Prognostic Impact of Mast Cell Infiltration Detected by Flow Cytometry on Excised Sentinel Lymph Nodes in Dogs With Newly Diagnosed Mast Cell Tumours

Lymph node (LN) metastasis is a negative prognostic factor in canine mast cell tumours (MCTs). Flow cytometry (FC) can identify and quantify mast cells (MCs) in LNs. This tri-institutional prospective study aimed to evaluate the impact of previously reported MC cut-off values in sentinel LNs (SLNs) detected by FC on clinical outcomes in dogs with cutaneous or subcutaneous MCTs, and to determine a prognostically significant cut-off. Dogs with newly diagnosed, previously untreated, single MCT scheduled for primary tumour excision and sentinel lymphadenectomy were enrolled. The SLNs of enrolled dogs were analysed using cytology and FC after excision. MCs were quantified as a percentage using FC, and SLNs were cytologically and histologically classified according to the Krick and Weishaar systems, respectively. The influence of potential prognostic variables, including MCs cut-offs > 1.1% and > 4%, on tumour progression and tumour-specific survival (TSS) was evaluated using Cox regression and log-rank analysis. The optimal cut-off for predicting tumour-related death was determined via ROC curves. A total of 64 dogs were enrolled. Dogs with nodal MC infiltration exceeding 1.1% and 4% were 10 and 40 times more likely, respectively, to experience tumour progression. SLN MC infiltration > 4% and HN3 SLN (observed in 9 out of 64 dogs) were associated with shorter TSS (median, 327 versus not reached; p < 0.001). The optimal SLN cut-off for predicting tumour-related death was 16%. These findings suggest that previously established cut-offs may hold prognostic value. Additional studies performing in vivo sampling with a larger number of events are necessary to validate this proposed cut-off.