Benign masses called “meibomian gland adenomas,” which originate from normal glands on the eyelids, are the most commonly diagnosed mass on the eyelid in dogs. The second most common type of mass found on the eyelid contains melanocytes, which are cells that produce pigment. Approximately 67 to 82% of these masses are benign, and 13-37% are malignant melanomas (skin cancer). Eyelid melanoma in cats is rare; therefore, little research is available.
Pigmented masses on the inner eye tissue or conjunctiva are usually more aggressive melanomas. The first treatment of choice in these cases is surgery with cryotherapy (freezing of the tissue).
Melanomas can also occur inside the eye itself. These often look like brown discoloration of the iris or a mass in the eye. The majority of the time, they are benign (less likely to spread throughout the body). However, even benign growths can still impact the eye’s function and result in vision loss. Some of these melanomas are malignant and can spread, often to the lungs. There is no way to differentiate between benign or malignant growths without a pathologist analyzing them by surgically removing the eye. Therefore, it is often recommended to have an ophthalmologist monitor the mass for any changes or progression prior to moving ahead with surgery.
For cats, the most common primary ocular tumor is “anterior uveal melanoma,” which can present as discoloration of the iris. Benign changes can appear similarly in the iris; therefore, predicting an appropriate treatment course can be challenging. Other types of tumors, such as ciliary body adenomas and adenocarcinomas, may appear similar to this type of melanoma.
Staging tests are often recommended prior to surgery, in order to determine the extent of cancer spread. Such tests often include aspirates of the local lymph nodes, chest radiographs, bloodwork, and an abdominal ultrasound.
If surgery to remove the mass is selected, a melanoma vaccine can be used afterward. This vaccine uses human DNA to stimulate an immune response against melanin-producing cells in an effort to prevent the spread of disease. This vaccine can safely be used in cats and dogs and often extends survival in patients with malignant melanoma.
Overall, most ocular melanomas in cats and dogs have a good prognosis for survival time.
