What is the role of P40 (p40 protein) testing in diagnosing lung cancer?

P40 Testing in Lung Cancer Diagnosis

P40 immunohistochemistry is a highly specific marker for identifying squamous cell carcinoma in lung cancer, with superior specificity (98-100%) compared to p63, and should be used as part of a minimal immunohistochemical panel alongside TTF-1 to accurately subclassify non-small cell lung cancer (NSCLC) on small biopsy specimens. 1

Primary Role: Squamous Cell Carcinoma Identification

P40 is specifically used to identify squamous differentiation in NSCLC, not as a general lung cancer screening or diagnostic tool. 1

• P40 detects the N-terminal-truncated protein isoform (ΔNp63) of the p63 gene, which is expressed by basal or progenitor cells of bronchial epithelium 1
• In surgically resected specimens, P40 demonstrates 100% sensitivity and 98-100% specificity for identifying squamous cell carcinoma 1
• Using tumor tissue microarrays, P40 shows 80.95% sensitivity and 90.0% specificity for lung squamous cell carcinoma 1

Superiority Over p63

P40 has markedly superior specificity compared to the traditional p63 marker, eliminating a critical diagnostic pitfall. 1, 2

• While p63 is positive in 100% of squamous cell carcinomas, it also shows reactivity in 31-37% of adenocarcinomas and 54% of large cell lymphomas 1, 2
• P40 maintains 100% sensitivity for squamous cell carcinoma but is positive in only 3% of adenocarcinomas and 0% of lymphomas (specificity 98%) 2
• P40 has higher specificity (90.0%) but lower sensitivity (80.95%) than p63 (specificity 80.0%, sensitivity 93.5%) in identifying lung squamous cell carcinoma 1
• The American College of Chest Physicians notes that p40 is superior to p63 and other markers of squamous differentiation, with sensitivity and specificity reaching 100% 1

Recommended Clinical Application

Use P40 in combination with TTF-1 as a minimal two-marker panel for efficient NSCLC subclassification on small specimens. 1, 3

Diagnostic Algorithm:

• P40-negative/TTF-1-positive = Adenocarcinoma 4
• P40-positive (strong, >50%)/TTF-1-negative = Squamous cell carcinoma 4
• P40-positive/TTF-1-positive or P40-positive (<50%) = Consider adenosquamous carcinoma 4
• P40-negative/TTF-1-negative = NSCLC not otherwise specified or consider alternative diagnoses 4

Tissue-Sparing Advantages:

• The minimal P40/TTF-1 panel preserves precious tissue for molecular testing, which is critical for small biopsy specimens 1, 4
• This approach has been shown effective in correctly subtyping 89% of lung cancers on small samples 4
• Triple marker combinations (TTF-1, Napsin A, P40) demonstrate 100% sensitivity and 97.1% specificity for identifying squamous cell carcinoma while using minimal tissue 1

Critical Pitfalls and Caveats

Be aware that P40 can show non-specific immunoreactivity with normal bronchial basal epithelial cells, which should not be confused with tumor cells. 1

• When P40 is positive in adenocarcinoma (rare), reactivity typically involves no more than 5% of tumor cells with low intensity 2
• P40 immunopositivity can rarely occur in trophoblastic tumors, creating a potential diagnostic pitfall in the lung where squamous cell carcinoma is common 5
• Different clones of the same antibody may show different staining patterns, affecting interpretation 1

Context Within WHO Guidelines

Specific subtyping of all NSCLCs is necessary for therapeutic decision-making, as adenocarcinoma and squamous cell carcinoma have different treatment responses and side effect profiles. 1

• The 2021 WHO classification recommends using terminology such as "non-small-cell carcinoma, probably/favour squamous" when undifferentiated morphology is present but P40 immunohistochemistry is positive 1
• P40 testing should be performed when routine histopathologic differentiation is difficult to ascertain on small samples 1
• This distinction is especially critical in advanced-stage disease where cytologic material and small biopsies guide chemotherapy selection 1