Histiocytoma is correct!
This benign round cell tumor is common in young dogs. Microscopic examination of histiocytoma reveals a uniform population of round cells with fairly abundant pale-grey cytoplasm; centrally located, round, or slightly indented nuclei; and indistinct nucleoli. An inflammatory infiltrate may be present in regressing lesions.
Mast cell tumors exhibit round to oval cells with round, central nuclei. The cytoplasm typically contains distinct purple granules that may obscure the nucleus. Cutaneous lymphoma exhibits a uniform population of round cells with scant cytoplasm and vesiculate nuclei that often contain prominent and irregularly shaped nucleolar whorls. Plasmacytomas exhibit a relatively uniform population of discrete oval to round cells with abundant cytoplasm and eccentric round nuclei with tightly clumped chromatin and a clear perinuclear area. Some multinucleated cells may be present.
Histiocytomas usually spontaneously resolve within six to 12 weeks in immunocompetent animals. Surgical excision is curative for lesions that do not resolve spontaneously. Corticosteroid administration should be avoided, since suppression of the normal immune response will delay or prevent tumor regression.
In this puppy, withdrawal of corticosteroid therapy allowed normal regression of the histiocytoma within six weeks.