Adamantinoma

Christina Gutowski

Adamantinoma is a rare, low-grade malignancy of bone with mesenchymal and epithelioid features. Adamantinoma almost always occurs in the diaphysis of the tibia (more than 90% of cases). Adamantinoma is slow-growing and is characterized clinically by the gradual development of pain and swelling at the tumor site in young adults. Radiographically, adamantinoma resembles osteofibrous dysplasia (also known as cortical fibrous dysplasia), a benign bone lesion of the tibia in pediatric patients.  

Epidemiology

Adamantinoma is an extremely rare bone tumor, accounting for 0.1-0.5% of all malignant bone tumors. Adamantinoma is slightly more common in males and tends to develop in patients 20-40 years old.

Clinical Features

Adamantinoma most commonly develops in the anterior cortex of the tibial diaphysis, leading to swelling and deformity of the tibial crest with progressive pain. Adamantinoma occasionally presents as pathologic fracture. Adamantinoma can be confused with fibrous dysplasia of the tibia, which must be distinguished by a high index of clinical suspicion and appropriate biopsy. In pediatric patients, osteofibrous dysplasia presents similarly to adamantinoma.

Radiologic Features

On radiographs, adamantinoma displays a classic multicystic, “soap-bubbly” appearance along the anterior tibial cortex, characterized by mixed lytic and sclerotic regions. The average length of the lesion is approximately 10 cm. Radiographs may reveal bowing of the tibia due to chronic bone remodeling, and an extraosseous soft-tissue mass (Figure 1).

Figure 1. Lateral X-ray of adamantinoma of the tibia, demonstrating a mixed lytic/sclerotic lesion with a soap-bubbly appearance. The anterior cortex is destroyed, and a bowing deformity is present.

On computed tomography (CT) scans, adamantinomas are characterized by cortical thinning and reactive sclerosis. There may frank cortical disruption, soft-tissue masses, and involvement of the medullary canal.

Magnetic resonance imaging (MRI) will demonstrate cortical thinning, expansion, and marrow replacement on T1 sequences. Fluid-sensitive sequences will demonstrate areas of cyst formation, which can be blood-filled and lead to fluid-fluid levels, as well as perilesional or intralesional edema (Figure 2). Adamantinomas enhance with contrast.

Figure 2. Sagittal short tau inversion recovery MRI of adamantinoma of the tibia, demonstrating an anterior soft-tissue mass, cortical destruction, and fluid-fluid levels within the cystic component.

Pathology

Grossly, adamantinoma is a rubbery, well-defined tumor that may contain areas of cyst formation and hemorrhage. The tumor is predominantly intracortical, although expansion both out of bone and into the intramedullary space are common.

Microscopically, the defining feature of adamantinoma is a biphasic pattern of bland spindle cells and epithelioid cells. The spindle cells are found in a fibrous stroma which stains positively for vimentin; the epithelioid cells are found in nests which stain positively for cytokeratin. The epithelial component is thought to represent the primary neoplastic cell population, which in turn stimulates reactive fibrous growth and cortical thinning (Figure 3).

Figure 3. H&E stain of adamantinoma, demonstrating bland spindle cell stroma surrounding nests of epithelioid cells.

In studies of adamantinoma pulmonary metastasis, only the cytokeratin-positive, epithelial-cell population was present in the nodules.

Differential Diagnosis

The radiographic appearance of adamantinoma of the tibia can be confused with that of osteofibrous dysplasia, fibrous dysplasia, and osteomyelitis, an intracortical abscess or osteosarcoma. Histologically, the epithelial cells of adamantinoma can be misdiagnosed as metastatic carcinoma, although a recent finding that the epithelial cells are uniformly positive for keratin 14 and keratin 19 can assist in the correct diagnosis. The spindle cells of adamantinoma can be misdiagnosed as primary bone sarcoma especially if epithelial cells are not seen.

Disease Course: Treatment and Prognosis

Adamantinoma is typically treated with surgery. Wide en bloc excision of the lesion is necessary to achieve durable local control. Reconstruction is based on the size and location of the lesion and may involve an endoprosthesis, bulk allograft, autograft, alloprosthetic composite, or distraction osteosynthesis (Figure 4). Chemotherapy is not indicated due to the low-grade nature of this tumor, and radiation has not proven useful for treating either primary or recurrent disease.

Local recurrence after wide excision occurs in up to 15% of cases and is thought to be due to satellite lesions that exist beyond the primary tumor mass. Metastasis occurs in up to 20% of cases, oftentimes many years after a margin-negative excision.

Figure 4. (Left) AP and (right) lateral X-ray demonstrating intercalary endoprosthetic reconstruction of the tibia after wide excision of adamantinoma.

Long-term surveillance is necessary for both local and distant recurrences. The mean duration to local recurrence has been reported at 5 to 15 years after treatment; distant recurrences have been reported at even 20 years post-operatively. Local recurrences are treated surgically, with overall limb salvage rates of approximately 70%.

Commonly Tested on Exam

1. Classic appearance and location on X-ray
2. Biphasic histologic characteristics, with cytokeratin-positive, epithelial cell nests
3. Surgical treatment with wide excision, no chemotherapy or radiation