Cutaneous mastocytosis in 8 young dogs and review of literature.

Cutaneous mastocytosis (CM) is a rare condition in young dogs characterized by multicentric cutaneous proliferation of neoplastic mast cells. Clinical data from 8 dogs that met inclusion criteria (age of onset less than 1.5 years, greater than 3 lesions) were obtained via a standardized survey. Biopsy samples were classified by the Kiupel/Patnaik grading systems and analyzed for c-KIT mutations. The median age of onset was 6 months (range: 2-17 months). Dogs had 5 to more than 50 lesions characterized as nodules, plaques, and papules. Seven dogs were pruritic. Clinical staging in 2 dogs did not reveal visceral involvement. No dogs had systemic illnesses at diagnosis. Histologically, CM was similar to cutaneous mast cell tumor (cMCT). Two dogs had neoplasms classified as high-grade/grade II while 6 dogs had low-grade/grade II neoplasms. No dogs had mutations in c-KIT exons 8 and 11. Treatment included antihistamines (8/8), corticosteroids (7/8), lokivetmab (3/8), and toceranib (1/8). Six dogs were alive with lesions at the end of the study with a median follow-up time of 898 days, while 2 dogs were euthanized. In dogs with high-grade/grade II neoplasms, one continued to develop lesions at 1922 days post-diagnosis, while the other dog was euthanized at 56 days post-diagnosis. One dog was euthanized 621 days post-diagnosis due to rupture of a neoplasm. CM occurs in young dogs and is histologically indistinguishable from cMCT. Current histologic grading systems did not apply uniformly to the dogs of the study and further studies are needed.