Melanoma is a cancer originating from melanocytes, the pigment-producing cells found in the skin and other tissues. While widely recognized in humans and dogs, melanoma is rare in felines, accounting for less than one percent of all cancer diagnoses. The disease can manifest in various locations, and early detection is a significant factor in achieving a favorable outcome. Feline melanoma can be either benign or malignant, which dictates the necessary treatment and overall prognosis.
Occurrence and Characteristics in Felines
Melanoma most frequently occurs in the eyes, the oral cavity (mouth), and the nail beds or toes (digital). It is typically seen in older cats, generally those over ten years of age, though no specific breed or sex has been identified as having a higher risk. Tumors originating from melanocytes can be categorized as either benign melanocytomas or malignant melanomas.
Benign melanocytomas are usually slow-growing and localized, often requiring only monitoring or simple surgical removal if they are causing irritation. Malignant melanoma, however, is aggressive and locally invasive, especially when it arises in certain locations. Oral melanomas are particularly concerning because they are rarely benign and tend to be rapidly destructive to local tissues, frequently spreading to the bone.
Ocular melanoma (specifically diffuse iris melanoma) has a variable course but a reported metastatic rate ranging from 19% to 70%, commonly spreading to the liver or lungs. Digital melanoma, while less common, tends to be aggressive and is associated with discomfort and inflammation. Cutaneous melanoma, which forms on the skin of the head, tail, or legs, is often more likely to be benign compared to internal forms.
Recognizing the Signs of Melanoma
The observable signs of melanoma are diverse and depend entirely on the tumor’s location within the body. For tumors developing in the oral cavity, owners may notice a visible mass on the gums, tongue, or palate, which can sometimes appear dark or black. Other common indicators of oral melanoma include difficulty chewing or swallowing, excessive drooling, a foul odor (halitosis) from the mouth, or unexplained weight loss.
Ocular melanoma, which often starts as flat, darkened areas on the iris, can cause subtle changes in the eye’s appearance. As the tumor progresses, the eye may develop dark spots, an irregular pupil shape, or appear enlarged, sometimes leading to vision loss or secondary inflammation. While many cats develop benign iris freckles, any new or changing pigmented spot in the eye warrants a professional ophthalmic examination.
Melanoma affecting the toes or nail beds typically presents as lameness, persistent swelling of a digit, or a dark mass visible under the nail. Any new, pigmented, or non-pigmented mass on the skin or mucous membranes should be promptly evaluated by a veterinarian, especially if it is rapidly changing, ulcerated, or bleeding.
Diagnosis and Treatment Options
A definitive diagnosis of melanoma requires obtaining a tissue sample for microscopic examination. This is typically done through a fine-needle aspiration (FNA) or a surgical biopsy, which determines whether the tumor is a benign melanocytoma or a malignant melanoma. Following a diagnosis of malignancy, the next crucial step is cancer staging to determine if the disease has spread to other parts of the body.
Staging protocols involve imaging procedures such as chest X-rays or CT scans to check for metastasis to the lungs, a common site of spread. An abdominal ultrasound may also be performed to screen for metastasis in organs like the liver or kidneys, and samples from nearby lymph nodes are often collected for analysis. The size of the primary tumor and the presence of spread are important factors in establishing a prognosis.
Surgical removal is the primary treatment for localized malignant melanoma, aiming to excise the mass with wide, tumor-free margins. For oral or digital lesions, this often necessitates aggressive surgery, such as partial jaw removal or toe amputation, to achieve complete tumor control. If complete surgical removal is not possible due to the tumor’s location or size, secondary treatments are employed.
Radiation therapy uses high-energy beams to damage cancer cells and is frequently recommended as an adjuvant treatment after incomplete surgery or for tumors in hard-to-reach areas. Chemotherapy is reserved for cases where the cancer has metastasized or has a high risk of spreading, working systemically to target cancer cells throughout the body. Immunotherapy, including a melanoma vaccine developed for dogs, is an emerging treatment that stimulates the cat’s immune system to fight the cancer cells. The prognosis for malignant feline melanoma is generally guarded due to the high risk of metastasis, and long-term survival depends on the tumor’s location and whether it has already spread at the time of diagnosis.