Answer

Subungual keratoacanthoma.

Discussion

Keratoacanthoma is a rare, rapidly growing, and often spontaneously regressing, cutaneous neoplasm characterized by squamous proliferation with a keratin-filled crater centrally [1,2,3,4,5,6]. Subungual keratoacanthoma (SUKA) is a rare, painful subtype of keratoacanthoma involving the nail bed or nail folds, often resulting in hyperkeratosis, paronychia, and erosion of the underlying bone [1, 2, 4, 5, 7,8,9].

Keratoacanthoma and SUKA typically present as a rapidly enlarging, scaly, red nodule that can grow to 1–2 cm within several weeks [2,3,4, 7, 10]. SUKA, as the name implies, develop beneath or adjacent to the nail plate, and exhibit swelling, induration, tenderness, pruritus, and erythema [1,2,3,4, 8,9,10] (Fig. 1). It most commonly affects the first through third digits of the hand with characteristic local destruction and onycholysis. Drainage, when present, can vary from keratinaceous to purulent and paronychia can develop [8, 10]. These are more common in men, predominantly affect 40–60-year-old patients, and those with very fair skin and more exposed to sunlight [2,3,4, 6, 7, 10]. Importantly, SUKA differs from other keratoacanthomas in that it rarely regresses spontaneously; thus, early identification and treatment is critical [2, 4, 7, 10].

Though the exact etiology is unknown, proposed etiologies include trauma, sun exposure, burns, and occupational exposure to heat, coal tar, oils, and chemical carcinogens [2,3,4, 7, 10]. Immunologic etiologies including viruses, immunocompromised states, transplant grafts, and increased Langerhans’ cell activity have also been proposed [2,3,4, 7, 10]. Furthermore, a myriad of other clinical conditions (e.g., eczema, atopic dermatitis, psoriasis, scleroderma, xeroderma pigmentosum) have also been implicated [2,3,4, 6, 7, 10].

While the exact pathophysiology is not understood [7], keratoacanthoma initially develop with minimal keratinization and a variable inflammatory infiltrate [3]. Once fully developed, it is characterized by squamous proliferation, acanthosis, hyperkeratosis, inflammation, microabscess formation, eosinophilic (“glassy”) cytoplasm, and collar-like surrounding epithelium [2,3,4, 9, 10]. Since there are no follicles in the subungual region, SUKA presents with less inflammation, increased dyskeratotic eosinophils, no collar-like surrounding epithelium, and deeply invasive burrowing [7, 8].

The imaging findings in SUKA are remarkable. Radiographically, SUKA appears as a crescent-shaped mass in the distal phalanx, with corresponding cup-shaped osteolysis of the underlying bone (without sclerosis or periosteal changes); the defect is hypothesized to be the result of pressure erosion [2, 4, 5, 7, 8] (Fig. 2). On MRI, SUKA demonstrates T1W intermediate signal, mixed to hyperintense T2W signal, and peripheral rim enhancement, with minimal central enhancement due to the keratin content and necrosis [1, 2, 4, 7] (Fig. 3).

Treatment of SUKA involves excision, curettage, cytostatic agents, and Mohs surgery. Amputation, as was performed in our case, is utilized for significant bone destruction, aggressive growth, or multiple recurrences [2, 3, 5,6,7,8].

The imaging differential for SUKA includes other expansile and erosive lesions of the distal phalanx, including osteomyelitis, glomus tumor, epidermoid cyst, acro-osteolysis, and squamous cell carcinoma (SCC) [2, 4, 8]. Radiographic findings may help differentiate these entities, as SUKA tends to be clearly delineated and less invasive of the bony trabeculae [1, 2, 9]. However, differentiating SUKA from SCC is particularly challenging and cannot be made solely by imaging [4]. Histological (ulceration, pleomorphism, several mitoses, anaplasia) and clinical (slow growth, older population) features of SCC aid in distinguishing it from the more rapid and painful SUKA [2, 4, 5, 8,9,10].

In our case, an initial punch biopsy by dermatology revealed squamous proliferation without anaplasia or several mitoses. These pathologic features in combination with the imaging and clinical findings resulted in a diagnosis of SUKA. The patient was then referred to hand surgery and underwent an amputation at the level of the first interphalangeal joint. The surgical specimen was sent to pathology and the final diagnosis was also SUKA. The margins were clear.

SUKA is a rare neoplasm with unique imaging and histologic features, however, with significant overlap with other clinical presentations. Knowledge of this entity and its findings is critical.