Mast cell tumours are one of the most common cutaneous canine tumours seen in clinical practice. They usually exfoliate readily producing moderate to large numbers of cells with characteristic dark red/ purple granules usually occupying most of the cytoplasm. The granules tend to take up most of the stain such that nuclear detail is often not obvious. Mast cell tumours involving the GI tract and mast cell leukemias are less frequently seen.
The most common laboratory cytological stains used for staining slides are Romanowsky stains which includes Diff Quik (aqueous based). Very occasionally and for poorly understood reasons, Diff Quik does not stain mast cell granules or stains them very poorly. When this occurs cells may present as large round cells with a large round nucleus, possibly with nucleoli, and a moderate amount of mostly clear cytoplasm. The cytoplasm of some of these cells may sometimes contain a few small pink granules. These largely agranular cells may then be confused with other round cell tumours such as histiocytomas. When additional smears from these cases are stained with methanolic based stains such as the Leishman’s stain, the granules usually then stain well.
When examining mast cells either cytologically or histologically, the degree of intracytoplasmic granulation is important in helping to make decisions as to whether the tumour is low grade or high grade. This decision is therefore difficult with some Diff Quik stained smears.
When aspirating a mass, it is therefore preferable to make several smears. If you wish to do your own cytology, then do so. But reserve at least one smear unstained for sending and staining in the laboratory should your clinic stained smear not give clear results. When the same blood sample was later sent to the laboratory, and further blood smears made and stained with Leishman’s stain, cells containing characteristic mast cell pink granules. Making multiple smears and reserving at least one smear for commercial laboratory staining is also suggested for masses/ lesions other than mast cell tumours if problems arise with clinic stained smears.
Dog with a mast cell leukemia:
Blood was taken from a sick dog presented at a veterinary clinic. The practitioner stained a blood smear in the clinic using the Diff Quik stain (Figure 1). This revealed moderate numbers of large round cells with a large round nucleus and moderate amount of clear cytoplasm. The cytoplasm of some cells contained a few small granules were revealed (Figure 2).
Figure 1 (below): Mast cells (arrows) in the feathered edge of a canine blood smear. The cells have small numbers of small faint pink intracytoplasmic granules. The smear was stained with Diff Quik.
Figure 2 (below): Mast cells with characteristic granules in a canine blood smear stained with Leishman’s stain. This smear was made from blood from the same blood sample as the smear in figure 1.
Horse with a nasal mass:
A practitioner was called to a horse with a mass on the nose. A fine needle aspirate was taken and two smears were made. The practitioner stained one of the smears in the clinic using Diff Quik and sent the remaining smear to the laboratory for staining and examination where Leishman’s was used. The Diff Quik stained smear from the clinic revealed round cells with a round nucleus and moderate amount of slightly pink hued cytoplasm. Eosinophils were also present (Figure 3). Heavily granulated mast cells and eosinophils were very obvious in the Leishman’s stained smear, stained in the laboratory (Figure 4).
Figure 3 (below): Mast cells and eosinophils in a scraping from the same horses nose stained with Diff Quik. Mast cells (yellow arrows) and eosinophils (orange arrows) are present, along with some very dark staining unidentifiable cells.
Figure 4 (below): Mast cells (yellow arrows) with characteristic granules, plus eosinophils (orange arrows) in a scraping from the same aspirate as the smear in figure 3. Stained with Leishman’s stain.
Jackson DE, Kim A. Selting, Spoor MS, Henry CJ, Wiedmeyer CE. Evaluation of fixation time using Diff-Quik for staining of canine mast cell tumor aspirates. Veterinary Clinical Pathology 42, 99-102, 2013
Garrett LD. Common neoplastic skin lesions in cats and dogs. Today’s Veterinary Practice 28-35, January/ February 2016