Weishaar's classification system for nodal metastasis in sentinel lymph nodes: Update on clinical outcome in 94 dogs with mast cell tumour

Introduction: This retrospective study aims to update the impact of histological nodal metastasis classes (HNMC) on clinical outcome in dogs with mast cell tumor (MCT), that underwent tumor excision and radiopharmaceutical-guided sentinel lymph node (SLN) extirpation Materials and methods: Histologically-confirmed treatment-“naïve” MCT without distant metastases, SLN enlargement, and simultaneous cutaneous/subcutaneous MCT were included. Clinical (signalment, tumor location, dimension, ulceration, SLN and lymphocenters number, adjuvant chemotherapy), pathological (Patnaik, Kiupel and Thompson classification, HNMC, margins status and Ki67), outcome variables (local, nodal, distant relapse) and de novo MCT presentation were collected. Correlation between these variables, Disease Free Interval (DFI), Survival time (ST) and HNMC were analyzed. Results: Seventy-one cutaneous, 22 subcutaneous and 1 mucocutaneous MCT, were enrolled. Twenty-seven dogs were HNMC0, 19 HNMC1, 37 HNMC2 and 11 HNMC3. Thirteen cases (2 HNMC0, 4 HNMC2 and 7 HNMC3) received adjuvant chemotherapies. Only number of SLN and lymphocenters resulted correlated with HNMC. Median follow-up was 590 days (92-2034 days): no nodal relapse was registered, and fourteen dogs developed de novo MCTs. Five dogs died for MCT- related causes: one low-grade (HNMC0) and one subcutaneous (HNMC3) had a local relapse, two high-grade had distant relapse (HNMC3-HNMC0) and one dog developed disease progression from a new subcutaneous MCT. There were no significant differences in DFI and ST among HNMC. Conclusions: In contrast to previous publication, outcome is comparable after complete excision of low grade/low risk MCTs with HNMC2 and HNMC0-1 SLN, also without adjuvant chemotherapy. As previously reported low-grade MCT with HNMC3 showed a better outcome if adjuvant chemotherapy was administered.