Age unknown castrated male mongrel dog, (Canis familiaris).A solitary subcutaneous mass was surgically excised from the perianal region.
The mass was approximately 6 mm in diameter with a red surface.Â The cut surface (after fixation) was smooth, firm and white.Â
The well-circumscribed dermal/subcutaneous mass is adjacent to several islands of normal perianal glands.Â The mass is composed of round cells arranged in cords, nests and sheets on a fibrous stroma.Â Neoplastic cells often separate and surround pre-existing perianal glands.Â Neoplastic cells are round, with a centrally placed round to oval nucleus, indistinct nucleoli, and a small amount of eosinophilic to clear cytoplasm.Â Histochemical staining with periodic acid-Schiff (PAS) and silver impregnation stain demonstrates that neoplastic cells are often surrounded by a prominent basement membrane that resembles chicken-wire.Â
Immunohistochemically, neoplastic cells are diffusely, strongly positive for vimentin, multifocally, weakly positive for desmin, and negative for alpha-smooth muscle actin, calponin, cytokeratins (AE1/AE3, 7, 14, CAM5.2 and CK-MNF), chromogranin A, synaptophysin, PGP9.5, NSE, CD31, Factor VIII, melan-A, PNL2, S-100, MHC Class II, Iba-1and CD18.Â The cytoplasmic border of each neoplastic cell is strongly positive for type IV collagen.
Perianal region; glomus tumor.
Glomus tumors are rare, benign neoplasms in humans and animals, which originate from the glomus cells that compose the glomus body, an arterioveneous anastomosis.Â Glomus tumors typically occur in areas that reflect the normal anatomic location of the glomus body; most tumors occur in the subungual region of the fingers.(10) In humans, these tumors are occasionally reported in various other locations, including the precoccygeal area, head, neck, bone, nerve, stomach, colon, nasal cavity and trachea.(1,3,5,10) Human glomus tumors are classified, based on the proportion of glomus cells, vascular structures and smooth-muscle components, into three subtypes: the classical glomus tumor (solid type), glomangioma (angiomatous type), and glomangiomyoma (myxoid type).(5,10) The classical (solid) is type most common and accounts for approximately 75 percent human glomus tumors.(10)
Glomus tumors are rare in animals, though there are a few case reports in dogs and cats.(2,3,8,9) Similarly to humans, most tumors in dogs and cats arise on the lower extremities and the digit.(2,3,8,9) Although glomangioma has been reported n a cow and a dog,(4,7) most glomus tumors in animals are most consistent with the classical glomus tumor (solid type) in humans.(2,3,8,9) Histopathologically, glomus tumors are composed of small round cells with round nucleus and eosinophilic cytoplasm, arranged in sheets, nests, cords, ribbons or duct like structures.(2,3,8-10) Nests and individual tumor cells are surrounded by a basement membrane.(1,9,10) Variably sized vascular structures are present within the neoplasm, and tumor cells often palisade along vessel walls.(1-3,6-8,10) Immunohistochemically, nearly all glomus tumors express vimentin and alpha-smooth muscle actin.Â Desmin is variably expressed.(1-3,8-10) The basement membrances surrounding neoplastic cells typically express type IV collagen and laminin.(1,10)
All glomus tumors reported in dogs and cats demonstrate palisading of tumor cells along vessel walls, as well as positive immunoreactivity for vimentin and alpha-smooth muscle actin.(2,3,8,9) Although the present case does not display all of these features, the morphological and immunohistochemical characteristics are most consistent with a glomus tumor.Â In particular, the basement membrane around each tumor cell is strongly suggestive of a glomus tumor.Â Glomus tumors in humans and animals must be distinguished from epithelial tumors (such as trichoblastoma), canine hemangiopericytoma, synovial sarcoma and epithelial leiomyoma.(2,3,8-10) In this case, all of these tumors were ruled out based on the histopathological and immunohistochemical features of the neoplastic cells.
Haired skin, perianal region: Glomus tumor.
The glomus is a convoluted segment of arteriovenous shunt, composed of an afferent arteriole and an efferent venule with multiple communications, enveloped by collagenous tissue.Â Blood flow in these shunts is controlled by the glomus body, which (in humans) is typically found in the dermis of the fingertips and is involved in regulating body temperature.(7,11) Glomus tumors arise from modified smooth muscle cells of the glomus body.(1) In veterinary medicine, glomus tumors are most frequently described in dogs, but have also been reported in cats, horses, non-human primates and a cow.(1,6,7) This tumor typically arises on the distal extremities; however three of the four reported equine glomus tumors occurred on the head or neck.Â Nevertheless, more cases must be examined in order to determine whether there is a true predilection for this site in horses.(1) Interestingly, the single bovine case report describes multiple glomus tumors of the urinary bladder, associated with bovine papillomavirus type 2 (BPV-2) infection.(7) Malignant glomus tumors in humans are defined by being larger than 2 cm with a deep location (below the muscular fascia), having marked nuclear atypia, having more than five mitoses per 50 high-power fields, or having atypical mitotic figures.(1) Due to the small number of identified cases in animals, there are no firmly established criteria of malignancy, although there are rare reports of aggressive, biological behavior.
This case is challenging, in that it demonstrates some, but not all of the classic histomorphologic and immunohistochemical features of a glomus tumor.Â After extensive debate within conference and consultation with pathologists from the Join Pathology Center soft tissue subspecialty, we are unable to definitively diagnose a glomus tumor; however we concur with the contributors conclusion that the microscopic and staining characteristics are most consistent with this entity.
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