+ General Considerations
- Dermal melanoma is common in dogs and accounts for 5%-7% of all canine skin tumors
- No sex predilection
- Breed predisposition: Scottish Terrier, Airedale Terrier, Boston Terrier, and Cocker Spaniel
- Mean age 9 years
- Sites: face (especially eyelids), trunk, and extremities
- Gross appearance: solitary with variable pigmentation
- 2 histologic variants reported in dogs: pilar neurocristic melanoma and clear cell melanoma
- Dermal melanomas are usually benign and are well-defined, deeply pigmented, small, and mobile
- Malignant dermal melanomas appear larger (> 2.5 cm) with ulceration, secondary infection, and rapid growth
- Fibrous dermal melanoma is not malignant
- Junctional melanoma and cellular dermal melanoma can undergo malignant transformation
- Metastasis occurs through both the vascular and lymphatic systems
TREATMENT
+ Surgery
- ± gene therapy, radiation therapy, local hyperthermia, photodynamic therapy, and intralesional cisplatin
- Durable response to course-fraction radiation therapy
- Responses are short-lived for photodynamic therapy, hyperthermia, and intralesional cisplatin
- Chemotherapy is usually ineffective (agents include mitoxantrone, doxorubicin, platinums, and melphalan)
+ PROGNOSIS
- Excellent prognosis if benign but guarded if malignant characteristics as metastatic rate between 30%-75%
- 65% of dogs with malignant dermal melanoma are euthanased within 2 years due to local recurrence or metastasis
- Prognostic factors in dogs include breed, tumor size, infiltration into adjacent tissue, mitotic index, Ki-67 count, DNA ploidy, and monoclonal antibody (MAB IBF9) reactivity
- 75% melanomas in Doberman and Miniature Schnauzer are benign v 85% melanomas in Miniature Poodle are malignant
- Malignant dermal melanomas < 4 cm have a significantly better MST (12 months v 4 months) and 2-year survival rate (46% v 0%) than tumors ≥ 4 cm
- Ki-67, proliferation marker to assess growth fraction of tumors, has higher predictive value for biologic behaviour of dermal melanomas than histology
- DNA ploidy can be used to differentiate benign from malignant melanoma but is not prognostic
- Histology does not predict malignant behaviour in cats compared to dogs (ocular melanoma more likely to be malignant and dermal melanoma more likely to be benign)
DERMAL MELANOMA
Stages
| Mitotic Index per High Power Field | Survival Time | Death Rate | |
| Well Differentiated | ≤ 2 | 104 weeks | 10% |
| Poorly Differentiated | ≥ 3 | 30 weeks | 73% |