This is the third painting series in collaboration with Francesco Cian. He is a veterinary pathologist and passionate about teaching cytology (visit his website here). Together we composed a series of five cytology cases – all about the differential diagnosis of round cell tumors.
As always Francesco had provided me with expressive cytology references. My aim was to capture the nuances of the differential diagnoses in the following paintings.

Lymphoma is a common neoplasia that affects all mammals and may involve virtually any organ. When confined to the skin, it is defined as cutaneous and may be small or large cell, epitheliotropic, or non-epitheliotropic, depending on cell size and localization, respectively. Lymphoid cells typically exhibit a round shape with an increased nucleus-to-cytoplasm ratio. Mitotic figures may be observed, especially in aggressive forms.

Lymphoma

Histiocytoma (HCT)
Histiocytoma (HCT) is a benign neoplasm described in dogs and originating from the Langerhans cells. Despite their rapid appearance that may be alarming, they tend to grow rapidly over a few weeks before often spontaneously regressing and disappearing within a few months, even without treatment. A distinctive feature of histiocytomas is the presence of a blue background, typically darker than the cell cytoplasm.
Transmissible venereal tumour (TVT) is a very fascinating neoplasm as it is one of the very few transmissible tumors described in mammals. TVT likely originated thousands of years ago in a single dog and behaves like a parasitic organism, persisting and spreading through populations across continents and over generations. From a cytology standpoint, TVT cells appear as discrete elements with characteristic, well-defined clear vacuoles, often aligned along cell borders.

Transmissible venereal tumour (TVT)

Mast cell tumor (MCT)
The Mast cell tumor (MCT) is often referred as the great pretender as its macroscopic appearance may vary significantly and may range from a lipoma like mass to a larger, irregularly shaped mass. Cytologically, it can be easily recognised by the presence of characteristic intracytoplasmic purple granules, also referred as magenta or metachromatic, which may sometimes obscure nuclear details.
Plasma cell tumour (PCT) is a fairly common neoplasia in dogs, primarily observed in adult and elderly animals. While marked atypia of cell criteria may be present, it behaves as a benign neoplasia when confined to the skin. Plasma cells, as depicted in the painting, exhibit characteristic morphology when well-differentiated, featuring a blue cytoplasm, perinuclear clear halo, and prominently eccentric and round nuclei.

Plasma cell tumour (PCT)