When discussing treatment options for recurrent or metastatic head and neck cancer, patients often encounter the term PD-L1. While it may sound like technical jargon, this biomarker is a critical piece of the puzzle in modern oncology. It serves as a primary indicator for whether immunotherapy —a treatment designed to empower the body’s own defenses—will be an effective strategy for a specific patient.
The “Cloaking Device” of Cancer Cells
To understand why PD-L1 testing is necessary, it is helpful to understand how cancer evades the immune system.
In a healthy body, the immune system uses proteins to distinguish between “self” (healthy tissue) and “non-self” (invaders like bacteria or viruses). PD-L1 (programmed death-ligand 1) is a protein that normally acts as a safety switch, preventing the immune system from attacking the body’s own healthy cells.
Cancer cells often hijack this mechanism. By expressing PD-L1 on their surface, tumor cells can bind to PD-1 proteins on immune cells. This interaction essentially sends a “do not attack” signal to the immune system.
“It essentially helps cancer cells ‘hide’ from the immune system,” explains Dr. Jeff Yorio, a medical oncologist at Texas Oncology.
By identifying how much PD-L1 is present, oncologists can determine if immune checkpoint inhibitors —medications that block this “hide” signal—can strip away the cancer’s camouflage and allow the immune system to recognize and destroy the tumor.
Deciphering the Scores: CPS vs. TPS
PD-L1 results are not a simple “yes” or “no.” Instead, they are reported as numerical scores that describe the intensity and prevalence of the protein. There are two primary ways these scores are calculated:
1. Combined Positive Score (CPS)
The CPS is the most common metric used for head and neck cancers. Rather than looking only at the tumor, it measures the percentage of both tumor cells and surrounding immune cells that carry the PD-L1 protein.
* Scale: 0 to 100.
* Interpretation: A score below 1 is considered negative; 1–19 is low; and 20 or above is considered high.
* Why it matters: Because it accounts for the entire tumor environment, it provides a more comprehensive view of how the cancer is interacting with the immune system.
2. Tumor Proportion Score (TPS)
The TPS is more narrow in its focus. It only measures the percentage of actual cancer cells that express PD-L1, ignoring the surrounding immune cells.
* Interpretation: A score of 1% or higher is positive, while 50% or higher is considered high.
The Testing Process: What to Expect
Determining a PD-L1 score requires a physical sample of the tumor, typically obtained through a biopsy or surgical procedure.
Once the tissue is collected, it undergoes a laboratory process called immunohistochemistry. During this process, special stains are applied to the tissue, making the PD-L1 proteins visible under a microscope so they can be accurately counted.
Key considerations for patients:
* Timeline: Results typically take between five to seven days, though they can sometimes take up to two weeks.
* Sample Quality: The accuracy of the test depends heavily on the sample. If a biopsy does not contain a sufficient amount of cancer cells, it may be impossible to perform a reliable test.
Summary: PD-L1 testing is a vital diagnostic tool that measures how much a tumor is using “cloaking” proteins to hide from the immune system. By analyzing these scores (CPS or TPS), doctors can make informed, personalized decisions about whether immunotherapy is the most effective path forward for treatment.
