Canine nail bed keratoacanthoma diagnosed by immunohistochemical analysis

Abstract

A 10-year-old, Shih Tzu dog was presented with an enlarged, curled 2nd nail in the left forelimb. Digital amputation was performed and the mass was diagnosed as a nail bed keratoacanthoma (infundibular keratinizing acanthoma) histopathologically. There was no recurrence postoperatively. This is the first case report of a canine nail bed keratoacanthoma diagnosed by histologic and immunohistochemical examination including Ki-67 and p53 expression.

Primary nail bed tumors are uncommon in dogs (1). There are no known breed predilections, and affected dogs typically present with signs of lameness associated with a mass (2). Squamous cell carcinoma (SCC) is the most common cause of primary nail bed tumor, followed by melanoma, osteosarcoma, soft-tissue sarcoma, and mast cell tumor (2). Subungual keratoacanthoma (KA, infundibular keratinizing acanthoma) is a rare and benign neoplasm in humans (3). It was first reported by Lamp et al (4) and differs from the common cutaneous KA in its origin from non-haired skin, less inflammation, typically deeper invasion, and more dyskeratotic eosinophilic cells histologically (3). Canine nail bed KA is also rare and appears to be similar to human subungual KA (1).

In this report, we describe the radiographic and histopathologic features, including immunohistochemical characteristics of canine nail bed KA. This is the first report of nail bed KA diagnosed by histologic examination including immunohistochemical analysis using Ki-67 and p53 in a dog.

Case description

A 10-year-old, intact male Shih Tzu dog, weighing 9 kg, was presented to the Veterinary Teaching Hospital of Konkuk University, Korea, for evaluation of left forelimb lameness secondary to an enlarged 2nd nail. There had been trauma to the nail 1 year before, and the mass developed 2 mo after the injury. On physical examination, the associated claw was overgrown, measured 1-cm thick and 4.1-cm long, was rolled inwards, and was covered by keratinized sheaths (Figure 1A). The dog expressed pain on palpation. The results of the remaining physical examination and blood tests were within normal limits.

Figure 1.

A — The overgrown claw, which was 1-cm thick and 4.1-cm long, appears rolled down and covered by keratinized sheaths. B — Radiographically, the dorsoventral view shows the elongated and thickened 2nd nail, and extensive osteolysis in the distal phalanx. There is no periosteal proliferation in the other phalanges. There is reduced opacity in the 2nd proximal phalanx of the affected limb, which presumably reflects disuse atrophy.

Radiography confirmed an elongated and thickened nail and extensive osteolysis in the distal phalanx of the 2nd digit. There was no periosteal proliferation associated with the other phalanges, but the proximal phalanx had mildly decreased bone opacity, presumably due to disuse atrophy (Figure 1B).

The dog was premedicated with midazolam (Vascam; Hana Pharm, Korea), 0.3 mg/kg body weight (BW), IV, and butorphanol (Butophan; Myungmoon Pharm, Korea), 0.2 mg/kg BW, IV, anesthetized with propofol (Provive Inj. 1%; Claris Lifesciences, India), 6 mg/kg BW, IV, and maintained on inhaled 2% isoflurane (Ifran Liq; Hana Pharm). The left ulna and radial nerves were blocked with bupivacaine HCl (Bupivacaine HCl inj. 0.5%; Hana Pharm, Korea), 1 mg/kg BW combined with lidocaine HCl (Lidocaine HCl inj. 2%; Dai-Han Pharm, Korea), 1 mg/kg BW. After making a sagittal incision, the entire 2nd digit was surgically amputated. Hemostasis was confirmed, and the subcutaneous tissue was sutured in a simple continuous pattern using monofilament polyglyconate suture (4-0 Maxon; Covidien, USA), followed by the skin in a simple interrupted pattern using monofilament polyamide suture (3-0 Dafilon; B. Braun Melsungen, Germany). Two weeks after the amputation, the dog was walking normally, and there has been no disease recurrence in the 34 mo since.

On histopathologic examination, the nail was extensively enlarged by a circumscribed, symmetrical, and unencapsulated neoplastic mass with irregular borders (Figure 2A). The mass had a central core of keratin surrounded by centripetal keratinization of squamous epithelial cells similar to that observed in the superficial epidermis. This core comprised amorphous keratin and parakeratotic cells without nuclear atypia (Figure 2B). Neoplastic epithelial cells expressed mild anisocytosis, distinct cell borders, and bright eosinophilic cytoplasm. The nuclei were enlarged and vesicular with prominent nucleoli. A few inflammatory cells were present, and low mitotic activity of the tumor cells was found at the peripheral region. There was no evidence of tumor infiltration into the middle and proximal phalanges. The tumor cells were positive for Periodic Acid-Schiff (PAS) stain (Figure 2C). On immunohistochemical staining, Ki-67 expression was weak and limited to the basal cell layer (Figure 3A), and p53 expression was rare (Figure 3B). Based on the radiographic and histologic examination, the mass was diagnosed as a nail bed keratoacanthoma.

Figure 2.

Histological appearance of the mass after amputation. A — The neoplastic mass has an irregular border (arrowheads) (H&E stain, ×100 magnification). B — The keratin central core shows centripetal keratinization of squamous epithelial cells, which resemble the superficial epidermis (H&E stain, ×40 magnification). C — The cells which have PAS-positive cytoplasm are numerous (arrowheads) (PAS stain, ×200 magnification).

Figure 3.

Immunohistochemical examination of the mass after amputation. A — Ki-67 expression is weak and limited to the basal cell layer (Ki-67 immunohistochemical stain, ×100 magnification). B — p53 expression is rare (p53 immunohistochemical stain, ×100 magnification).

Discussion

Nail bed KA affects dogs aged 3 to 14 y and has no clear breed or gender predilection (5). The mass typically emerges as a single, enlarged, and twisted claw (5); the neighboring epidermis may be ulcerated; the nail may break (1,5). Affected dogs experience lameness and pain associated with the mass (1).

Canine nail bed KA occurs uncommonly, and there are few published studies describing it. Six of 35 (17.1%) digit amputations were performed to remove benign tumors, and only 2 (5.7%) were diagnosed as KA (6). In another report, 15 of 428 (3.5%) amputated digits were KA (7). The reason underlying this low incidence rate and few published reports may be that diagnosis of digital masses is difficult, particularly when differentiating KA and SCC. There was disagreement among pathologists 19 times in 21 diagnoses involving KA (8). Hematoxylin and eosin (H&E) staining alone was used for diagnosis, and a well-differentiated SCC was the most common variant diagnosis for KA (8). More reports of canine nail bed KA diagnosed by more discriminatory methods are needed.

Because nail bed KA is benign, while nail bed SCC is malignant, differential diagnosis is important. Keratoacanthoma grows more rapidly and less commonly causes ulceration than does SCC (1,3,9,10). Notably, KA typically affects a single digit whereas SCC may invade multiple digits (1). However, the growth rate and degree of ulceration are subjective, and SCC can occur in a single digit. Radiographically, periosteal proliferation or local invasion to the middle and proximal phalanges is common in SCC but not in KA (1). However, it can be difficult to observe radiographic evidence depending on the stage of SCC, and this may be insufficient for differential diagnosis. On H&E staining, the extent of disagreement in the diagnosis of both lesions between pathologists is high (8). Most neoplastic epithelial cells of cutaneous KA have a richer cytoplasmic glycogen concentration than those of cutaneous SCC and normal epidermis in humans, and are stained positively by PAS (11). Unfortunately, the glycogen concentration within an individual tumor cannot distinguish cutaneous KA from SCC (11), and well-differentiated nail bed SCC appears markedly PAS-positive (1). On immunohistochemical analysis, Ki-67 expression is limited to the basal layer in human subungual KA but more dispersed in human subungual SCC (12,13). In addition, p53 expression is rare in KA but diffuse in SCC lesions (12). In the present case, the minimal Ki-67 and p53 expression on immunohistochemical staining were important in distinguishing nail bed KA from nail bed SCC. Immunohistochemistry with Ki-67 and p53 is essential for diagnosis of nail bed KA in dogs.

In conclusion, this case report shows the radiographic, histologic, and immunohistochemical features of nail bed KA in a dog. Above all, immunohistochemical analysis is important when diagnosing canine nail bed KA. Digital amputation is the primary treatment for nail bed KA and has a good prognosis; recurrence and metastasis are unlikely. This is the first report of the rare canine nail bed KA differentiated from SCC by Ki-67 and p53 expression. CVJ