Small Cell Carcinoma of the Lung: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC and Matt Cecchini MD PhD FRCPC
April 28, 2026

Small cell carcinoma of the lung is an aggressive type of lung cancer that develops from neuroendocrine cells — specialized cells that release hormones and chemical signals and are found in small numbers throughout the airways of the lungs. Small cell carcinoma is also called small cell neuroendocrine carcinoma or, in older literature, oat cell carcinoma. Unlike the other main types of lung cancer, which are grouped together as non-small cell lung cancer (NSCLC), small cell carcinoma behaves very differently: it grows and spreads rapidly, responds well to chemotherapy and radiation initially, but has a high rate of recurrence. 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 small cell carcinoma of the lung?

Tobacco smoking is by far the most important cause of small cell carcinoma of the lung. The link between smoking and small cell carcinoma is stronger than for almost any other lung cancer type — the vast majority of people diagnosed with this cancer have a significant history of smoking. The harmful chemicals in cigarette smoke damage the DNA inside lung cells, and over many years, these changes accumulate and can cause a cell to become cancerous.

Other risk factors include:

• Radon gas exposure — Radon is a naturally occurring radioactive gas that can accumulate in homes and buildings. Long-term radon exposure is a recognized cause of lung cancer and may contribute to a small proportion of small cell carcinoma cases.
• Occupational exposures — Prolonged contact with substances such as asbestos, silica dust, nickel, chromium compounds, or arsenic in certain workplaces increases the risk of developing lung cancer.
• Prior chest radiation — People who have received radiation therapy to the chest for a previous cancer have a modestly increased risk of developing a second lung cancer, including small cell carcinoma.

Small cell carcinoma in people who have never smoked is rare but does occur. In never-smokers, the underlying cause is often unclear.

What are the symptoms of small cell carcinoma of the lung?

Small cell carcinoma most often arises in the central portion of the lung, near the larger airways and the major blood vessels that supply the heart. As the tumor grows, it can press on or invade nearby structures, causing symptoms such as:

• A persistent or worsening cough.
• Coughing up blood.
• Shortness of breath or wheezing caused by partial airway blockage.
• Chest pain or discomfort.
• Hoarseness occurs when the tumor compresses the nerve that controls the voice box.
• Swelling of the face, neck, or arms if the tumor presses on the large vein that returns blood from the upper body to the heart (superior vena cava syndrome).

Because small cell carcinoma is made up of neuroendocrine cells — cells that normally produce hormones and chemical signals — the cancer cells can sometimes release hormones or trigger immune reactions that affect parts of the body far from the lung. These are called paraneoplastic syndromes, and they are more common in small cell carcinoma than in almost any other cancer type. Examples include:

• SIADH (syndrome of inappropriate antidiuretic hormone) — The tumor releases a hormone that causes the kidneys to retain too much water, leading to abnormally low sodium levels in the blood. Symptoms can include confusion, nausea, headache, or seizures in severe cases.
• Ectopic ACTH secretion — The tumor produces a hormone that overstimulates the adrenal glands, causing a condition called Cushing syndrome, with symptoms including weight gain, high blood pressure, high blood sugar, and muscle weakness.
• Lambert-Eaton myasthenic syndrome (LEMS) — The immune system, in its attempt to fight the cancer, produces antibodies that mistakenly attack the junctions between nerves and muscles. This causes muscle weakness — particularly in the legs and trunk — that tends to improve briefly with repeated use but returns with sustained activity.
• Paraneoplastic encephalomyelitis — Immune-mediated inflammation of the brain and spinal cord that can cause memory problems, confusion, unsteady walking, or other neurological symptoms.

Paraneoplastic syndromes can sometimes appear before the cancer itself is detected on imaging, and their presence in someone with a history of smoking is an important clinical signal to investigate for small cell carcinoma.

How is the diagnosis made?

The diagnosis of small cell carcinoma is usually made from a tissue sample obtained by biopsy. Because the tumor most often arises near the central airways, it is frequently accessible via bronchoscopy — a procedure in which a thin, flexible tube is passed into the airways — or endobronchial ultrasound (EBUS). For tumors or lymph node deposits in less accessible locations, a CT-guided needle biopsy or fine needle aspiration (FNA) may be used. Because small cell carcinoma so commonly spreads to lymph nodes and distant sites before diagnosis, the biopsy is sometimes performed on a lymph node or on a distant site, such as the liver, bone marrow, or brain, rather than on the lung itself. Surgery to remove the tumor is only occasionally performed (see staging section below) and is not the standard means of making the diagnosis.

Under the microscope, a pathologist identifies small cell carcinoma by its characteristic appearance. The tumor cells are much smaller than most cancer cells — roughly the size of three lymphocytes placed side by side — and contain very little cytoplasm (the material surrounding the nucleus inside the cell). This sparse cytoplasm makes the cells look densely crowded and gives them a dark blue color under the microscope, a feature pathologists describe as hyperchromatic. The genetic material (chromatin) inside each cell’s nucleus has a fine, evenly dispersed, salt-and-pepper appearance — distinct from many other cancer types. Large, prominent nucleoli (the dark spots within a nucleus that direct protein production) are characteristically absent. The tumor cells divide very rapidly, and numerous mitotic figures — cells caught in the act of dividing — are typically visible throughout the tumor.

To confirm the diagnosis and distinguish small cell carcinoma from other types of lung cancer that can look similar under the microscope — particularly large cell neuroendocrine carcinoma and poorly differentiated non-small cell carcinomas — the pathologist performs immunohistochemistry (IHC), a laboratory technique that uses antibodies linked to colored dyes to detect specific proteins within the cells. Small cell carcinoma typically shows positive staining for chromogranin, synaptophysin, and CD56 (proteins produced by neuroendocrine cells) and positive staining for TTF-1 (a lung lineage marker). Because tumor cells divide so rapidly, the Ki-67 index — a measure of the proportion of cells actively dividing — is typically very high, often exceeding 70% and approaching 100%. This extremely high proliferation rate is one of the features that distinguishes small cell carcinoma from lower-grade neuroendocrine tumors of the lung, such as carcinoid tumors.

Once the diagnosis is confirmed, imaging — including CT of the chest and abdomen, brain MRI, and often a PET scan — is performed to determine how far the cancer has spread throughout the body. This staging information guides treatment decisions.

Histologic grade

Small cell carcinoma of the lung is not assigned a histologic grade in the conventional sense. Grading systems used for other lung cancers — such as the well/moderately/poorly differentiated classification used in adenocarcinoma — do not apply here. Small cell carcinoma is by definition a high-grade cancer: its very rapid cell division rate, aggressive behavior, and tendency to spread early are defining features of the diagnosis rather than variables that differ between individual tumors. All small cell carcinomas are treated as high-grade disease regardless of any subtle microscopic differences between cases.

Surgical margins

Surgical margins are the cut edges of the tissue removed during an operation. The pathologist examines all margins under the microscope to determine whether the tumor was completely removed. Margins are reported only when surgery has been performed — they are not relevant after a biopsy, which, by design, removes only a small sample of tissue.

• Negative margin — No cancer cells are seen at the cut edge of the tissue. This indicates that the tumor was fully removed, which is the most favorable outcome.
• Close margin — Cancer cells are present very near the cut edge but do not reach it. Further treatment may be recommended depending on the distance and location.
• Positive margin — Cancer cells are present at the cut edge. This raises concern that some tumor remains and often leads to discussion of additional radiation therapy to the surgical site.

Lymph nodes

Lymph nodes are small structures that are part of the immune system and are distributed throughout the chest. Small cell carcinoma spreads to lymph nodes early and frequently — nodal involvement is present in the majority of patients at diagnosis. When surgery is performed (in rare, very early-stage cases), the surgeon removes lymph nodes from specific locations within the lung and central chest 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 cancer cells, and the size of any deposits found. The location of involved nodes determines the nodal stage (N stage) and has important implications for whether the disease is classified as limited or extensive stage (see staging section below). When cancer cells are found in a biopsy performed on a lymph node rather than the lung itself, the report will describe the biopsy site and confirm the presence of small cell carcinoma.

Biomarker and molecular testing

Biomarker testing plays a role in the management of small cell carcinoma of the lung, though the landscape is very different from that of non-small cell lung cancer. Small cell carcinoma does not harbor the targetable driver mutations — such as EGFR mutations, ALK rearrangements, or KRAS mutations — that are central to treatment decisions in lung adenocarcinoma. Instead, the most clinically relevant biomarkers in SCLC relate to immunotherapy eligibility and emerging targeted approaches.

PD-L1

PD-L1 (programmed death-ligand 1) is a protein that some cancer cells display on their surface to hide from the immune system. Drugs called checkpoint inhibitors block this mechanism, allowing the immune system to recognize and attack the cancer. In small cell carcinoma, PD-L1 expression tends to be low overall, and unlike in non-small cell lung cancer, the PD-L1 score does not reliably predict which patients will respond to immunotherapy. Nevertheless, immunotherapy is now a standard part of first-line treatment for extensive-stage small cell carcinoma regardless of PD-L1 level. Atezolizumab (Tecentriq), combined with carboplatin and etoposide chemotherapy, was approved based on the IMpower133 trial, which showed improved overall survival compared to chemotherapy alone. Durvalumab (Imfinzi) combined with platinum-etoposide is an alternative, supported by the CASPIAN trial. Your report may describe the PD-L1 result as a percentage or score, but the treatment decision to add immunotherapy is not based on a specific PD-L1 threshold in SCLC.

Tumor mutational burden (TMB)

Tumor mutational burden (TMB) is a measure of the number of mutations in a cancer cell’s DNA. Small cell carcinoma — strongly associated with decades of tobacco exposure — typically carries a very high TMB, reflecting the extensive DNA damage caused by cigarette smoke. Pembrolizumab is FDA-approved for any solid tumor with TMB ≥10 mutations per megabase of DNA that has progressed after prior treatment. This approval includes small cell carcinoma, and TMB testing by next-generation sequencing may be performed to determine eligibility. Your report will state the numerical TMB value and whether it meets the ≥10 mut/Mb threshold.

DLL3

DLL3 (delta-like ligand 3) is a protein that is highly expressed on the surface of small cell carcinoma cells in approximately 80–85% of cases, but is largely absent from normal adult tissues. This makes it an attractive therapeutic target. Tarlatamab (Imdelltra), a bispecific antibody that bridges DLL3-expressing cancer cells to cancer-killing immune cells, received accelerated FDA approval in 2024 for patients with extensive-stage small cell carcinoma that has progressed after platinum-based chemotherapy. In the DeLLphi-301 trial, tarlatamab produced response rates of approximately 40% in heavily pre-treated patients, with some responses lasting more than a year. DLL3 expression is measured by immunohistochemistry and is typically reported as the percentage of tumor cells staining positive. Because the great majority of small cell carcinomas express DLL3, testing may be performed at diagnosis to document eligibility for this therapy if the cancer progresses after first-line treatment.

Mismatch repair (MMR) and microsatellite instability (MSI)

Mismatch repair deficiency (dMMR) and microsatellite instability-high (MSI-H) are uncommon in small cell carcinoma but can occur. When present, dMMR/MSI-H tumors are eligible for pembrolizumab under its tumor-agnostic approval — meaning the drug is approved for any solid tumor with these features, regardless of cancer type. Testing is performed by immunohistochemistry for the four MMR proteins (MLH1, PMS2, MSH2, MSH6) or by PCR or next-generation sequencing. Your report will describe the result as MMR intact (pMMR) or MMR deficient (dMMR).

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

Pathologic stage

Small cell carcinoma of the lung is staged using two systems: the limited-versus-extensive-stage system, which is the primary system used to guide treatment decisions, and the TNM system, which applies when surgery has been performed.

Limited versus extensive stage

The limited/extensive staging system was developed specifically for small cell carcinoma and divides the disease into two categories based on how far the cancer has spread at the time of diagnosis.

• Limited stage — The cancer is confined to one side of the chest and can be encompassed within a single radiation treatment field. This includes tumors in one lung and lymph nodes on the same side of the chest, and may include lymph nodes in the center of the chest (mediastinum). Approximately 30–35% of patients present with limited-stage disease at diagnosis. Standard treatment is concurrent chemotherapy and radiation, with the goal of long-term disease control.
• Extensive stage — The cancer has spread beyond one side of the chest to the other lung, to lymph nodes on the opposite side, to fluid around the lung or heart, or to distant organs such as the brain, liver, adrenal glands, or bone. The majority of patients — approximately 65–70% — have extensive-stage disease at diagnosis, reflecting how quickly and silently small cell carcinoma spreads. Standard treatment is systemic chemotherapy combined with immunotherapy.

TNM pathologic stage (pTNM)

The TNM staging system is used when surgery has been performed to remove the tumor — a situation that applies only to a small minority of patients with very early-stage, limited-stage small cell carcinoma. Surgery is not the standard treatment for most patients with this cancer; it is considered only when the tumor is very small, confined to the lung, and there is no lymph node involvement. When surgical staging is performed, the AJCC 8th edition criteria are used.

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 main airway junction (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 cancer cells found in any lymph nodes examined.
• pN1 — Cancer 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 — Cancer cells found in lymph nodes in the central chest on the same side (ipsilateral mediastinal or subcarinal nodes; stations 4–9).
• pN3 — Cancer 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 small cell carcinoma of the lung depends primarily on the stage at diagnosis. Small cell carcinoma is one of the most aggressive lung cancers — it grows quickly and spreads early — but it also responds well to chemotherapy and radiation initially. The challenge is that most patients relapse after their initial response.

• Limited stage — With concurrent chemotherapy and radiation, median survival is approximately 15–25 months, and roughly 15–25% of patients are alive at five years. Patients who achieve a complete response to chemoradiation are offered prophylactic cranial irradiation (PCI) — low-dose radiation to the brain — to reduce the risk of brain metastasis, which is a common site of relapse in this disease.
• Extensive stage — With modern first-line chemotherapy plus immunotherapy regimens, median overall survival is approximately 12–14 months. Long-term survivors exist but are uncommon; five-year survival rates in extensive-stage disease remain below 5%.

When the cancer relapses after initial treatment, the timing and nature of relapse matter. Tumors that relapse more than 90 days after completing first-line treatment are described as sensitive relapse and tend to respond better to second-line chemotherapy. Tumors that progress during or very shortly after first-line treatment are described as refractory and are associated with a worse prognosis and fewer treatment options.

Overall health, performance status, and the presence of paraneoplastic syndromes also influence outcomes. Smoking cessation is strongly encouraged even after diagnosis, as it is associated with improved treatment tolerance and better overall survival.

What happens after the diagnosis?

After the pathology report is finalized, your doctor will review the findings together with your imaging results and overall health to develop a treatment plan. Small cell carcinoma is managed by a multidisciplinary team including a medical oncologist, radiation oncologist, respirologist, and pathologist.

For limited-stage disease, the standard treatment is concurrent platinum-based chemotherapy (carboplatin or cisplatin) and etoposide, given alongside radiation therapy to the chest. This combined approach is more effective than either treatment alone. For patients who achieve a complete or near-complete response to treatment, prophylactic cranial irradiation (PCI) — low-dose radiation delivered to the entire brain — is often offered to reduce the risk of brain relapse, which is otherwise common. In highly selected patients with very early-stage disease (small tumor, no lymph node involvement), surgical resection followed by adjuvant chemotherapy may be considered, but this applies to a small minority of patients.

For extensive-stage disease, treatment is systemic. The current standard first-line regimen is carboplatin or cisplatin combined with etoposide and an immune checkpoint inhibitor — either atezolizumab (Tecentriq) or durvalumab (Imfinzi). Most patients respond well initially, with response rates of approximately 60–70%, but the cancer typically progresses within 6–12 months. For patients with brain metastases at diagnosis, brain radiation (either whole-brain radiation or stereotactic radiosurgery to individual lesions) is often added to systemic treatment.

When the cancer relapses after first-line treatment, second-line options include topotecan (Hycamtin), lurbinectedin (Zepzelca), or — for patients with DLL3-positive tumors — tarlatamab (Imdelltra). Clinical trials are an important option at relapse, and your oncology team can advise whether any are open and appropriate for your situation.

Follow-up after treatment involves regular CT imaging of the chest, abdomen, and pelvis, as well as brain MRI, to monitor for signs of recurrence. The frequency of follow-up imaging will be guided by your care team based on your stage and treatment response.

Questions to ask your doctor

• Is my cancer limited stage or extensive stage, and what does that mean for my treatment options?
• How was the diagnosis made — from a biopsy of the lung, a lymph node, or another site?
• What biomarker tests were performed on my tumor, and what were the results?
• Is my tumor DLL3-positive, and does that affect my treatment options if the cancer comes back?
• Will immunotherapy be part of my first-line treatment, and which drug will be used?
• What is prophylactic cranial irradiation, and is it recommended for me?
• What are the signs of relapse I should watch for, and what should prompt me to contact you between appointments?
• What are the second-line treatment options if the cancer comes back?
• Are there clinical trials I might be eligible for, now or if the cancer progresses?
• Do any of my symptoms suggest a paraneoplastic syndrome, and how will that be managed?
• What is my follow-up schedule after treatment is complete?
• Can you connect me with resources to help with smoking cessation?