A splenic mast cell tumor in an 11-year old Siamese cat: Anatomic pathology perspective

Dr. Jade FisherHistologically, the mast cells that comprise feline splenic mast cell tumors resemble their cutaneous counterparts. These cells are round with distinct cell borders and a moderate amount of eosinophilic cytoplasm that contains very fine, vague, cytoplasmic granules that vary in color from amphophilic to basophilic (Figure 9) and are metachromatic on toluidine blue and Giemsa stains.

Figure 9. A photomicrograph of the spleen from the patient in this case that contains neoplastic mast cells with fine, vague, basophilic, cytoplasmic granules (hematoxylin-eosin stain; 400x).

Nuclei are round with finely stippled chromatin and can contain one to several nucleoli. Anisocytosis and anisokaryosis are typically mild with rare to few mitoses. Within the spleen, neoplastic mast cells can form aggregates or be diffusely distributed in sheets throughout the parenchyma (Figure 10).

Figure 10. A photomicrograph of a section of spleen from the patient in this case that is effaced by coalescing sheets of neoplastic mast cells (hematoxylin-eosin stain; 20x).

Currently, there is no published grading scheme that can be used to predict behavior or survival of cats with mast cell tumors of the spleen or other primary locations. Previously published reports of canine cutaneous mast cell tumors have investigated gain of function mutations in c-kit, which is a proto-oncogene that encodes for the tyrosine kinase receptor KIT.1 These studies have identified a significant correlation between prognosis and the pattern of KIT staining within neoplastic cells,1 which has prompted similar studies looking at immunohistochemical biomarkers with feline splenic mast cell tumors. One such study found that within the feline splenic mast tumors examined, 86% had type II or III cytoplasmic KIT staining, indicating focal or diffuse cytoplasmic expression, and that there was a significant correlation between these types of KIT cytoplasmic expression and missense mutations within exon 8 and 9 of feline KIT.2 However, no significant correlation was found between these mutations and tumor di?erentiation, mitotic activity or survival.2

Another study including feline splenic mast cell tumors looked at possible aberrations in serotonin and histamine production within the cytoplasmic granules of neoplastic mast cells that had type II KIT staining but found no such correlation.3 Possible grading criteria to consider in future studies may include degree of pleomorphism, mitoses, multinucleation and invasion into the smooth muscle trabeculae and capsule.

References

1. Kiupel M, Webster JD, Kaneene JB, et al. The use of KIT and tryptase expression patterns as prognostic tools for canine cutaneous mast cell tumors. Vet Pathol 2004;41:371-377.

2. Sabattini S, Barzon G, Giantin M, et al. Kit receptor tyrosine kinase dysregulations in feline splenic mast cell tumours. Vet Comp Oncol 2016. (Epub ahead of print.)

3. Mallett CL, Northrup NC, Saba CF, et al. Immunohistochemical characterization of feline mast cell tumors. Vet Pathol 2013;50:106-109.