Dogs: 

Splenic masses are very common and ~50% are benign (often hematoma or nodular hyperplasia). The “rule of 2/3”: ~2/3 malignant, and of those malignant, ~2/3 are hemangiosarcoma (HSA)—but that also means many are NOT HSA.

 

Cats:

Splenic diffuse enlargement is more common than a discrete mass. True primary splenic tumours are rare. When neoplastic, mast cell tumour or lymphoma are most common. Cats often do very well after splenectomy for splenic MCT.