Navigating an often difficult and confusing type of cancer
Mast cell tumors occur due to an uncontrolled proliferation of malignant mast cells in various organs. Mainly seen in middle aged to older dogs, the approach to this cancer can often be challenging and confusing. Most patients present with a single cutaneous mass; however, this tumor can often have a variable appearance such as ulceration, inflammation, bruising in the form of petechiae and ecchymosis or edema, each of which can be minor or extensive. Some patients may have multiple masses all of which can have varied appearances also.
The history presented by owners also varies and may include a lump which has been present for a long time with no change, or recently it may have changed. A mast cell tumor is always suspected with any lump reported to be increasing and decreasing in size over time. Other patients may have a history of mast cell tumors. And others may have a large region of edema, wheal formation or bruising with no obvious mass.
Given that these cells exfoliate easily, an aspirate is usually all that is required to confirm diagnosis. Aggressive and repeated sampling is not necessary. Staining with inhouse Diff-Quik is usually sufficient to confirm a mast cell tumor, revealing a round cell with the characteristic purple cytoplasmic granules. Occasionally a round cell with a “fried egg” appearance is noted with no granules, this sample will need to be sent to the pathologist for additional staining, such as with Wright-Giemsa or toluidine blue stain. After aspiration, the mass should be re-evaluated given that these tumors tend to bleed easily and for a prolonged period. Degranulation may also occur resulting in the mass increasing in size. The erythema and wheal formation (Darier’s sign) and prolonged bleeding are also used to confirm your diagnosis.
After cytological confirmation, the next consideration is if staging is necessary. Typically, if the mass is in a location which carries a favorable prognosis and, in a location, considered easy to remove, then surgical removal is recommended. Various studies have confirmed that mast cell tumors in different locations are of different prognostic value. Anatomical locations which may carry a poor prognosis include perineal-perianal region, head and neck, inguinal area, scrotum, digit, and axilla.1 Once the tumor is removed further staging and treatment is dependent on the histopathology results and the grade. If a mass is difficult to remove (eg, due to location or size), if a patient has a history of mast cell tumors or if the mass is in a location associated with a negative prognosis, then staging tests are recommended. Staging tests involve chest radiographs and an abdominal ultrasound. Aspirate of the local draining lymph node is recommended. Willmann et al. has described the spread of mast cell tumors to regional lymph nodes (stage 2), then to the spleen and liver (stage 3) and then to other visceral organs and occasionally to the bone marrow (stage 4). Pulmonary metastatic disease is exceedingly rare. Staging tests are also recommended if the patient is clinically unwell, e.g., lethargic or vomiting or if the patient has had a history of mast cell tumors. It is never wrong to stage a patient.
Treatment with ancillary medications to decrease degranulation effects are recommended before treatment with surgery or radiation therapy. Antihistamines such as the H1 blocker diphenhydramine (2 mg/kg BID) and a H2 blocker famotidine (0.5-1 mg/kg SID) or the proton pump inhibitor omeprazole (0.5-1 mg/kg SID) are recommended.
Traditionally, treatment options include surgery, radiation therapy, chemotherapy, or a combination of these treatments. Most commonly surgery is the first line of treatment. Removal of any enlarged or concerning lymph nodes is recommended. Recently it has been recommended to remove any regional lymph node associated with mast cell tumors >3cm, given that at this size, metastatic disease was more likely.2 Prednisone and Chemotherapy can be used to cytoreduce a mass to make it more appealing for surgery. These tumors are radiation responsive and so radiation therapy can be used for bulky disease or to address microscopic disease post-surgery.
Newer therapies are also currently available and include Stelfonta, Vidium, and Torrigen. Stelfonta (tigilanol tiglate) is an intra-tumoral mast cell tumor treatment. Vidium involves genomic based-precision testing using next-generation sequencing to identify mutations in 120 relevant cancer genes and Torrigen produces an experimental Autologous prescription cancer immunotherapy vaccination.
Kathleen Tsimbas, BVSc received her veterinary school training at the University of Sydney, Australia. Before completing a rotating internship, specialty internship and residency at the University of Wisconsin in Madison. She is currently treating small animals at Advanced Veterinary Care in Salt Lake City, Utah.