Heterologous Elements

Heterologous elements are areas inside a tumor that look like a different tissue, such as bone, cartilage or muscle. In mesothelioma they occur mainly in sarcomatoid and biphasic tumors, and they can lead a pathologist to mistake mesothelioma for a primary bone or soft-tissue sarcoma.

Origin

The term comes from general pathology, where heterologous means derived from a tissue not normally found at that site. Pathology studies of mesothelioma have described tumors with bone-forming (osteosarcomatous), cartilage-forming (chondrosarcomatous) and muscle-type (rhabdomyosarcomatous) areas.

Significance in Asbestos Context

Mesothelioma with heterologous elements is uncommon. Because the bone or cartilage areas may stain weakly or not at all for keratins, the usual markers of mesothelioma, a small biopsy from one of these areas may be read as a sarcoma. The correct diagnosis usually depends on finding typical sarcomatoid mesothelioma elsewhere in the tumor, the pattern of spread along the pleura, and supporting tests such as loss of BAP1 or MTAP or deletion of CDKN2A. This matters because sarcomatoid mesothelioma and primary sarcomas are treated differently and have different causes. In a claim, defendants may argue that a tumor with bone or cartilage in it is a sarcoma unrelated to asbestos, so a report from a pathologist experienced in mesothelioma, ideally reviewing all available tissue, carries weight.

Example Sentence

The biopsy showed malignant bone formation, but the resection specimen contained keratin-positive spindle cells spreading along the pleura, and the final diagnosis was sarcomatoid mesothelioma with heterologous elements.

Related Terms

Sarcomatoid, Biphasic Mesothelioma, Sarcoma, Broad-spectrum Cytokeratin, Immunohistochemistry, Synovial Sarcoma, Pathology Review, Mesothelioma Misclassification

Notes

• If a chest tumor is called a sarcoma, ask whether mesothelioma with heterologous elements was considered.
• Ask that all tissue, not just one biopsy, be reviewed by a mesothelioma pathologist.
• Keep the original and any consult pathology reports.