Histologic grade is the most useful prognostic indicator, but it has its limitations, including inter-pathologist variation in interpretation.This cannot be performed cytologically. Interpretation is generally straight forward. Most rapid staining methods as well and Giemsa and wrights stains will stain up mast cell granules (metachromatic - blue/purple). And these coloured granules are not seen in any other skin tumour. Mast cells are not found in abundance in inflammatory lesions. Tumour cells are often accompanied by plump reactive spindle cells (yellow arrowhead) and eosinophils (black small arrow).

Mitotic index (mitoses per 10 high power fields) has been identified as a strong predictor of overall survival for dogs with cutaneous mast cell tumours. The median survival time for dogs with a MI  equal to or less than 5 was significantly longer (70 months) than for those with a MI >5 (2 months), regardless of tumour grade. For those  Grade II or intermediate grade tumours with a MI equal to or less than 5 the median survival time was 70 months, compared with 5 months for tumours of a MI>5. Immunohistochemistry is the most recent addition / adjunctive predictor of overall survival and has been suggested to be performed in cases of intermediate grade tumours. Ki67 is a cellular marker for proliferation and closely correlates with mitotic activity.

Most low grade tumours are cured by adequate local excision and most high grade ones metastasise, but intermediate grade (IG) tumours exhibit variable behaviour. Metastatic rate for IG tumours is estimated at 20-50% but a recent study showed metastases in only 5 of 55 dogs with IG tumours. Other prognostic factors include breed (Boxers have more benign tumours whilst Shar-peis have more aggressive ones), site (perianal, inguinal, subungual and mucocutaneous tumours seem to have a poorer outcome), clinical signs (rapid tumour growth and systemic disease are associated with poorer prognosis), and clinical stage (0 and 1 do better than higher stage disease). Recent literature indicates that multiple synchronous mast cell tumours carry a poorer prognosis. Older and male dogs also had shorter survival times. The depth of the tumour had no effect on survival time. 

First published: Wed, Oct 6 2010


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