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Mast cell tumors of the skin in cats

Melville K, Smith KC, Dobromylsky MJ. Feline cutaneous mast cell tumours: a UK-based study comparing signalment and histological features with long-term outcomes. J Feline Med Surg. 2015 Jun; 17(6):486-493.

Mast cell tumours (MCTs) are the second most common skin tumour in cats, comprising up to 21% of all skin tumours in cats. The etiology of MCTs is unknown though some prior studies cite a possible genetic predisposition in Siamese cats.

There is currently no histological grading system for MCTs in cats, unlike in dogs, therefore a more accurate prognosis for disease outcome is difficult to determine. The tumours are classified as one of three histological types: well-differentiated mastocytic, pleomorphic mastocytic and atypical (previously referred to as histiocytic). The majority of feline cutaneous MCTs are usually considered a single well-differentiated and benign mass cured by complete surgical excision. A small yet significant portion of these tumors are more aggressive where they recur and/or metastasize to local lymph nodes or other distant sites within months of excision. The investigators in this study aimed to use a large number of histological samples of cutaneous MCTs to examine signalment and anatomical location of the tumours in an attempt identify potential indicators of the course and outcome of the disease.

The numbers of cutaneous MCTs submitted were from 287 cats with 308 tumours. Eighty-six tumours from 69 cats with a known outcome were then evaluated by follow up. The anatomic locations were categorized as one of the following: head (including the ear), neck and trunk, or extremities (including limbs, paw, and tail). The locations were then subcategorized as head, ear, neck, trunk, limbs, paw or tail. The results of the study in regards to anatomical location of lesions showed the head, including the ear, was the most common site over all (38.1%), followed by the neck and trunk (33.3%). The extremities (limbs, paws and tail) accounted for the remaining number (28.5%). Of the 11 Siamese cats (other breeds considered at increased risk are Burmese, Russian Blue and Ragdoll cats) in the study population, the head was the overwhelming primary location of MCTs. In cats seven years of age or less, MCTs were more likely to be found on the head; with older cats 8 years of age or older were more likely to have lesions arise on the trunk.

On histological evaluation, fifty-seven (66.3%) of MCTs were classified as well-differentiated mastocytic type. A novel subcategory (five cats, 5.8%) of the well-differentiated MCT type was noted with prominent multinucleated cells. These cells were larger and most often present to the periphery of the mass and contained more than two nuclei per cell.  Seventeen (19.8%) of MCTs were classified as pleomorphic mastocytic followed by seven (8.1%) classified as the atypical type. Multiple cutaneous MCTs (either arising simultaneously, sequentially or a mix of both) occurred in 10 cats.

Some of the key findings from the study:

  • The study confirms several other reports that cutaneous MCTs tend to arise in older cats, most often affect the head region and Siamese cats are at increased risk. 
  • The reported mean age for presentation is within the range of 8 – 10.5 years.
  • The novel, but relatively rare histological subtype of MCT (described previously) occurred in older cats (8 – 16 years) and sometimes in conjunction with other MCTs of different histological types.
  • There was no significant difference noted in the prevalence of the histological types between those who died to MCT-related disease and those who did not. This suggests that MCTs have a wide-spectrum of behavior and it is not possible to predict the degree of malignancy based on histological type alone.
  • This study confirmed prior reports that there is a correlation between higher mitotic rates and poorer outcomes. Yet due to significant overlap of the mitotic index between groups, mitotic index should not be used as a sole prognostic indicator.
  • This study found that there is no correlation between the number of tumours and outcome.
  • There is still a need for more accurate prognostic indicators in these cases. (VT)

See also:
Mallett CL, Northrup NC, et al. Immunohistochemical characterization of feline mast cell tumors. Vet Pathol. 2013 Jan;50(1):106-109. (Free article)