Corneal stromal invasive squamous cell carcinoma: a retrospective morphological description in 10 horses

To describe the pathomorphological features of corneal stromal invasive squamous cell carcinoma (CSI-SCC) in horses.

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.

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.

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.