Ki67 Index in Patnaik Grade 2/Kiupel Low-Grade Canine Cutaneous Mast Cell Tumors with Early Lymph Node Metastasis: A Descriptive Study

Simple Summary Patnaik grade 2/Kiupel low-grade (G2/LG) represent the majority of diagnosed canine cutaneous mast cell tumors (cMCTs), with a reported incidence of 53.6-57.6%. It has been reported that a variable percentage of dogs (3-17%) with G2/LG die from tumor-related causes; therefore, it could be hypothesized that a subset of G2/LG tumors could manifest an aggressive behavior. Ki67 and mitotic count (MC) are the two most investigated proliferation markers in cMCTs. However, different Ki67 cut-offs have been proposed by previous studies, and have always been assessed in cMCTs of different histological grade in heterogeneous canine populations; moreover, the lymph nodes' (LNs) metastatic status has never been included. This retrospective study investigated the usefulness of the Ki67 index in predicting cMCT behavior in a homogeneous cohort of G2/LG with HN2 LNs. Thirty-nine dogs with primary G2/LG cMCTs and HN2 LNs treated with surgery alone were enrolled. The results of the present study report that all dogs treated with surgery alone had a good prognosis; no local, nodal or distant metastasis occurred in the timespan considered, and the Ki67 index did not have prognostic impact. Several studies evaluating Ki67 in canine cutaneous mast cell tumors (cMCTs) have reported its prognostic value when tumors of all histological grades are included. This study aims to evaluate whether the Ki67 index has a predictive value in a homogeneous cohort of G2/LG cMCTs with HN2 lymph nodes (LNs) and to describe the clinical outcome. The second goal was to explore the correlation between the Ki67 index and MC. The medical databases of three institutions were retrospectively searched for dogs undergoing surgical treatment for cMCT and LN extirpation, with a histological diagnosis of G2/LG with HN2 LNs. Information about histological margins, MC, Ki67 index, local recurrence, nodal relapse, distant metastasis, de novo cMCT occurrence and date and cause of death were included. A total of 39 cases were identified. None of these developed local and nodal relapse or metastatic distant disease. Median MC was 1 (0-2). Median Ki67 index was 3.5 (0.7-14.3). Ki67 and MC were not significantly correlated. At the end of the study, 32 (82%) dogs were alive, 7 (18%) dogs were dead from unrelated causes and 4 (10.2%) dogs were lost to follow-up. The median ST was not reached, and the mean was 893 days (104-2241 days). Considering the strict inclusion criteria, dogs affected by G2/LG with HN2 LNs treated with surgery alone may have a good oncologic outcome; the Ki67 index does not have prognostic impact.