Mast cell tumors in dogs

Mast cell tumors are the most common tumors in our dogs. These tumors originate from mast cells: Mast cells are cells of the immune system that can be found normally throughout the body and in almost every tissue in healthy animals. They are responsible for defense reactions, but also play an important role in allergic reactions. Most commonly, mast cell tumors occur in the skin and subcutaneous tissue, but internal organs such as the digestive system can also be affected.

How to detect a mast cell tumor?

Mast cell tumors in the skin present as nodular changes. They are often discovered while petting or brushing the dog. But not every lump in or under the skin is immediately a mast cell tumor, a thorough and as early as possible examination by the veterinarian can provide clarity. Sometimes these areas are hairless and reddened, especially after manipulation. These symptoms are grouped together as Darier's sign. Special attention should be paid to open, or ulcerated, mast cell tumors. It is not uncommon for the tumor to change size rapidly. It may become significantly larger one day and spontaneously smaller the next. These changes are typical of mast cell tumors and result from messenger substances released from the mast cells. The main role is played by histamine, which is stored in the granules of the mast cells.

A diagnosis should be made for any newly palpated circumferential growth. To differentiate between a benign tumor, such as a lipoma, and a mast cell tumor, a fine-needle aspiration (FNA) can be performed. This examination is quick, simple, painless, and a mast cell tumor can thus be diagnosed very reliably.

Mast cell tumors: Always malignant?

Mast cell tumors represent a broad group of tumors that show varying degrees of aggressiveness. If left untreated, they can further increase their aggressiveness. Basically, we distinguish different groups, so-called grades. Depending on the system, there are two grades (low grade and high grade) or grade 1 to grade 3. First-grade mast cell tumors are less aggressive tumors, i.e. they infiltrate the surrounding tissue only to a minor degree and rarely form metastases. In contrast, a third-degree mast cell tumor is highly aggressive, infiltrates the surrounding tissue, and forms metastases in 50%-90% of cases. Therefore, the grade of the tumor should be known for the selection of the right therapy. Whether surgery is performed or a biopsy, i.e. a tissue sample is taken, depends on the localization. In the case of tumors on the legs, often only marginal surgery can be performed without a safety margin in the surrounding tissue, so in many cases a tissue sample is not taken. In the case of a mast cell tumor in the chest area, a biopsy may be taken first for better surgical planning. Depending on the grade or aggressiveness, surgery is then performed. While grading describes the aggressiveness of the tumor, staging is a metastasis search. In the case of mast cell tumors, the regional lymph node is punctured if possible. In addition, an ultrasound examination of the abdominal cavity with puncture of the liver and spleen should always be performed.

How to treat a mast cell tumor?

The treatment of the mast cell tumor depends on the aggressiveness and already existing metastases. If the tumor is highly aggressive, the treatment must also be aggressive. A large-scale surgery is performed with 2-3 cm of safety margin in the healthy tissue and one to two fasciae in depth. When surgery is performed, the removed tumor material should always be sent for histopathologic examination, even if an FNA has been done earlier. The benefit of this examination is usually much greater than the cost. In addition, the veterinarian can request a more precise differentiation here if necessary, for example by an immunohistochemical examination. If large-scale surgery is not possible, typically in the limb area, local tumor control can be improved by additional, i.e. adjuvant, radiotherapy. If metastases are suspected or there is a high risk of metastasis, a chemotherapeutic agent is administered. If the tumor is already very large or metastasis has already occurred at the time of diagnosis, palliative radiotherapy combined with chemotherapy is used.

In which breeds do mast cell tumors occur particularly frequently?

Mast cell tumors are more common in brachycephalic breeds such as pugs or French bulldogs. These dogs may even suffer from multiple mast cell tumors at the same time or develop multiple mast cell tumors during their lifetime. However, these mast cell tumors are often grade 1, meaning they are not very aggressive. Other predisposed breeds are for example Labrador Retriever and Golden Retreiver, Weimaraner or Vizsla.