Fluorine‐18 Fluorodeoxyglucose‐Positron Emission Tomography/Computed Tomography for Detection of Lymph Node Metastasis in Canine Mast Cell Tumour

Lymph node (LN) metastasis has been associated with shorter survival times in dogs with mast cell tumour (MCT), and treatment of metastatic LN with lymphadenectomy or irradiation has been demonstrated to improve outcomes. Identification of metastatic LN in dogs with MCT is therefore of both prognostic and therapeutic significance. The aim of this prospective, exploratory study was to investigate whether fluorine-18 fluorodeoxyglucose-positron emission tomography/computed tomography (¹⁸F-FDG-PET/CT) is a useful staging tool for the detection of metastatic LN in dogs with cutaneous or subcutaneous MCT, using histopathology as the gold standard. Sixteen client-owned dogs with cytologically or histologically confirmed cutaneous or subcutaneous MCTs underwent full-body ¹⁸F-FDG-PET/CT followed by surgical removal and histopathology of the primary tumour and regional LN(s). The maximum standard uptake value (SUV_max) of the tumour and LN(s) was measured. Primary tumours were graded using both the Patnaik and Kiupel grading systems, and mitotic count was tabulated. LNs were categorised based on Weishaar's histologic criteria for nodal metastasis. Eighteen primary tumours were excised: six subcutaneous and 12 cutaneous MCTs. Of 33 excised regional LNs, 18 (55%) were categorised as metastatic (≥ HN2). There was no difference between the median SUV_max of metastatic (3.88) and nonmetastatic LNs (3.16) (p = 0.41). SUV_max was positively correlated with the mitotic count of the primary tumour (p = 0.02). The results of this exploratory study suggest that ¹⁸F-FDG-PET/CT may not be useful for identifying metastatic LNs in canine MCT.