Mast Cell Tumors in Dogs

Quick Facts at a Glance

20-25% of all skin/subcutaneous tumors in dogs are mast cell tumors (MCT)
10-15% of canine MCT are clinically indistinguishable from subcutaneous lipomas (fatty tumors)
Definitive diagnosis of MCT cannot be made without cytologic (aspirate) or histologic (biopsy) evaluation of the lesion
Behavior of individual MCTs is difficult to predict but should always be considered aggressive until proven otherwise
MCT of the perineal and inguinal regions tend to behave more aggressively than their tumor grade would predict
Most dogs with MCT have a normal CBC(complete blood count)
20% of dogs with MCT will have multiple primary tumors in their lifetime
Aggressive surgical resection remains the cornerstone of treatment

What are the clinical features?

Mast cell tumors in dogs occur primarily as either a skin or subcutaneous mass. It is important to remember that mast cell tumors are extremely variable in their clinical presentation. They can resemble any other type of skin or subcutaneous tumor, both benign (i.e. lipoma) and malignant. Most canine MCT are solitary although multiple primary tumors develop in 20% of patients. Approximately 50% of canine MCT are located on the trunk and perineum, 40% on the extremities and 10% on the head and neck.

Regional lymphadenopathy (lymph node enlargement) may occur when high-grade mast cell tumors metastasize (spread) to draining lymph nodes. Hepatomegaly (liver enlargement) and splenomegaly (spleen enlargement) are features of advanced stage, metastatic MCT. Malignant mast cells may be detected in the blood and bone marrow of dogs with advanced stage disease.

Cytology vs. histopathology?
The diagnosis of MCT is often made simply with cytologic evaluation of a fine needle aspirate of the mass. Although cytology may allow us to confirm the diagnosis, it provides little prognostic information. Histopathology (biopsy) allows us to grade the tumor so we can predict biologic behavior and make appropriate treatment recommendations.

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  Survival Times of Dogs After Surgery Based on Histologic Grade
  Grade                                    #of Dogs                        Alive 6 Months Post-Surgery  
  I                                               39                                           77% 
  II                                              30                                           45% 
  III                                             45                                           13%
  
  
  Surgery: 3cm margins!!!
  
  The most frustrating feature of MCT is how aggressively they can behave, regardless of their gross appearance and histologic grade. MCT are notorious for their invasive nature and tendency to extend far beyond visible margins. If 2-3 cm surgical margins, both lateral and deep, are not obtained then chances are good that the tumor will not be adequately excised. While it may be difficult to remove 2-3 cm deep margins in a patient without much body fat, removing a layer of muscle below the tumor is advised. A surgical margin of a couple of millimeters is not adequate. When surgical margins are very close, a second surgery should be recommended as soon as possible. It is impossible to comment on completeness of surgical excision and predict biologic behavior of mast cell tumors without histopathologic evaluation of the entire tumor.
  
  Depending on where the tumor is located, it may not be possible to obtain 3 cm surgical margins (i.e. leg). In those cases, it is appropriate to perform a “debulking” surgery followed immediately by radiation therapy. Radiation therapy in this setting can be highly effective at achieving long-term local control. Depending on the situation, chemotherapy may be recommended in conjunction with or in lieu of radiation therapy. Administration of prednisone alone (1mg/kg every 24 hours) for 5-7 days prior to surgery may result in sufficient reduction of the tumor mass and associated inflammation thereby increasing success of surgery.
  
  What about lymph nodes?
  Whenever possible, the draining lymph nodes (lymph nodes in close proximity) should be biopsied or excised for histopathologic analysis, regardless of gross appearance and tumor grade. The presence of lymph node metastasis worsens the prognosis and necessitates treatment with chemotherapy.
  
  When is it essential to stage the patient with mast cell tumor?
  Diagnostic tests performed in the staging of MCTs include cytology or biopsy of the draining lymph node, abdominal ultrasound with ultrasound guided aspirate and cytology of the spleen and liver, bone marrow aspirate and cytology, complete blood count (CBC) and chest radiographs. Although the staging process can provide us with essential information, we are very limited by the low sensitivity of these diagnostic tests. In the case of a grade I or low grade II MCT that has been completely excised with wide surgical margins, it is unlikely that metastasis has occurred. Early metastasis would not likely be detected during the staging process.
  
  Staging is essential prior to undergoing an extensive or expensive treatment procedure (i.e. amputation or radiation therapy). Staging is also recommended for any grade III or high-grade II MCT, even if completely excised with wide margins. Histologic evaluation of draining lymph nodes is preferred over cytology in this situation.
  
  Radiation therapy
  Mast cell tumors are generally very sensitive to radiation. Radiation is most effective at achieving long-term (greater than 5 years) tumor control when it is delivered to microscopic disease, grade 2 intermediate or lower and in a well-defined radiation field. Radiation therapy can be used to control local disease after incomplete resection of high-grade 2 or grade 3 MCT. The size of the tumor prior to surgical resection has great impact on the effectiveness of radiation.
  
  Chemotherapy: When to treat?
  As with any cancer, chemotherapy for the treatment of MCT is most effective when combined with other treatment modalities or used early in the course of the disease. Chemotherapy will not be helpful in the treatment of a large mast cell tumor that cannot be excised and has become resistant to prednisone therapy. Chemotherapy is appropriate when tumor cells extend to surgical margins and a second surgery and radiation therapy are not feasible treatment options. Chemotherapy is recommended in all cases with lymph node involvement. Chemotherapy is recommended for all patients with high-grade II or grade III MCT, even when the tumor is excised with wide margins.
  
  Prednisone, vinblastine and lomustine are used in combination to obtain the best results with minimal side effects. Adjuvant chemotherapy results in survival times of greater than 2 years in patients with high-grade II MCT with metastasis to regional lymph nodes, that have been treated with aggressive surgical resection resulting in complete resection and are negative for visceral (organ, i.e. spleen) metastasis. Chemotherapy is not of much benefit when staging confirms systemic metastasis.