Recall from Section 11.1.3 that mast cells are an expected population of cells in all patients. These cells play an important role in inflammatory and immune responses as a direct result of the mediators that they contain within granules.
When mast cells are called into action, they undergo degranulation; that is, they release the contents of their cytoplasmic granules into the surrounding tissues.
The granules of mast cells contain proteoglycans. This is important because certain cytological stains and dyes are capable of binding these proteoglycans, thus making it possible to identify mast cells easily in tissue, even under light microscopy [91, 92]. Mast cells appear under the microscope to be round cells that contain a variable number of purple‐red cytoplasmic granules [91, 93–96] (see Figures 11.9a, b).
Degranulation serves a purpose in the normal patient.
The release of cytokines by mast cells recruits other cells to the area of injury. In this way, mast cells mediate the body’s response to inflammation [17].
However, mast cell tumors (MCTs) can also develop in which this cell type is overrepresented. MCTs can form anywhere, including the liver and the spleen; however, the skin is a common location for dogs [17, 97].
Some clinicians call cutaneous MCTs the “great pre-tender” because they have no distinct pathognomonic appearance. They truly can look like any other tumor type (see Figures 11.10a–d).
Cutaneous MCTs that are low grade and well differen-tiated often, but not always, appear as solitary, slow‐ growing nodules [96, 98]. Undifferentiated cutaneous MCTs tend to grow more rapidly and ulcerate [96, 98].
Sometimes lesions are poorly defined, particularly when they are located on distal extremities [91]. Here, they may be mistaken for acral lick dermatitis based upon gross appearance [91].
Because cutaneous MCTs do not have a classic appear-ance, the differential diagnosis list for any cutaneous mass should always include MCT [91, 99]. Fine‐needle aspira-tion is an important diagnostic tool because it allows for cytological examination, which is diagnostic [91].
Any dog can develop a MCT. However, certain breeds appear to have a predisposition for their development.
These breeds include Boxers, Boston terriers, English Bulldogs, Bull terriers, Staffordshire terriers, Cocker spaniels, Labrador retrievers, Golden Retrievers, and Schnauzers [17, 91, 94, 96, 100–105].
There does not appear to be predisposition for MCTs based upon gender [91, 98, 100, 104–107].
Most dogs are between 7.5 and 9 years of age at the time of diagnosis [91, 93, 94, 98, 99, 104–111]. However, MCTs have been diagnosed in canine patients less than six months of age [108, 112, 113]. Boxers also appear more likely to develop MCTs at younger ages than other breeds [105, 109].
Patients with cutaneous MCTs often present for evalu-ation of a newly discovered growth rather than for the local or systemic effects that result from mast cell degranulation [19, 91, 94, 114]. The trunk and extremi-ties are the most common anatomic locations for cuta-neous MCTs to appear in dogs [94, 102, 106, 115, 116].
The scrotum and perineum are less frequently involved [103]. Cutaneous MCTs in cats tend to concentrate on the head, neck, and trunk [117, 118].
A significant concern associated with MCT is that they can degranulate spontaneously, for example, during palpation or even during surgical manipulation
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