Splenic HS in a 10-year-old female neutered cross-breed dog. (A) Ultrasonographic still of a large (maximum diameter, 59 mm) well-vascularized (as demonstrated by Doppler color flow) mass lesion of mixed echogenicity within the tail of the spleen. (B) Histopathology of the splenic mass revealed a poorly encapsulated neoplasm showing both round and spindle morphologies of large tumor cells forming sheets and streams. Diagnosis of HS was confirmed using immunohistochemical stains (not shown; hematoxylin and eosin stain; ×20 objective).