Surgery STAT: An overview of cutaneous mast cell tumors in dogs
This common skin neoplasm may vary widely in its clinical appearance, but maintaining a consistent diagnostic and treatment approach offers the best chance for an optimal outcome.
Mast cell tumors (MCT) are one of the most common cutaneous neoplasms in dogs, representing up to 21 percent of all skin tumors.1 Mast cells originate in the bone marrow and migrate to become tissue mast cells in the dermis and subcutis. They possess metachromatic cytoplasmic granules containing histamine, heparin and proteases, and stain positive with toluidine blue stain. Known as the “Great Imitators,” MCT have a variety of clinical appearances, but most commonly present as a pink, raised hairless mass on the trunk and limbs.
Approximately 10 percent of dogs that present with one cutaneous MCT will have at least one other cutaneous MCT present at the time of diagnosis.1 Any dog presented with a MCT should have cytology performed on all other skin and subcutaneous masses, and each tumor should be treated independent of the others. Metastasis occurs most commonly to regional lymph nodes, followed by the liver and spleen, mesenteric lymph nodes and bone marrow.
MCT should be staged prior to surgical excision. Routine staging includes cytology or biopsy to confirm the diagnosis, a minimum database (complete blood count and serum chemistry profile) and regional lymph node aspiration. Metastatic lymph nodes are not always enlarged, so any local lymph nodes that are palpable should be aspirated for cytological evaluation. Local imaging (e.g. ultrasound, computed tomography, magnetic resonance imaging) should be considered if the tumor is large or firmly adhered to deeper tissues.
Additional staging should be performed postoperatively in dogs with high-grade tumors or preoperatively in dogs with evidence of lymph node metastasis, large infiltrative tumors or tumors in unfavorable locations. This includes abdominal ultrasound with spleen and liver aspirates and bone marrow aspiration. Pulmonary metastasis is very rare, and thoracic radiographs are not routinely performed except in cases of high-grade tumors. Buffy coat smears have not been shown to be of prognostic value and are not recommended routinely.2
Surgery is the primary treatment for MCT when possible. Remember that the first surgery is the best chance for complete excision. Dogs should receive diphenhydramine (2.2 mg/kg intramuscularly) at the time of anesthetic induction to reduce the risk of histamine-related complications. This does not need to be continued after surgery unless there is a concern about incomplete excision. In cases where the tumor is large and resection with wide margins would be difficult, prednisone can be given at a dose of 1-2 mg/kg/day for one to two weeks prior to surgery to help shrink the tumor and reduce peritumoral inflammation.3
Historically, surgical margins of 2-3 cm and one fascial plane have been recommended for MCT. Recent studies have shown that most grade 1 and 2 tumors can be completely excised with 1-2 cm lateral margins (Figure 1).4 In locations where no deep fascial plane exists, such as the axillary or inguinal folds, a minimum of 4 mm of deep tissue should be excised.5 Surgical margins should extend at least 2 cm around all biopsy tracts (Figure 2).
Figure 1: Using a sterile marking pen, circle the palpable tumor margins and mark 2 cm margins in all directions, including biopsy and drain tracts. Use this as a guide to create your elliptical incision. (All photos courtesy of Dr. Steven Neihaus.)
Figure 2: Remember that the cut skin will retract. Maintain the incision along the same plane to prevent narrowing the deep margins.
Drains should be avoided during closure in the event that excision is incomplete, in which case re-excision will require 2 cm margins around the surgical scar and all drain tracts (Figure 3). After excision, tumor margins should be inked to assist the pathologist with margin evaluation (Figures 4 and 5).
Figure 3: The deep margins should include at least one fascial plane (fascia or skeletal muscle). Avoid drain placement if possible.
Figure 4: A suture tag can be placed to give directionality to the pathologist (i.e. suture tag dorsally).
Figure 5: Ink tumor margins.
Postoperative concerns with MCT excision include delayed wound healing, dehiscence and hemorrhage. Complications can be avoided by use of gentle surgical technique, meticulous hemostasis and appropriate preoperative planning. In cases where tumor excision is incomplete, options include re-excision or radiation therapy to prevent local recurrence and chemotherapy in cases of high-grade tumors with an increased risk of metastasis.
Client communication is paramount when discussing surgical excision of MCT. In certain locations, such as the dorsum of the paw, wide and deep excision is not possible. Owners need to be aware of the risk of local recurrence and plans for follow-up treatment should be discussed in advance (Figure 6). Consultation with a veterinary oncologist should be performed preoperatively if there is concern about about large, invasive or suspected high-grade tumors. When radiation will be performed postoperatively, consider the placement of radiographic markers such as hemoclips at the deep surgical margins to aid the radiation planning (Figure 7).
Figure 6: Warn owners that a small tumor excised with wide margins will result in a long incision.
Figure 7: Place hemoclips at deep margins when radiation is to be performed postoperatively.
Prognostic factors for MCT include grade, location, tumor size, breed and the presence of clinical signs.1 Tumor grade can only be assessed histologically and is based on several factors, including degree of differentiation, location in the dermis and number of mitotic figures. The prognosis for low-grade (1 and 2) tumors with complete excision and no evidence of metastasis is good. The risk of recurrence and metastasis are increased with increasing grade and incomplete excision.
1. Withrow SJ, David MV. Mast cell tumors. In: Withrow and MacEwen’s small animal clinical oncology. 4th ed. St. Louis, MO: Saunders-Elsevier, 2007;402-424.
2. McManus PM. Frequency and severity of mastocytemia in dogs with and without mast cell tumors: 120 cases (1995-1997). J Am Vet Med Assoc 1999;215(3):355-357.
3. Stanclift RM, Gilson SD. Evaluation of neoadjuvant prednisone administration and surgical excision in treatment of cutaneous mast cell tumors in dogs. J Am Vet Med Assoc 2008;232(1):53-62.
4. Simpson AM, Ludwig LL, Newman SJ, et al. Evaluation of surgical margins required for complete excision of cutaneous mast cell tumors in dogs. J Am Vet Med Assoc 2004;224(2):236-240.
5. Schultheiss PC, Gardiner DW, Rao S, et al. Association of histologic tumor characteristics and size of surgical margins with clinical outcome after surgical removeal of cutaneous mast cell tumors in dogs. J Am Vet Med Assoc 2011;238(11):1464-1469.
Steven Neihaus, DVM, DACVS, practices at Chicago Veterinary Emergency and Specialty Center. His professional interests include oncologic surgery, skin and reconstructive surgery, and minimally invasive surgery (laparoscopy and arthroscopy).