Cutaneous Mast Cell Tumors
(Mastocytomas,
Mast cell sarcomas) |  |
| These tumors are the most frequently recognized malignant or potentially malignant neoplasms of dogs. In addition, leukemic and visceral forms can occur. A viral etiology has been speculated but remains controversial. Tumors may be seen in dogs of any age (average 8-10 yr). They may develop anywhere on the body surface as well as in internal organs, but the limbs (especially the posterior upper thigh), ventral abdomen, and thorax are the most common sites; ~10% are multicentric.
Location on mucocutaneous junctions or on the ventral surface of the body is associated with a more aggressive biologic behavior. Many breeds appear to be predisposed, especially Boxers and Pugs (in which tumors are often multiple), Rhodesian Ridgebacks, and Boston Terriers. The tumors vary markedly in size, and clinical appearance alone cannot establish a diagnosis. Most commonly, they appear as raised, nodular masses that may be soft to solid on palpation. Although they often seem
encapsulated, mast cell tumors in dogs are seldom discrete. Rather, they consist of a highly cellular center surrounded peripherally by a halo of smaller numbers of mast cells that palpate as normal skin. Dogs can also develop clinical signs associated with the release of vasoactive products from the malignant mast cells. Most common is gastroduodenal ulceration that may be present in up to 25% of cases. Cytologic evaluation of Wright-stained, fine-needle aspirates or impression
smears can be used to establish the diagnosis of mast cell tumors in dogs. All skin tumors should be examined by fine needle aspiration cytology prior to excision to rule out mast cell cancer. If the surgeon is aware that the tumor is a mast cell, a surgical plan for wide and deep excision will yield the best results. All mast cell tumors need to be submitted for biopsy to determine margins and grade because cytology is not a substitute for histopathology—only the latter has
been correlated with prognosis. Two systems of histopathologic grading have been defined, and to avoid confusion, it is essential to know which system is being used.
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| Although there is believed to be a benign variant of canine mast cell tumor, there is no clinical or microscopic means of identifying it. In addition, small mast cell tumors may remain quiescent for long periods before becoming aggressive. Thus, all should be treated as at least potential malignancies. |
| Treatment depends on the clinical stage of the disease and the predicted aggressive biologic behavior. For Stage I tumors (a solitary tumor confined to the dermis without nodal involvement), the preferred treatment is complete excision with a wide margin; at least 3 cm of healthy tissue surrounding all palpable borders should be removed in an attempt to excise both the nodule and its surrounding halo of neoplastic cells. Intraoperative cytology (examination of impression smears at
the excised tissue margins) can guide the surgeon, who should continue to remove tissue until the margins are free of mast cells. If histologic evaluation suggests that the tumor extends beyond the surgical margins, reexcision should be attempted. Alternatively, because mast cells are sensitive to radiation, intraoperative radiation therapy or followup external beam radiation therapy may be curative if the remaining tumor is small or can only be seen microscopically. Combined
radiation and hyperthermia may be more effective than radiation alone. |
| At present, there is no agreed upon mode of therapy for Stage II-IV mast cell tumors. For Stage II tumors (a solitary tumor with regional lymph node involvement), options include excision of the mass and the affected regional node (if feasible), prednisolone, and radiotherapy, used either singly or in combination. Intracavitary injections of triamcinolone or medroxyprogesterone acetate in a sesame seed oil or safflower oil emulsion placed evenly into the open tumor bed at the time
of surgery may also help, especially when combined with intraoperative radiation therapy and followup external beam therapy. Treatment of Stage III (multiple dermal tumors with or without lymph node involvement) or Stage IV (any tumor with distant metastasis or recurrence with metastasis) tumors is generally palliative. One recommended therapy is prednisolone (2 mg/kg, PO, for 5 days, followed by a maintenance dose of 0.5 mg/kg, sid) or intralesional injections of
triamcinolone (1 mg/cm diameter of tumor, every 2 wk). Treatment with H1- and H2-receptor antagonists for the peripheral and gastric effects of histamine, respectively, may be indicated for animals with systemic disease or clinical signs referable to histamine release. Chemotherapy with vinca alkaloids (vincristine, vinblastine), L-asparaginase, and cyclophosphamide has also been used with some effectiveness. Prednisone and vinblastine
used as adjuvant chemotherapy to incomplete surgical resection conferred an apparent improvement over historical survival data employing surgery alone, yielding a 57% 1- and 2-yr disease-free state and a 45% survival at 1 and 2 yr for dogs with Grade III tumors. In 19 dogs on a high dose of lomustine given every 21 days, 42% of mast cell tumors showed measurable responses, ranging from stable to partial with one complete response. Neutropenia appears 7 days after treatment with
neutrophil counts of 1,500 cells/µL. |
| In cats, cutaneous mast cell tumors are common. In addition to cutaneous tumors, systemic, leukemic, and GI forms have been recognized. Two distinct variants of the form occur—a mast cell type analogous to, but not identical with, cutaneous mast cell tumors in dogs, and a histiocytic type unique to cats. |
| The mast cell type is most common. It is found primarily in cats >4 yr old and may develop anywhere on the body but most commonly on the head and neck. The tumors are single, alopecic nodules, generally 2-3 cm in diameter, that occasionally extend into the subcutaneous fat. Lymphoid nodules are common; eosinophils are rare. Unlike mast cell tumors in dogs, those in cats are benign, and generally, atypia and clinical behavior are poorly correlated. Surgical excision is the
treatment of choice; <20% of tumors recur after surgery and of those that do, considerably fewer metastasize. Cryotherapy may be a good option to treat multiple recurrent small lesions while avoiding anesthesia. |
| The histiocytic type of cutaneous mast cell tumor in cats is recognized primarily in Siamese cats <4 yr old. Lesions may develop anywhere on the body and appear as multiple, small (generally 0.5-1 cm in diameter), firm, subcutaneous papulonodules. Usually, the older the cat, the fewer the lesions. This variant may be difficult to distinguish morphologically from a granulomatous inflammatory response. Because these tumors are reported to resolve spontaneously, no treatment is
necessary. |
| In horses, mast cell tumors are uncommon, benign tumors. There is debate as to whether they are actually a neoplastic process or an unusual inflammatory response. Lesions may develop anywhere on the body but are most common on the head and legs. Typically, there is a single, solitary mass in the dermis or subcutaneous fat that may expand to involve the underlying musculature. The tumor begins as a nodule composed of a generally monomorphic proliferation of mast cells. As the lesion
evolves, the mast cells are limited to aggregates in a fibrous stroma that surrounds large foci of liquefactive necrosis containing numerous eosinophils. In the late stages, the necrotic foci undergo dystrophic mineralization, and mast cells may be very difficult to identify. Once mineralization occurs, the lesion is gritty on sectioning. Alopecia and ulceration are variable features. Excision is the treatment of choice. These lesions do not metastasize. A variant of cutaneous mast
cell tumor is seen in newborn foals, in which the lesions may become generalized but regress over time, suggesting an equine equivalent of urticaria pigmentosa in humans. |
| In pigs and cattle, mast cell tumors are rare. In pigs, most appear as discrete, solitary, cutaneous nodules. Most are benign but disseminated, and leukemic variants do occur. In cattle, most are malignant and characterized by multiple cutaneous nodules often accompanied by systemic involvement; purely cutaneous forms have been recognized occasionally. |
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