Patterns of nodal metastases, biological behavior and prognosis of canine mast cell tumors of the pinna: a multi-institutional retrospective study.

Canine cutaneous mast cell tumors (cMCTs) of the pinna have been associated with an aggressive biological behavior, although data remain scarce. The knowledge acquired over the past years on histologic gradings, and the value of lymph node (LN) staging, may help in better characterizing this anatomical presentation. The first aim was to describe the frequency, location, and histologic appearance of LN metastases in cMCT of the pinna. A second aim was to evaluate prognosis. Medical records of dogs with cMCT of the pinna, that underwent tumor and sentinel (SLN) or regional LN (RLN) excision, were reviewed. The influence of potential prognostic variables on time to progression (TTP) and tumor-specific survival (TSS) was investigated. Thirty-nine dogs were included: 19 (48.7%) had Kiupel high-grade (K-HG) and 20 (51.3%) had low-grade (K-LG) MCTs. Eighteen (46.1%) dogs underwent SLN mapping: the superficial cervical LN was at least one of SLN in 17 (94.4%) cases. Twenty-two (56.4%) dogs had LN metastases; the superficial cervical LN was always involved. On multivariable analysis, only K-HG was associated with increased risk of progression (p=0.043) and tumor-related death (p=0.021). Median TTP and TSS were 270 and 370 days in K-HG, respectively; these were not reached in dogs with K-LG tumors (p<0.01). cMCTs of the pinna are often K-HG and are also associated with a higher frequency of LN metastasis; however, we confirmed the independent prognostic value of histologic grading. A multimodal treatment may lead to favorable long-term outcome. Moreover, the superficial cervical LN is most often the SLN.