34th Annual Scientific Meeting proceedings

Periocular neoplasms in horses represent a diverse group of tumours affecting the tissues surrounding the eye, including the eyelids, conjunctiva, and periorbital skin (1-3). These tumours may grow rapidly and can spread to invade adjacent tissues, potentially compromising vision and ocular health (1-3). 

The most common periocular masses in horses include squamous cell carcinoma (SCC), sarcoids, papillomas, melanomas, and lymphosarcomas (1-3). In this lecture, the main focus will be on (peri)ocular SCC as this is the most prevalent neoplasm of the equine eye and it is a frequently encountered ophthalmic disease in equine practice (1-3). (Peri)ocular SCC particularly occurs in horses with unpigmented periocular skin such as Appaloosas and Cremello horses (1). Risk factors include UV radiation exposure, genetic predisposition, and chronic inflammation (1-6). SCC may involve the cornea, conjunctiva, third eyelid, limbus and/or the eyelids (1-3). The average age of horses diagnosed with periocular SCC is around 10-11 years (range 3-26 years) (1).

Treatment of periocular neoplasms is often therapeutically challenging and frustrating and treatment options vary based on tumour type, size, location, and invasiveness. In (peri)ocular SCC, surgical excision alone carries a high risk of recurrence (1,3). Tumour free margins of two centimetres have been recommended when excising SCC, but this is often impossible in the periocular region. Therefore, the mainstay of treatment is surgical excision combined with adjunctive therapy (1,3). Many adjunctive therapies have been described in literature, like (intralesional) chemotherapy, cryotherapy, hyperthermia, radiation or carbon dioxide laser ablation (1-3). Recent advancements have introduced promising new treatment options aimed at improving outcomes and reducing recurrence rates (3, 7-9).

Photodynamic therapy (PDT) is an emerging modality for the management of equine periocular neoplasms, particularly SCC. PDT involves the use of photosensitizing agents, light, and oxygen (1,3,7). After surgical excision of the tumour, a photosensitizing agent is administered to the wound bed, followed by light irradiation with a specific wavelength of light. This activates the photosensitizer, leading to selective tumor cell apoptosis, necrosis, vascular damage and inflammation, while minimizing collateral injury to adjacent healthy tissues. PDT is generally performed under standing sedation with local or regional anesthesia. It offers a tissue-conserving, localized treatment with favorable cosmetic and functional outcomes (1,3,7,8). PDT is a very promising treatment option for periocular SCC as recent studies have shown a 100% nonrecurrence rate at 25 months (8,9).

References

1. Giuliano, E.A. Diseases of the adnexa and nasolacrimal system. In: Gilger, B.C. editor. Equine Ophthalmology. 3rd edn. John Wiley & Sons Incorporated. 2017; 197-251.
2. Montgomery, K.W. Equine ocular neoplasia: A review. Equine Vet. Educ. 2014; 26(7): 372-380.
3. Giuliano, E.A. Equine periocular neoplasia: current concepts in aetiopathogenesis and emerging treatment modalities. Equine Vet. J. Suppl. 2010; 37:9‐18.
4. Bellone, R.R., Liu, J., Petersen, J.L., Mack, M., Singer-Berk, M., Drögemüller, C., Malvick, J., Wallner, B., Brem, G., Penedo, M.C., and M. Lassaline. A missense mutation in damage-specific DNA binding protein 2 is a genetic risk factor for limbus squamous cell carcinoma in horses. Int. J. Cancer. 2017; 141:342-353.
5. Crausaz, M., Launois, T., Smith-Fleming, K., McCoy, A.M., Knickelbein, K.E., and R.R. Bellone. DDB2 Genetic Risk Factor for Ocular Squamous Cell Carcinoma Identified in Three Additional Horse Breeds. Genes. 2020; 11(12):1460.
6. Chen, L., Bellone, R.R., Wang, Y., Singer-Berk, M., Sugasawa, K., Ford, J.M., & Artandi, S.E. A novel DDB2 mutation causes defective recognition of UV-induced DNA damages and prevalent equine squamous cell carcinoma. DNA Repair. 2020; 97:103022.
7. Giuliano, E.A., Ota, J. and Tucker, S.A. Photodynamic therapy: basic principles and potential uses for the veterinary ophthalmologist. Vet. Ophthalmol. 2007; 10:337-343.
8. Giuliano, E.A., MacDonald, I., McCaw, D.L., Dougherty, T.J., Klauss, G., Ota J., Pearce, J.W., Johnson, P.J. Photodynamic therapy for the treatment of periocular squamous cell carcinoma in horses: a pilot study. Vet Ophthalmol. 2008;11 Suppl 1:27-34.
9. Giuliano, E.A., Johnson, P.J., Delgado, C., Pearce, J.W., Moore, C.P. Local photodynamic therapy delays recurrence of equine periocular squamous cell carcinoma compared to cryotherapy. Vet Ophthalmol. 2014;17 Suppl 1:37-45.