Small-cell lung cancer (SCLC) causes cells in lung tissue to mutate and grow uncontrollably. When these cells look different than healthy cells under a microscope, doctors call this poorly differentiated SCLC. Poorly differentiated cells can spread more quickly than well-differentiated cells.

Differentiation helps doctors assign a grade to tumors, which can tell them important information about the cancer and how it may spread.

In this article, we look at how poorly differentiated SCLC differs from other tumor grades, how doctors diagnose it, and how diagnosis may affect prognosis and treatment.

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Pathologists use differentiation to describe how cancerous cells look under a microscope.

There are four types, or grades, of differentiation:

  • Well-differentiated: This is where cancerous cells look similar to healthy lung tissue cells and appear relatively organized.
  • Moderately differentiated: Here, cancerous cells share some similarities with healthy lung tissue cells but also differ in some ways.
  • Poorly differentiated: These cancerous cells look different from healthy lung tissue cells.
  • Undifferentiated: These cells differ greatly from healthy lung tissue cells.

Pathologists assign a number to each grade. The numbers range from 1 (well-differentiated) to 4 (undifferentiated). There can also be grade X, which pathologists use when the grade is unknown.

Tumor grades are different from cancer staging. Cancer staging determines how advanced the cancer is, based on how far it has spread throughout the body. For SCLC, doctors often use a two-stage system: limited stage and extensive stage.

Limited stage SCLC is present on one side of the chest and typically causes cancer in one lung and sometimes the lymph nodes on the same side of the chest.

In extensive stage SCLC, the cancer has spread to both lungs or to other body parts.

Tumor grades and cancer staging will both appear on a pathology report for people with SCLC. A doctor will be able to explain what the results mean and how they may affect treatment decisions and outcomes.

The American Cancer Society (ACS) provides a useful guide on understanding the contents of a pathology report for people with lung cancer.

SCLC is a serious condition that is difficult to cure. According to the National Cancer Institute, current treatments will not cure lung cancer for most people.

Treatment can slow or prevent the tumor from growing in people with SCLC, but the cancer is likely to return.

On average, people with poorly differentiated SCLC typically have worse outcomes than those with a lower grade. Poorly differentiated SCLC can grow and spread more quickly than well-differentiated SCLC and requires intensive treatment.

The overall prognosis for SCLC depends on several factors, in addition to differentiation. A person’s age, health status, and cancer stage also play a role.

The ACS estimates that around 2 in 3 people with SCLC have extensive stage cancer by the time doctors diagnose it. Extensive stage SCLC is more difficult to treat than limited stage SCLC, as it has spread to multiple parts of the body.

The 5-year relative survival rate for people with SCLC that is only in one area of the chest is 27%. This means that people with localized SCLC have a 27% chance of surviving for at least 5 years from the time they receive their diagnosis, compared with the rest of the population.

If the cancer has spread outside the lung to nearby tissues, this rate falls to 16%. If SCLC reaches distant organs and tissues, the 5-year relative survival rate is 3%.

To diagnose SCLC, a doctor will ask about a person’s symptoms and any risk factors for the condition, such as smoking.

They will also ask about family history and perform a physical examination. If lung cancer is a possibility, the doctor may order diagnostic tests.

There are a range of tests doctors may use, depending on the case. These include:

  • medical imaging tests, such as an X-ray and CT scans, to see inside the lungs
  • a tissue biopsy, which involves a doctor taking a small sample of cells from lung tissue to send to a laboratory
  • sputum cytology, which involves taking a sample of mucus from the lungs for analysis
  • bronchoscopy, where a doctor uses a small camera to see inside the airways
  • pulmonary function tests

If a person receives an SCLC diagnosis, a pathologist can look at lung tissue cells from a biopsy under a microscope to determine the tumor grade.

They will also assess the stage of the cancer and other important information and compile this into a report. Doctors use this to guide treatment decisions.

In people with SCLC, cells in lung tissue mutate and grow uncontrollably. When these cells look different from healthy lung tissue cells under a microscope, doctors call this poorly differentiated SCLC.

Pathologists use cell differentiation to assign tumors a grade, based on the extent to which cancerous cells differ from healthy cells in the same part of the body.

Poorly differentiated SCLC can spread more easily than lower-grade SCLC tumors. This is because poorly differentiated cells can grow faster. However, every case is different, and differentiation is not the only factor that determines the outlook.

People should contact a doctor, such as an oncologist, if they have any questions about tumor grades and what they mean.