Mast Cell Tumor (Mastocytoma) in Cats Leave a comment

Connective Tissue Tumors in Cats

Mast cell tumors (or mastocytomas) are graded according to their location in the skin, presence of inflammation, and how well they are differentiated. Grade 1 cells are well differentiated with a low potential for metastasis; Grade 2 cells are intermediately differentiated with a potential for locally invasive metastasis; and Grade 3 cells are poorly differentiated or undifferentiated with a high potential for metastasis. Differentiation is a determination of how much a particular tumor cell looks like a normal cell; the more differentiated, the more like the normal cell. In general, the more differentiated the mast cell tumor is, the better the prognosis is.

Siamese cats appear to be more susceptible to mast cell tumors than other breeds, and are susceptible to histiocytic cutaneous (skin) mast cell tumors (where histiocytes are mature macrophages that reside in connective tissue). The mean age for the development of the mastocytic (mast cell) form occurs at mean age of 10 years in cats; the histiocytic form occurs at a mean age of 2.4 years. It has been reported in animals less than one year of age and in cats as old as 18 years of age.

Symptoms and Types

Symptoms may be dependent on the location and grade of the tumor.

  • Tumor on the skin or under the skin (subcutaneous), may have been present for days to months
  • Tumor may appear to fluctuate in size
  • Recent rapid growth after months of inactive or subtle growth is common
  • Recent onset of redness and fluid build-up is most common with high-grade skin and subcutaneous tumors
  • Extremely variable; may mimic or resemble other types of skin or subcutaneous tumors (benign and cancerours); may resemble an insect bite, wart, or allergic reaction
  • Primarily occurs as a single skin mass or subcutaneous mass, but may have multiple masses located throughout the body
  • Approximately 50 percent of all mast cell tumors are located on the trunk and perineum (the area between the anus and vulva in females, or the anus and scrotum in males); 40 percent are found on the extremities, such as the paw; and 10 percent  are found on the head and neck region
  • Lymph nodes may be enlarged around the area of the tumor and may develop when a high-grade tumor spreads to the lymph nodes
  • Masses may be itchy or inflamed due to the higher level of histamines in the tumor
  • Enlarged liver and enlarged spleen are characteristic of wide-spread mast cell cancer
  • Vomiting, loss of appetite, and/or diarrhea may occur, depending on the stage of the disease

Symptoms are also dependent on the stage of the disease:

  • Stage 1 is characterized by a single tumor without metastasis
  • Stage 2 is characterized by a single tumor with metastasis into the surrounding lymph nodes
  • Stage 3 is characterized by multiple skin tumors, or by a large tumor that has invaded subcutaneously
  • Stage 4 is characterized by the presence of a tumor, with metastasis to an organ or wide spread mast cell presence in the blood




You will need to give a thorough history of your cat’s health, including a background history of symptoms. The history you provide may give your veterinarian clues as to which organs are being affected.

The most important preliminary diagnostic test will be an examination of the cells taken from one of the tumors. This will be performed with a fine needle aspirate and will determine the presence of an abnormal amount of mast cells in the blood. A surgical tissue biopsy will be necessary for definitive identification of both the grade of the cells occupying the mass, and the stage the disease is in. Additionally, your veterinarian may examine a sample from a draining lymph node, from the bone marrow, or from the kidney and spleen. X-ray and ultrasound images of the chest and abdomen will also be component of determining the exact location and stage of the tumor’s development.


Manipulation of the tumor may result in the release of histamines from the tumor due to the mast cells releasing from the tumor into the blood stream. Antihistamines may be prescribed to alleviate some of the symptoms related to this effect. This same behavior can come into play as a result of surgical intervention; antihistamines will be used under the circumstances, as a large release of histamines on the body can have a drastic effect on the organs.

Surgery is the treatment of choice for mast cell tumors of the skin. Surgical removal of the spleen is the treatment of choice for mast cell tumors of the spleen. Surgical removal of the spleen along with chemotherapy may be beneficial when mast cells circulating in the blood accompany mast cell tumors of the spleen.

Excisional biopsy with wide margins is reasonable for very small tumors. Incisional biopsy of large mast cell tumors is recommended to obtain a tumor grade, to predict prognosis, and to establish a treatment plan. Your veterinarian will probably consider pretreatment with antihistamine therapy prior to an incisional biopsy. Biopsy of lymph nodes and other suspicious internal organs is appropriate. Complete surgical removal with 3-cm margins in all planes is recommended for all moderate Grade 2, high Grade 2, and Grade 3 tumors; margins of 2 cm or less may be adequate for Grade 1 and low Grade 2 tumors. Surgical removal of regional lymph nodes is recommended for all high Grade 2 and Grade 3 tumors.

Living and Management

Your veterinarian will want to microscopically evaluate any new masses and evaluate the lymph nodes at regular intervals in order to detect spread of Grade 2 or Grade 3 tumors. Your doctor will also want to perform a complete blood count at regular intervals if your cat is receiving chemotherapy. Immunity can be affected by cancer fighting drugs, so it will be important to protect your cat from illness and communicable diseases during this period, as well as sticking closely to a healthy, immune boosting diet.


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