Carolina Veterinary Surgical Service
Mast cell tumors are the very common in the dog, and are often classified according to histologic characteristics into one of three grades. Grade III mast cell tumors (Patnaik classification) are poorly differentiated and have the highest potential for metastatic disease. Metastatic potential of mast cell tumors also appears to be associated with location, as histologically low-grade tumors of the perineum and head frequently metastasize. Cytoplasmic granules within the tumor cells contain vasoactive substances, including histamine. Degranulation of the tumor cells, as may occur with excessive tumor palpation can result in hemodynamic shock, and is thought to contribute to gastrointestinal ulceration. Vasoactive amines released by mast cells may also impair macrophage function, and as a result, increase the risk of wound dehiscence following incomplete mast cell tumor excision.
Staging of clinical disease takes into account histopathologic characteristics, number of tumors present, regional lymph node involvement, and presence/absence of metastasis, and is useful in determining a therapeutic plan and for prognostication. Stage I tumors are solitary dermal tumors without regional node involvment. Stage II tumors are solitary dermal tumors with regional node involvement. Stage III disease includes large infiltrating tumors such as the one presented in this case report. Stage IV tumors include any mast cell tumor where metastasis is present.
Treatment of mast cell tumors in the Stage III category usually consists of wide surgical excision, corticosteroid therapy (Pred 30mg/m2/d x 14d, then 20mg/m2/day x 14d, then 20mg/m2 eod x 3-5 months), anti-ulcer drug therapy (Cimetidine, Pepsid), +/- Vincristine/Vinblastine.
In performing surgery for mast cell disease, it must be kept in mind that microscopically, tumor cells extend well beyond the palpable cutaneous tumor margins, and as such a very aggressive surgical resection is necessary. In general, a minimum 3-cm cutaneous margin is recommended. The deep margin should include a minimum of one fascial or muscle plane. Surgical margins must be examined histologically.
Attempting a primary closure of a defect in the lateral abdominal region as large as that seen in this case would likely create excessive wound tension leading to dehiscence. Reconstruction of the defect with a pedicle flap is clearly the better alternative. Advancement and rotational flaps may be used and rely upon the subdermal vascular plexus for survival. Axial pattern flaps include a direct cutaneous artery/vein which increases the size of flap that can be safely translocated. The superficial epigastric flap is very versatile in its application, and is often used for cutaneous reconstruction of the hindlimb. As the name implies, direct cutaneous vascular supply for this flap is derived from the superficial epigastric vascular pedicle. Surgical technique is described in detail in the published literature (Pavletic, 1990).
References: MM Pavletic, Vet Clin NA: SAP: 20(1) 1990; Macy and MacEwen (1989) "Mast Cell Tumors" in Clinical Veterinary Oncology (Withrow and MacEwen), Lippincott.
Special thanks to Dr. Donald Hoover and Michelle Argersinger of Westside Animal Hospital (Durham, NC) for referring Maddie to CVSS.
If you have any comments or questions regarding this case, please contact Dr. Clary.
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