A pathology report is one of the most important documents in your cancer journey. It contains the results of laboratory analysis of your tumour tissue - information that directly determines what treatments are available to you, including whether you are eligible for pembrolizumab. Here is how to read it.
What a pathology report contains
A standard cancer pathology report covers several sections. The diagnosis section confirms the tumour type and histological subtype - for example, "adenocarcinoma of the lung" or "squamous cell carcinoma of the head and neck". The grade describes how abnormal the cancer cells look under the microscope (Grade 1 is most similar to normal; Grade 3 or 4 is most abnormal). The stage describes how far the cancer has spread - usually using the TNM system (Tumour, Node, Metastasis).
For pembrolizumab eligibility, the section you need is the immunohistochemistry (IHC) results - specifically the PD-L1 staining report.
Reading the PD-L1 IHC result
The PD-L1 IHC section will state the antibody clone used (22C3 is the one validated for pembrolizumab eligibility assessment), the scoring method (TPS or CPS depending on cancer type), and the numerical result. A report might read: "PD-L1 IHC 22C3: TPS 65%" or "CPS 18". The number is what matters for eligibility thresholds.
If your report uses a different antibody clone or scoring system, it may not be directly applicable to pembrolizumab eligibility criteria - which are validated specifically with the 22C3 clone. An oncologist can advise on whether retesting is needed.
Other relevant results
For lung cancer patients, the report should also include EGFR mutation status and ALK rearrangement testing - as positive results in these would typically direct treatment toward targeted therapy rather than pembrolizumab. For any tumour type, MSI/MMR testing results may appear: MSI-H or dMMR status can qualify patients for pembrolizumab regardless of PD-L1 score.
Tumour mutation burden (TMB) may also be reported, particularly for patients assessed for pan-tumour pembrolizumab eligibility on the basis of high TMB. This is a separate biomarker from PD-L1.
How to use your report
You are entitled to a copy of your pathology report. If you do not have one, ask your oncologist or GP - they can provide it. When completing our eligibility check, having this report to hand means you can give us the most accurate information and receive the most useful preliminary assessment. If you are unsure what your results mean, the Welcome Call is a good opportunity to go through them with a clinician.
Have your pathology report and want to understand your eligibility?
Our questionnaire takes three minutes. An oncologist reviews every submission within 24 hours and can interpret your results in clinical context.
Check your eligibility