Oral Mass in a Cat
History and Physical Exam: An 11-year-old, castrated male domestic shorthair cat presented for evaluation of a rapidly growing mass along his left rostral maxilla. The owners reported that hyperemia of the mucus membranes had been noticed in that region at his annual physical exam by his local veterinarian. The lesion progressed over the next 5 months and caused loosening of his maxillary canine and incisor teeth. The cat also had a history of hypertrophic cardiomyopathy and hyperthyroidism. Physical examination revealed an approximate 3.5 x 3 x 2 cm gingival mass in the left rostral maxilla displacing the lip. Additional findings included loose teeth, blood-tinged saliva, a moderately enlarged left mandibular lymph node, a palpable thyroid slip and a grade III/VI heart murmur.
What are your ranked differential diagnoses for this lesion?
What is your plan to diagnose and stage this cat?
Figure 1: Oral tumor at initial presentation
The most common oral tumor in cats is squamous cell carcinoma, followed by fibrosarcoma. Less-common oral tumors encountered in cats include epulides (more common in young cats) and melanoma, both of which are rare in the cat as compared to the dog.
In order to obtain a diagnosis, the tumor must be sampled, either via fine needle aspiration or, preferably, a biopsy. Since cancer is the most likely differential for the lesion, the cat should also be staged. Staging tests for oral tumors include a minimum database (CBC, chemistry panel and urinalysis), three-view thoracic radiographs and regional lymph node evaluation via fine needle aspiration.
This cat underwent a workup including bloodwork, urinalysis, thoracic radiographs, mandibular lymph node aspiration and a punch biopsy of the mass. The minimum database was generally unremarkable. Three-view thoracic radiographs revealed a possible pulmonary metastatic nodule (only visible on one view and superimposed over the cardiac silhouette). Fine needle aspiration of the left mandibular lymph node revealed lymphoid hyperplasia. Histopathology of the biopsy revealed an amelanotic malignant melanoma.
Figure 2: Radiation therapy setup showing the radiation beam field. A soft tissue equivalent material (bolus) is taped over the tumor. The bolus functions to modify the radiation dose distribution and ensure full dosage to the tumor.
Given the extent of his tumor and the possible pulmonary metastatic disease, this cat was treated with palliative radiation therapy. The primary tumor and mandibular lymph nodes were treated with external beam radiation therapy using 6 MV photons in parallel opposed fields. He was given 8 Gy per fraction in four weekly fractions for a total dose of 32 Gy. The cat had a marked response to radiation therapy. At the time of the last fraction, his tumor was 1.7 x 1.3 x 2 cm, corresponding to approximately 75% reduction in volume. Since radiation therapy only treats the local oral disease, follow-up systemic chemotherapy and/or the melanoma vaccine were also recommended but declined by the owner. His tumor was well controlled until approximately 2.5 months after the completion of radiation therapy, when the tumor rapidly regrew. He was euthanized 2 weeks later, approximately 4 months after the initial diagnosis.
Figure 3: Oral tumor at final radiation therapy treatment
Oral melanoma is very rare in cats, accounting for approximately 0.3% of all feline oral tumors. The tumor is highly malignant and locally invasive. In one report of 4 cats, mean survival rate was only 61 days, with all cats being euthanized for metastatic disease. Radiation therapy is often helpful in local tumor control but the response is short-lived. The median survival for 5 cats treated with hypofractionated radiation therapy (24 Gy given in 8 Gy fractions on days 0, 7, and 21) was only 146 days (range 66-224 days).
Reference: Farrelly J, Deman DL, Hoenhaus AE, et al. Hypofractionated radiation therapy of oral melanoma in five cats. Vet Radiol Ultrasound 2004;45:91-93.