This installment of "Five Minute Pathology Pearls" addresses a critical diagnostic challenge: the non-specific staining of the endothelial marker CD31 in non-vascular tumors. Using a case of a highly hemorrhagic, high-grade pleomorphic sarcoma, the video demonstrates that CD31 frequently produces a misleading, weak, patchy, and granular positive stain in infiltrating histiocytes and sometimes in the adjacent tumor cells. The key teaching point is to utilize the internal control—the normal, crisp, and strongly staining endothelial cells of background vessels—to differentiate true vascular differentiation (typically seen as strong, diffuse positivity in angiosarcoma) from non-specific histiocytic uptake. Recognizing this distinction prevents the inappropriate overdiagnosis of non-vascular malignancies as angiosarcoma.
CD31 Staining Pitfalls in Pathology
0:08 Focusing on CD31 Pitfall: The video aims to explain how the vascular marker CD31 can stain histiocytes and tumor cells non-specifically, not indicating endothelial or vascular differentiation.
0:19 Case Example: The presenter uses a high-grade pleomorphic sarcoma (possibly UPS or high-grade myxofibrosarcoma) with significant hemorrhage, which initially raised clinical concern for angiosarcoma.
1:17 Recognizing Histiocyte Staining: Histiocytes commonly exhibit a characteristic weak, patchy, and granular staining pattern with CD31. This is normal and non-specific.
1:31 Internal Control is Key: When evaluating vascular markers, the pathologist must compare the tumor cell/histiocyte staining to the crisp, dark, strong staining present in the endothelial cells of the adjacent normal blood vessels, which serve as an internal control.
1:59 Differentiation Point: True vascular tumors like angiosarcomas are typically strong and diffuse positive for CD31 (with some noted exceptions like certain Kaposi sarcomas or PMEH). Weak, blushy, or granular staining is likely non-specific histiocytic uptake or background artifact.
2:26 Takeaway: Pathologists must recognize that weak, granular staining should not be interpreted as indicative of endothelial differentiation to avoid overcalling non-vascular tumors as angiosarcomas.
3:04 Recommended Adjunct Markers: If CD31 staining remains equivocal or "blushy," supplementary markers such as ERG (and potentially CD34, although it can be lost in some angiosarcomas) should be added to confirm the vascular lineage.