matrix, resembling mucus, containing mucopolysaccharides. Myxosarcoma develops commonly as cyst‐like masses filled with mucoid fluid. They are rare and mainly occur on the chest or limbs. Myxosarcomas have also been reported in the heart, eye, spinal canal, temporomandibular joint, brain, and bone. Biologically, they behave like any STS, with aggressive first surgery after diagnosis being the treatment of choice (Dennis 2008; Foale et al. 2003; Kirby et al. 2014; Levy et al. 1997; Manfredi et al. 2015; Parslow 2016). Pulmonary metastasis of a myxosarcoma in a dog has been reported (Headley et al. 2011).
Cutaneous Hemangiosarcoma
Hemangiosarcoma (HSA) is a malignant neoplasm arising from vascular endothelial cells (Liptak 2007). HSAs may be associated with ultraviolet exposure (Hargis et al. 1992). Prevalence appears to be higher in tropical and subtropical areas (Mukaratirwa et al. 2005). Skin HSAs account for less than 1% of all (sub)cutaneous tumors in dogs and 2–3% in cats (Goldschmidt and Shofer 1992; Miller et al. 1991; Schultheiss 2004). In dogs, 17–35% of all HSAs (including both visceral and nonvisceral) are cutaneous (Schultheiss 2004; Brown et al. 1985), and 50–77% of all HSAs in cats are (sub)cutaneous (Johannes et al. 2007; Scavelli et al. 1985). Dermal HSA is commonly observed in shorthaired dogs on the poorly pigmented skin of the ventral abdomen, medial thigh, inguinal region, and scrotum (Trappler et al. 2014). This is typically seen in “sun bathers” dogs that lie on their back in the sun. Cutaneous HSA has been reported in a Golden retriever appearing from the left elbow to the digits (Tsuji et al. 2013).
Subcutaneous HSAs are more commonly observed in dogs with variable hair coat and pigmentation. The most common primary sites for cutaneous HSAs in the cat are the pinna, lateral face, and the inguinal or abdominal subcutis.
Skin HSAs vary in size, are poorly circumscribed, dark and soft, and may resemble bruises or ecchymosis (Gross et al. 2005). Alopecia, ulceration, and hemorrhage are common, and animals may present with a hematoma.
Elevated serum big ET‐1 can be used as a diagnostic marker for canine HSAs. Serum big ET‐1 levels in dogs with HSA have been found to be significantly (P < 0.01) higher than in other dogs. High sensitivity (100%) and specificity (95%), for HSAs diagnosis were obtained using a cut‐off of 17 pg/mL (Fukumoto et al. 2015).
Canine cutaneous HSAs are staged according to tumor depth and invasion: superficial tumors, confined to the dermis (stage I), tumors extending into subcutaneous tissues (stage II), and tumors invading muscle and fascia (stage III) (Ward et al. 1994). Tumor‐free surgical margins are more likely to be achieved in cutaneous than subcutaneous lesions and are therefore associated with longer survival times. Completeness of excision was the most important prognostic factor in a large retrospective study, with disease‐free intervals (DFIs) of at least 1 year for completely excised skin HSAs. Similarly, dermal location was associated with more complete resections compared to the subcutaneous location of HSAs (Schultheiss 2004). Subcutaneous HSA (stage II) are more biologically aggressive than the cutaneous (stage I) form and are more likely to recur locally and result in euthanasia or death.
After attempted wide surgical excision of solitary HSAs in 10 cats, a favorable long‐term prognosis was achieved. In this study, complete or incomplete surgical excision of feline cutaneous HSA resulted in long survival times (mean survival time 622 days, range 90–1460 days) regardless of age, tumor size, or location. If the tumor was in the subcutaneous regions of the trunk or proximal pelvic limb, the mass was excised with a surface margin extending 1–3 cm beyond the tumor and a deep margin of at least one fascial plane. All the cats that had complete surgical excision were alive with no evidence of disease at their last evaluation. These five cats had a mean disease‐free interval of 479 days (median 420 days; range 120–1186 days). None of the cats with clean surgical margins received chemotherapy postoperatively. Cats that had surgery had a significantly longer median survival time (>1460 days) than cats that did not undergo surgery (60 days). In a study of 53 cats, subcutaneous tumors were associated with longer survival than visceral tumors, and cutaneous tumors were associated with longer survival than subcutaneous tumors. Completely excised tumors were associated with longer survival than incompletely excised tumors, and cats with incompletely excised tumors had longer survival times than those for which surgical resection was not attempted (Johannes et al. 2007; McAbee et al. 2005).
In general, surgical excision is the therapy of choice in cats and dogs with cutaneous HSA. In a study of 94 dogs with dermal HSA treated by surgical excision median overall survival time was 987 days. Locoregional recurrence occurred in 72/94 (77%) dogs (Szivek et al. 2012). Chemotherapy may be a viable adjunctive therapy, especially for incompletely resected tumors. Five dogs with subcutaneous HSA treated with surgery, doxorubicin, and cyclophosphamide had a median survival time of 211 days (Sorenmo et al. 1993). Seven dogs treated with surgery, vincristine, doxorubicin, and cyclophosphamide had a median survival of 425 days (Hammer et al. 1991). In a study of 17 dogs with subcutaneous HSA and 4 dogs with intramuscular HSA, receiving adequate local control and doxorubicin chemotherapy, reported a median DFI and survival of 1553 and 1189 days for subcutaneous HSA and 266 and 273 days for intramuscular HSA, respectively. Younger age (<9 years) was associated with longer DFI and survival times in dogs with subcutaneous tumors. Dogs that did not receive radiation therapy had a longer DFI, which may be due to a lack of standardization between groups. Distant metastasis occurred in 43% and local recurrence in 19% (Bulakowski et al. 2008). Therefore, if complete surgical excision of HSA cannot be achieved, it is advisable to start additional chemotherapy (Hammer et al. 1991).
Lipoma
Lipomas are, together with adenomas and mast cell tumors, the most common skin tumor types (Villamill et al. 2011). They are benign tumors originating from adipose tissue and are common in middle‐aged to older dogs. They usually appear as soft, well‐circumscribed, smooth, subcutaneous singular masses on the trunk or proximal extremities. Other locations (thoracic, abdominal, intermuscular, intraosseous, and pericardial) have also been documented (Brambilla et al. 2006; Ben‐Amotz et al. 2007; Nakladal et al. 2012; Mayhew and Brockman 2002; Miles and Clarke 2001).
Although histologically benign, lipoma can be life‐threatening in some anatomical locations; McLaughlin and Kuzma (1991) described intestinal strangulation caused by intra‐abdominal lipomas, and Kolm et al. (2002) described an intrapericardial lipoma in a dog. Lipomas rarely recur after marginal surgical resection. Multiple lesions in one patient should be considered as separate tumors, not metastatic disease.
Thoracic and pelvic limb intermuscular lipoma have a similar prevalence. Most intermuscular lipoma of the limbs are located in the axilla and caudal thigh. Surgical excision of infiltrative lipomas of the limbs are mostly done by hand dissection after the skin and subcutaneous incisions to avoid nerve and large vessel damage with sharp instruments. Surgical treatment of intermuscular lipoma is associated with an excellent prognosis in dogs (Case et al. 2012).
Liposuction has been reported as treatment in dogs (Böttcher et al. 2007; Hunt et al. 2011). Liposuction was completely effective in removal of 96% of simple, encapsulated lipomas of less than 15 cm in diameter. Giant lipomas containing fibrous trabeculae hinder liposuction resulting in poor fat retrieval. Giant lipomas have been associated with a high risk of bruising, hematoma, and seroma, especially when inguinal in location. Regrowth has been reported between 9 and 36 months in 28% of lipomas after liposuction (Hunt et al. 2011).
Infiltrative Lipomas
Infiltrative lipomas have been described in dogs and in cats. Histologically, they are composed of well‐differentiated fat cells. They are considered benign and do not metastasize. They tend to expand, however, and infiltrate between surrounding tissues including muscle, fascia, nerve, myocardium, joint capsule, periosteum, and bone. Clinically, they may be painful, are less well defined than lipomas, and palpate as a firm or solid mass because of their infiltrative behavior. Definitive diagnosis is made by the histologic evaluation of surgically
