OBJECTIVE To describe the pathomorphological features of corneal stromal invasive squamous cell carcinoma (CSI-SCC) in horses.
MATERIAL AND METHODS A total of 87 equine SCC in the Comparative Ocular Pathology Laboratory of Wisconsin database were retrieved. The signalment and anatomical distribution were summarized. Ten CSI-SCC out of 87 SCCs were further investigated focusing on pathomorphological description. All 10 cases were stained with H&E, periodic acid-Schiff stain and Verhoeff’s elastic stain.
RESULTS Four Appaloosas, two Quarter horses, two American Paint, one Pinto and one Thoroughbred horse were affected. The mean age at the time of enucleation/keratectomy was 16.7 +/- 5.2 years. Out of 10, five horses were clinically diagnosed as chronic stromal keratitis, 3 of 10 had a previous biopsy diagnosed as SCC, 1 of 10 was described as stromal mass, and 1 of 10 as invasive SCC. Previous keratectomies before enucleation were performed in 3 of 10 horses, of which 2 also had additional lasertherapy/cryotherapy. Seven of 10 cases showed tumor infiltration in the anterior-mid stroma, 3/10 in the mid-deep stroma. The anterior epithelium had no contact with the CSI-SCC in 8 of 10 cases, 7 of 10 had intact and normal epithelium, and 3 of 10 showed intact, dysplastic corneal and conjunctival epithelium. The limbus was not pigmented in 8 of 10 specimens. There was a mild-moderate lymphoplasmacytic inflammation between the neoplastic islands. Solar elastosis was present in 2 of 10 samples.
CONCLUSION The CSI-SCC shows a distinctive intrastromal tumor growth pattern with a smooth, intact corneal epithelium. The tumor can be underestimated and misdiagnosed as chronic active stromal keratitis. A deep biopsy is necessary for the definitive diagnosis.