The Role of Fine Needle Aspiration of Liver and Spleen in the Staging of Low-Grade Canine Cutaneous Mast Cell Tumor

SIMPLE SUMMARY: Mast cell tumor is one of the most common cutaneous tumors in dogs, representing 16–21% of all cutaneous tumors. Clinical staging is a fundamental step in the patients’ assessment, and it is considered essential to further refine the prognosis and to establish an ad hoc therapeutic plan. The recommended staging work-up includes a basic laboratory database, fine needle aspiration of the expected tumor draining lymph node/s, and diagnostic imaging with fine needle aspiration of the liver and spleen. The aim of this retrospective study was to investigate the incidence, at presentation, of hepatic and splenic metastases in dogs affected by low-grade cutaneous mast cell tumor referred for further investigations (Patnaik grade I–II, Kiupel low-grade). Only 1 out of 136 dogs had the presence of visceral metastases at diagnosis, suggesting that the prevalence of visceral metastases in low-grade cutaneous mast cell tumor is extremely low and that cytology of visceral organs may not represent an essential step in the clinical staging work-up in these dogs. ABSTRACT: Clinical staging is a fundamental step in the clinical assessment of canine cutaneous mast cell tumor (cMCT), and it is recommended to evaluate the tumor draining lymph node (eTDLN), perform diagnostic imaging, and fine needle aspiration (FNA) of the spleen and liver to determine the presence of metastatic disease, thereby refining the prognosis. The aim of this retrospective study was to evaluate the prevalence of splenic and hepatic involvement in newly diagnosed canine low-grade cMCT (Patnaik grade I–II, Kiupel low-grade). Medical records of dogs that underwent a clinical staging work-up and surgical excision for a low-grade cMCT between December 2019 and December 2021 were reviewed at five veterinary centers. Only dogs with a histological diagnosis of low-grade cMCT, FNA or histology of the eTDLN, FNA of the spleen and liver, and one year of follow up were included. One hundred and thirty-six dogs met the inclusion criteria. Only 1 out of 136 dogs (0.7%) had the presence of visceral metastases at diagnosis, suggesting that the prevalence of visceral metastases in low-grade cMCT is extremely low. The results of this study are consistent with previous literature and suggest that after a diagnosis of low-grade cMCT, cytology of visceral organs may not represent an essential step in the clinical staging work-up.