Tumor markers are substances found in the body that can provide information about cancer. These markers are often proteins or other molecules produced by cancer cells, or by the body in response to cancer. While they do not diagnose cancer on their own, they serve as one tool among several, complementing imaging scans, physical examinations, and biopsies. Tumor markers help medical professionals understand the disease’s characteristics and how it might respond to treatment.
Circulating Tumor Markers for Monitoring
Certain tumor markers can be measured in the blood, offering insights into lung cancer activity. For non-small cell lung cancer (NSCLC), common circulating markers include Carcinoembryonic antigen (CEA), Cytokeratin fragment 21-1 (CYFRA 21-1), and Squamous cell carcinoma antigen (SCCA). CEA is a protein typically found in low levels in healthy adults, with elevated concentrations observed in NSCLC. CYFRA 21-1 is a protein fragment expressed in normal squamous cells, and its presence in the blood can indicate NSCLC. SCCA is a protein that reflects the differentiation grade of squamous cell cancers.
For small cell lung cancer (SCLC), different circulating markers are used. Neuron-specific enolase (NSE) is an enzyme often elevated in SCLC. Pro-gastrin-releasing peptide (Pro-GRP) can also be elevated in SCLC, with levels rising rapidly in advanced stages. These markers are produced by the tumor cells and released into the bloodstream, making them detectable through blood tests.
The Role of Tumor Markers in Patient Care
Circulating tumor markers contribute to patient care by assisting in several aspects of disease management. Initially, elevated levels of these markers can offer a general idea about the aggressiveness of the disease, aiding in assessing prognosis. Higher baseline levels might suggest a more advanced or active cancer, helping clinicians anticipate the disease course.
During treatment, monitoring changes in marker levels helps evaluate how well therapies are working. A decrease in marker levels often indicates that the treatment is effective and the tumor is shrinking or responding. Conversely, stable or rising levels might suggest that the cancer is not responding as expected, or that the disease is progressing. This allows doctors to adjust treatment plans as needed.
Following initial treatment, regularly checking tumor marker levels can help detect early signs of cancer recurrence. A rise in previously normalized marker levels can signal that the cancer may have returned, sometimes before it is visible on imaging scans.
Tissue Markers for Targeted Therapy and Immunotherapy
Beyond circulating markers, specific genetic and protein markers found directly within the tumor tissue play a distinct role in guiding treatment decisions. This approach is central to personalized medicine, tailoring treatments to an individual’s cancer. These tissue markers are predictive, indicating which specific therapies are most likely to be effective.
For targeted therapy, mutations in certain genes can be identified. For example, mutations in the Epidermal Growth Factor Receptor (EGFR) gene are present in about 15-20% of NSCLC cases and can be treated with EGFR tyrosine kinase inhibitors. Similarly, rearrangements in the Anaplastic Lymphoma Kinase (ALK) gene, found in 2-7% of NSCLC, and ROS1 gene fusions, present in 1-2% of lung cancers, can be targeted with ALK or ROS1 inhibitors. Other mutations, such as those in KRAS and BRAF, also have targeted therapies available or under investigation. Identifying these genetic alterations means a patient may benefit from a drug designed to block the abnormal proteins driving the cancer’s growth.
For immunotherapy, the PD-L1 protein marker is assessed. PD-L1 is expressed on the surface of some NSCLC cells. When PD-L1 binds to its receptor, PD-1, on immune T-cells, it can inactivate the T-cell, preventing it from attacking the cancer. Tumors with high levels of PD-L1 expression respond well to immunotherapy drugs called checkpoint inhibitors, which block this interaction and allow the immune system to recognize and fight the cancer. Testing for these tissue markers helps ensure patients receive the most appropriate and effective treatment.
Interpreting Results and Limitations
Understanding tumor marker results requires considering them within the broader clinical picture. Marker levels are never interpreted in isolation; they are always evaluated alongside imaging scans and the patient’s clinical condition and symptoms. This comprehensive approach helps doctors make informed decisions, as a single number does not tell the whole story.
A notable limitation of tumor markers is their lack of specificity. Elevated levels can sometimes occur due to non-cancerous conditions, including smoking, chronic obstructive pulmonary disease (COPD), or various inflammatory processes. This can lead to “false positives,” where a marker is high but cancer is not present, underscoring why these tests are not used for initial diagnosis.
Not all lung cancers produce detectable levels of tumor markers. A person can have lung cancer even if their tumor marker levels remain normal, meaning a “normal” result does not rule out cancer. The trend of marker levels over time is more informative than any single measurement. A consistent rise or fall provides more meaningful data about disease progression or response to treatment than an isolated high or low value.