Placental Alkaline Phosphatase (PLAP)

Placental alkaline phosphatase is an enzyme normally produced by the placenta during pregnancy that can also be made by certain cancers, particularly germ cell tumors such as seminoma and embryonal carcinoma. Pathologists detect it in tissue using immunohistochemistry (a stain that highlights specific proteins) to help identify what kind of tumor a biopsy actually represents.

Origin

First characterized in the 1960s and named for its discovery in placental tissue, PLAP was recognized as a tumor marker in the 1970s when researchers, including the work of Fishman and colleagues, found that some cancers re-express this normally pregnancy-specific enzyme, a phenomenon known as oncofetal expression.

Significance in Asbestos Context

In the workup of a suspected mesothelioma, pathologists use a panel of immunohistochemistry stains to rule out cancers that can look similar under the microscope. PLAP helps exclude germ cell tumors, which can occasionally spread to the chest or abdomen and mimic mesothelioma on imaging or biopsy. A negative PLAP result, combined with positive mesothelial markers (such as calretinin, WT-1, and cytokeratin 5/6) and negative carcinoma markers, supports a mesothelioma diagnosis, and a firm, well-documented diagnosis is the foundation of both your medical care and any legal claim.

Example Sentence

The pathology report noted that PLAP was negative, which helped the pathologist rule out a germ cell tumor and confirm the diagnosis of epithelioid mesothelioma.

Related Terms

Immunohistochemistry, Calretinin, WT-1, Cytokeratin 5/6, Mesothelioma, Differential Diagnosis

Notes

• Ask your oncologist or pathologist for a copy of the full immunohistochemistry panel used on your biopsy. PLAP is one of several markers that together support or refute a mesothelioma diagnosis.
• If your diagnosis is unclear or based on a limited panel, request a second opinion from a mesothelioma specialty center (such as an NCI-designated cancer center) where expert thoracic pathologists routinely interpret these stains.
• Preserve the original pathology report and, if possible, the paraffin tissue blocks; defense experts in litigation sometimes request re-staining, and your legal team will want the original material protected.
• PLAP is most relevant in younger patients or unusual tumor locations where germ cell tumors are part of the differential. Ask whether it was included if your case has atypical features.