Mast-cell tumors (MCT) often present a therapeutic challenge to practitioners due to their varied biologic behavior. Although
many are cured with surgery, some MCTs require additional therapy for local and systemic disease control.
•The mast cell
Mast cells originate in the bone marrow and migrate to peripheral tissues, where they are essential in allergic and inflammatory
reactions. When mast cells are activated, they release preformed granules that contain histamine, heparin and proteases; this
action is termed degranulation. Degranulation results in pruritus and swelling of the tumor or peritumoral tissue, excessive
bleeding from the site of biopsy or fine-needle aspiration, delayed wound healing and gastrointestinal ulceration. The etiology
of MCT is unknown. Although MCT are most commonly found in the skin and subcutaneous tissues, reports of primary MCT in visceral
tissues and the nervous system exist.
Most dogs with MCT are asymptomatic. Mast-cell tumors are often red, raised and alopecic, but some can clinically mimic
subcutaneous lipomas or even insect bites. Some MCT have a history of shrinking and swelling, related to intermittent mast-cell
degranulation with histamine release. Uncommonly, MCT are surrounded by bruising or edema related to mast-cell degranulation
(termed Darier's sign); this usually indicates a poorly differentiated, aggressive tumor. If a skin or subcutaneous tumor
is thought to be MCT based on its clinical appearance, then administration of diphenhydramine (1-2 mg/kg, subcutaneously)
30-60 minutes prior to aspiration of the mass can minimize further swelling and degranulation post-aspirate. Mast-cell tumors
are generally straightforward to diagnose cytologically because of their round nucleus and characteristic intracytoplasmic
granules (Figure 1); however, in poorly differentiated tumors, granules might not be visualized.
Figure 1: Cytologic appearance of a mast-cell tumor.
A thorough examination of the entire skin surface is indicated in every patient with MCT to ensure that no tumors are overlooked.
Ten percent or more of dogs with MCT will develop additional MCT during their lifetime, so it is wise to suggest to clients
that any new skin mass in a dog with a history of these tumors should be examined and aspirated as soon as possible. Ideally,
patients with a history of MCT should have regularly scheduled recheck appointments (typically every three to six months)
to examine for new tumors.
Figure 2: Locally invasive and metastatic body wall mast-cell tumor in a Shar pei. Shar peis tend to develop aggressive mast-cell
tumors at a young age. This dog was 5 years old.
•Initial therapeutic plan
For most MCT, excisional biopsy after confirming the diagnosis via cytology is the initial treatment of choice. In dogs
with large, infiltrative MCT that appear non-resectable or in patients with signs of mast-cell degranulation, such as peritumoral
edema and bruising or gastrointestinal ulceration, additional staging is recommended prior to any attempt at surgical removal
(Figures 2 and 3). For tumors not amenable to complete surgical resection or in patients with metastasis at diagnosis, chemotherapy
and/or radiation therapy (RT) may be indicated prior to, following, or instead of surgery. In those cases, current therapeutic
options should be reviewed, and consultation with an oncologist is indicated when available.