Page 183 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Chapt er 10
170
• The cytoplasm is moderate in amount and clear to pale basophilic. Rarely, an increased
basophilia can be observed in high-grade tumours.
VetBooks.ir • In well granulated forms, the cytoplasm contains numerous thick purple granules, usu-
ally evenly distributed within the cell.
• In poorly granulated forms, granules are reduced in numbers and are usually finer. In
these cases they may be polarized to one side of the nucleus or may form clumps.
• Occasionally, neoplastic cells can display erythrophagia.
• Cytological features of atypia are prominent in high-grade forms. These include karyomeg-
aly, variable anisokaryosis and anisocytosis, bi- and/or multinucleation. Mitotic figures can
be observed.
• A concurrent eosinophilic inflammation is often present. Eosinophils are attracted by the
chemokines contained in the mast cell granules.
• Reactive fibroblasts are frequently seen. They may exhibit variable cytological pleomorphism
and may contain eosinophilic/pink granules.
• Collagen fibrils may also be present.

Differential diagnoses
• Mast cell rich inflammatory lesion (e.g. insect bite reaction)
• Round cell tumour of other origin (in poorly granulated forms or Diff-Quik slides)

Pearls and Pitfalls
• Histopathology is always required to differentiate between cutaneous and subcutaneous
forms and, in the case of a cutaneous MCT, for histopathological grading, which has been
reported to have a prognostic significance. The most widely used grading systems are the
three-tier Patnaik and the two-tier Kiupel systems. Nowadays, most anatomical patholo-
gists apply both grading systems to all canine cutaneous mast cell tumours. The Patnaik
system designates MCTs as grade I (low), II (intermediate), or III (high), based on depth
of invasion, cell and nuclear morphology and mitotic count. The Kiupel system desig-
nates MCTs as low-grade or high-grade based on mitotic count, presence of multinucle-
ation, bizarre nuclei and karyomegaly.
• As discussed above, the accurate grading of MCTs remains pertinent to the histopathology.
However, based on a study by Camus et al. (2016), a cutaneous MCT is defined as
high-grade on cytology when cells are either poorly granulated or when at least two of the
four following findings are observed: (i) mitotic figures; (ii) binucleated or multinucleated
cells; (iii) nuclear pleomorphism; and (iv) > 50% anisokaryosis. Claimed sensitivity and
specificity of this cytological grading scheme are 88% and 94%, respectively.
• Certain Romanowsky-type dyes (e.g. Diff-Quik) sometimes fail to stain mast cell gran-
ules. However, also in these cases, it should always be possible to find a few intracytoplasmic
granules in some of the cells upon careful observation of the slide.