Suspected canine primary lymph node mast cell tumor.

An 8-year-old spayed female Chihuahua dog was diagnosed with a suspected primary mast cell tumor (MCT) of the left popliteal lymph node (LN). Physical examination, computed tomography, and focused hind-limb ultrasound failed to identify a primary integumentary MCT. Asymmetry of the ipsilateral medial iliac LN was identified on abdominal ultrasound. Surgical excision of the left popliteal and left medial iliac LN was completed without complication. Histologic assessment confirmed overt MCT effacement of both LN. Postoperatively, the dog received adjuvant chemotherapy (vinblastine and prednisone). The dog was alive 562 d after surgery, without evidence of disease recurrence. To the authors' knowledge, this is the first report in the veterinary literature documenting a suspected canine primary LN MCT. Key clinical message: Albeit rare, primary LN MCT should be a differential diagnosis for lymphadenopathy in the absence of a primary integumentary tumor.