group. It is used when the tumor has grown or spread despite other treatments.

Germline testing for MEN1

Because atypical carcinoid tumors can occur as part of Multiple Endocrine Neoplasia Type 1 (MEN1) — an inherited condition caused by mutations in the MEN1 gene — your doctor may recommend referral to a genetic counselor, particularly if you have multiple lung neuroendocrine tumors, a personal history of MEN1-associated tumors (parathyroid, pancreatic, or pituitary), or a family history suggesting MEN1. Identifying a germline MEN1 mutation has important implications for you and your family members, who may benefit from surveillance for MEN1-related tumors.

For more information about biomarker testing in cancer, visit the Biomarkers and Molecular Testing section of MyPathologyReport.

Pathologic stage (pTNM)

Atypical carcinoid tumor of the lung is staged using the TNM system based on AJCC 8th edition criteria. The T category describes the size of the tumor and whether it has grown into nearby structures. The N category indicates whether cancer has spread to nearby lymph nodes. The M category — which describes spread to distant organs such as the liver, bone, or brain — is determined by imaging rather than the pathology specimen and is typically not reported in the surgical pathology report. Together, T, N, and M are combined to determine an overall stage, ranging from I (earliest) to IV (most advanced).

Tumor stage (pT)

• pT1a — Tumor is 1 cm or smaller, surrounded by lung or visceral pleura, with no involvement of the main bronchus.
• pT1b — Tumor is larger than 1 cm but no larger than 2 cm, otherwise meeting T1a criteria.
• pT1c — Tumor is larger than 2 cm but no larger than 3 cm, otherwise meeting T1a criteria.
• pT2a — Tumor is larger than 3 cm but no larger than 4 cm; or the tumor — regardless of size — has grown into the visceral pleura, involves the main bronchus without reaching the carina, or is associated with partial lung collapse.
• pT2b — Tumor is larger than 4 cm but no larger than 5 cm, otherwise meeting T2a criteria.
• pT3 — Tumor is larger than 5 cm but no larger than 7 cm; or the tumor has grown into the chest wall, phrenic nerve, or parietal pericardium; or a separate tumor nodule is present in the same lobe as the primary tumor.
• pT4 — Tumor is larger than 7 cm; or the tumor has grown into major structures such as the heart, large blood vessels, trachea, esophagus, or spine; or a separate tumor nodule is present in a different lobe of the same lung.

Nodal stage (pN)

• pNX — Lymph nodes were not examined.
• pN0 — No tumor cells found in any lymph nodes examined.
• pN1 — Tumor cells found in lymph nodes within the lung or near the main airway on the same side of the chest (intrapulmonary, hilar, or peribronchial nodes; stations 10–14).
• pN2 — Tumor cells found in lymph nodes in the central chest on the same side (ipsilateral mediastinal or subcarinal nodes; stations 4–9).
• pN3 — Tumor cells found in lymph nodes on the opposite side of the chest or in the lower neck (contralateral mediastinal, contralateral hilar, scalene, or supraclavicular nodes). This indicates advanced nodal disease.

What is the prognosis?

The prognosis for atypical carcinoid tumor is significantly worse than for typical carcinoid tumors but considerably better than for the high-grade neuroendocrine carcinomas. Overall, five-year survival is approximately 60–70%, though outcomes vary considerably by stage and the presence of specific pathologic features.

• Stage I (no lymph node involvement, no distant spread) — Five-year survival is approximately 85–90% with complete surgical resection, reflecting the favorable outcome achievable when the tumor is caught early and fully removed.
• Stage II–III (lymph node involvement) — Five-year survival falls to approximately 40–60% when lymph nodes are involved. The substantially higher rate of nodal spread compared to typical carcinoid — occurring in approximately 40–50% of cases — means that many patients present at this stage.
• Stage IV (distant metastasis) — Outcomes are worse, but patients can survive for many years given the slow-growing nature of atypical carcinoid even at the metastatic stage. The most common sites of distant spread are the liver, bone, and lung.

Pathologic features associated with a higher risk of recurrence include:

• Lymph node involvement — The most important predictor of recurrence in resected atypical carcinoid.
• Positive or close surgical margins — Raises concern for residual disease.
• Lymphovascular invasion — Associated with a higher risk of nodal and distant spread.
• Pleural invasion — Increases the T stage and is associated with a higher recurrence risk.
• Higher mitotic count within the Grade 2 range — Tumors at the upper end of the atypical carcinoid mitotic range (closer to 10 mitoses per 2 mm²) tend to behave more aggressively than those at the lower end.

Long-term surveillance is important because late recurrences — occurring more than five years after surgical resection — are well documented in atypical carcinoid. Annual imaging and regular clinical assessment for at least 10 years are typically recommended.

What happens after the diagnosis?

Surgery to completely remove the tumor is the primary treatment for atypical carcinoid tumor of the lung and offers the best chance of long-term cure. The type of surgery depends on the size and location of the tumor and the patient’s overall lung function. For most tumors, lobectomy (removal of an entire lobe of the lung) is the standard approach, performed using minimally invasive video-assisted thoracoscopic surgery (VATS) where possible. For central airway tumors, a bronchoplastic procedure — also called a sleeve resection — may preserve more functional lung tissue by removing and reconnecting the segment of airway containing the tumor. Systematic lymph node dissection is particularly important in atypical carcinoid given the high rate of nodal involvement, and thorough sampling of all relevant lymph node stations is standard practice.

Unlike typical carcinoid, adjuvant treatment after surgery is more actively considered for atypical carcinoid, particularly in patients with lymph node involvement (Stage II–III) or other high-risk pathologic features. Platinum-based chemotherapy (carboplatin or cisplatin combined with etoposide) is the most commonly used adjuvant regimen, though the evidence base remains limited and decisions are made on an individual basis in consultation with a multidisciplinary team. Radiation therapy to the chest may be considered in specific circumstances, such as a positive surgical margin that cannot be re-resected.

For patients with metastatic or progressive disease that cannot be surgically removed, treatment options include:

• Somatostatin analogs — Octreotide LAR or lanreotide, given by injection every 28 days, control carcinoid syndrome symptoms and have been shown to slow tumor growth in somatostatin receptor-positive neuroendocrine tumors.
• Everolimus (Afinitor) — An oral targeted therapy approved for progressive, well-differentiated neuroendocrine tumors of lung origin.
• Peptide receptor radionuclide therapy (PRRT) — A targeted radiation treatment using lutetium-177 DOTATATE (Lutathera) that delivers radiation directly to somatostatin receptor-positive tumor cells. PRRT is approved for gastroenteropancreatic NETs and is used in lung carcinoids in clinical practice for progressive, receptor-positive disease.
• Chemotherapy — Temozolomide-based regimens or platinum/etoposide combinations may be used for rapidly progressive atypical carcinoid tumors, particularly those with a higher Ki-67 index approaching the Grade 3 threshold.
• Clinical trials — New treatment approaches for neuroendocrine tumors are under active investigation. Your oncology team can advise whether any trials are appropriate for your situation.

After surgery, a typical surveillance protocol includes annual CT of the chest and abdomen and periodic DOTATATE PET-CT imaging for at least 10 years, given the documented risk of late recurrence. Blood and urine markers including chromogranin A and 5-hydroxyindoleacetic acid (5-HIAA) may also be monitored at follow-up visits.

Questions to ask your doctor

• Is my tumor confirmed as an atypical carcinoid (Grade 2), and what features — mitotic count or necrosis — led to this classification?
• What was the mitotic count, and where does it fall within the Grade 2 range?
• What is my pathologic stage, and what does it mean for my prognosis?
• Were the surgical margins clear? If not, what are the next steps?
• Were any lymph nodes found to contain tumor cells, and how many were examined?
• Was lymphovascular invasion identified in my pathology report?
• Was pleural invasion present, and how does it affect my stage?
• Was somatostatin receptor expression tested, and am I eligible for somatostatin analog therapy or DOTATATE PET imaging?
• Is adjuvant treatment recommended after surgery, and if so, what does it involve?
• Do my family or I have features that suggest I should be evaluated for MEN1 syndrome?
• What follow-up schedule is recommended, and for how long?
• What symptoms of recurrence should I watch for between appointments?
• Are there clinical trials I might be eligible for now or if the disease comes back?