Immunohistochemical characterization and EGFR expression of canine thymic tumors

Introduction: Thymoma is the most common primary mediastinal tumor in dogs; although derived from thymic epithelial cells, it often displays prominent lymphocytic infiltration (lymphocytic thymoma), which makes lymphoma its main differential diagnosis. Conversely, epithelial thymomas are morphologically undistinguishable from epithelial tumors in other organs; thymofibrolipomas are composed of cords of epithelial thymic cells admixed with fibroadipose tissue. The immunohistochemical profile of canine thymic tumors has been poorly investigated, and pathological markers for prognosis are also lacking. Materials & Methods: Immunohistochemistry was carried out on canine samples from 12 thymic tumors and 5 normal thymus, aiming at verifying and quantifying the expression of lymphoid markers (CD3, CD79), cytokeratins (CKAE1/AE3, CK5/6, CK14, CK19), vimentin, TTF-1, COX-2, tyrosine-kinase-receptors (EGFR and KIT) and Ki67. Results: There were 6 lymphocytic thymomas, 4 epithelial thymomas and 2 thymofibrolipomas; tumor size varied from 3 to 20 cm, and survival time after diagnosis ranged from one month to two years (mean 5 months). Three dogs underwent radiation therapy and one surgery. Infiltrating lymphocytes were mostly CD3+; thymic epithelial cells constantly and strongly expressed CKAE1/AE3 and CK19, while COX-2 and EGFR were expressed at a variable degree; KIT was labeled in sparse infiltrating mast cells and in one epithelial thymoma; Ki67 positivity was detected in 5-70% of epithelial cells and in 5-50% of lymphoid cells. There was no evident association between these markers and biological behavior. Conclusion: Cytokeratin immunolabeling revealed useful in detecting epithelial cells in lymphocytic thymomas. Although not directly related to the tumor behavior, EGFR expression suggests a possible indication for targeted therapy.