Tumor Proportion Score (TPS) is a specialized diagnostic tool in cancer treatment. It provides information about specific tumor characteristics, particularly in non-small cell lung cancer, helping medical professionals make informed decisions about suitable treatment pathways and tailoring therapies for improved patient care.
Understanding PD-L1 and Its Role
The immune system distinguishes between healthy and abnormal cells. Programmed Death-Ligand 1 (PD-L1) is a protein found on the surface of various cells, including some healthy cells. Its normal function is to act as a “brake” on the immune system, preventing immune cells, specifically T-cells, from attacking healthy tissues. When PD-L1 on a healthy cell binds to its partner protein, PD-1, on a T-cell, it sends a signal that tells the T-cell to stand down, thus maintaining immune tolerance.
Cancer cells, however, can exploit this natural mechanism for their survival. Many cancer cells display PD-L1 on their surfaces, engaging the PD-1 protein on immune T-cells. This interaction effectively “turns off” or deactivates T-cells that would otherwise destroy the cancer cells, allowing the tumor to evade the immune system.
The expression of PD-L1 on tumor cells is a significant factor in how cancer progresses and how it might respond to certain treatments. High levels of PD-L1 on tumor cells can create an immunosuppressive environment around the tumor. This environment hinders the body’s natural ability to fight the cancer, making it a target for specific therapies designed to counteract this immune evasion.
How Tumor Proportion Score is Determined
Determining the Tumor Proportion Score begins with a tissue sample from the patient’s tumor, typically collected via biopsy. Pathologists then prepare this tissue for analysis.
Pathologists use a specialized technique called immunohistochemistry (IHC) to identify PD-L1 protein in the tumor sample. This method applies specific antibodies that bind to the PD-L1 protein. These antibodies are linked to a colored stain, which makes the PD-L1-expressing cells visible under a microscope.
The Tumor Proportion Score is calculated by counting the percentage of viable tumor cells that show any detectable PD-L1 staining, regardless of color intensity. The pathologist visually assesses hundreds or thousands of tumor cells within the sample to arrive at this precise percentage.
Interpreting the Tumor Proportion Score
The Tumor Proportion Score (TPS) measures the percentage of tumor cells displaying the PD-L1 protein. A higher percentage indicates a greater presence of PD-L1 on the tumor cells. This numerical result helps categorize the tumor’s PD-L1 expression level.
Common classifications for TPS often include categories such as less than 1% positive, 1% to 49% positive, and 50% or greater positive. A TPS of less than 1% signifies very low or no PD-L1 expression. A score within the 1% to 49% range indicates moderate expression, while a score of 50% or greater suggests high expression of PD-L1.
A higher TPS suggests the tumor is more likely to use the PD-L1 pathway to evade the immune system. This indicates a greater potential for the tumor to respond to certain immunotherapies. The score therefore serves as an indicator of the tumor’s biological characteristics related to immune evasion.
TPS in Guiding Cancer Treatment
The Tumor Proportion Score guides treatment decisions for various cancers, particularly non-small cell lung cancer (NSCLC). Doctors use TPS to determine if a patient is a suitable candidate for specific immunotherapies, treatments that harness the body’s immune system to fight cancer. These often involve PD-1 or PD-L1 inhibitors.
For example, in NSCLC, a TPS of 50% or greater often indicates that a patient may benefit from receiving a PD-1 or PD-L1 inhibitor as a first-line treatment. This high score indicates the tumor relies heavily on PD-L1 to evade immune attack, making it a good target for these drugs. Patients with lower TPS values, such as 1% to 49%, might also be considered for these immunotherapies, often in combination with chemotherapy, or in later stages of treatment.
TPS serves as a valuable biomarker, helping to personalize treatment plans for individual patients. By understanding the level of PD-L1 expression on a tumor, doctors can select therapies that are more likely to be effective, potentially leading to better outcomes and reduced side effects compared to a generalized approach. While TPS is a powerful tool, it is generally considered alongside other patient and tumor characteristics to develop a comprehensive and tailored treatment strategy.