December 2008: A 28 y.o. woman with a rapidly growing mass in the thumb

History: A 28 year-old woman presented with a rapidly growing nodule on the right thumb. Radiographs taken prior to surgery demonstrated an ill-defined soft tissue mass without apparent calcification or periosteal reaction (Fig. 1).

Sections of the excised mass revealed loosely arranged, bland spindle cells in a myxoid matrix (Fig. 2,Fig. 3,Fig. 4). Areas with scattered osteoblast-rimmed osteoid and bone were present (Fig. 5). Nuclear pleomorphism was not prominent and mitoses were not seen. Immunostains were positive for vimentin and negative for CD99 (Fig. 6).

Diagnosis: “Fibroosseous Pseudotumor of the Digits”

Michelle Fajardo DO, Donald R. Chase MD
Department of Pathology, Loma Linda University and Medical Center
California Tumor Tissue Registry

Discussion: Fibroosseous pseudotumor of the digits is a rare, benign soft tissue tumor. It presents as a rapidly growing, painful, erythematous, fusiform swelling in the subcutaneous tissues of the fingers and less commonly, the toes. It primarily affects young adults but can occur in infancy through late adulthood.

Radiographic studies demonstrate soft tissue swelling often with irregular calcifications. There may be periosteal reactions or cortical erosion and destruction of bone which may misleadingly suggest a malignant lesion, along with its rapid growth.

Histologically, the lesion is composed of a proliferation of bland fibroblasts admixed with haphazardly distributed, osteoblast-rimmed osteoid and bone. The fibroblasts may show some variable degrees of atypia. The spindle cells express positive immunohistochemical staining with vimentin and actin, which suggests a myofibroblastic differentiation.

The differential diagnosis of fibroosseous pseudotumor includes:

• Extraskeletal osteosarcoma
• Myostitis ossificans
• Bizarre parosteal osteochondromatous proliferation (Nora’s lesion)
• Osteochondroma
• Calcifying aponeurotic fibroma

The most important differential diagnosis is extraskeletal osteosarcoma due to the potential amputation of the digit. However, extraskeletal osteosarcoma differs from fibroosseous pseudotumor in that it usually occurs in the lower extremity of older patients and rarely in the digits. It is also distinguished by prominent pleomorphism, hyperchromasia, and atypical mitoses.

Myositis ossificans is a morphologically similar lesion that may occur anywhere in the body, but most commonly involves the elbow, thigh, buttock or shoulder. This lesion is found commonly in the musculature and is characterized by a zonal pattern of osteoid and bone with central cellular areas in contrast with the random distribution of osteoid and bone seen in fibroosseous pseudotumor.

Bizarre parosteal osteochondromatous proliferation also occurs in the bones of hands and feet of young adults but unlike fibroosseous pseudotumor, displays a zonal architecture and a prominent cartilaginous component, which is disorganized and irregular.

Osteochondroma is a common, benign, cartilage-forming tumor characterized by a hyaline cartilage cap with enchondral ossification which is not seen in fibroosseous pseudotumor. It is present in the metaphyseal region of long bones, ilium, and scapula, but not in the digits.

Calcifying aponeurotic fibroma is an infiltrative lesion found in the palms of the hands and soles of the feet and characterized by a collagenized, fibroblastic proliferation with foci of calcification. Unlike fibroosseous pseudotumor, this lesion often demonstrates palisading polygonal cells around the areas of calcification and can show areas of cartilaginous metaplasia.

Fibroosseous pseudotumor is a benign, self-limited process with a good prognosis following local excision. Local recurrence is unlikely with complete resection of the lesion. Because of its rapid growth, radiographic, and histologic appearance, this lesion can be misdiagnosed as a malignant neoplasm, such as extraskeletal osteosarcoma, despite its benign clinical behavior. It is important to recognize this entity in order to avoid needless amputation of the digit.

Suggested Reading:

Chan KW, Khoo US, Ho CM. Fibro-osseous tumor of the digits: report of a case with immunohistochemical and ultrastructural studies. Pathology 25(2):193-196, 1993.

deSilva MV, Reid R. Myositis ossificans and fibroosseous pseudotumor of digits: a clinicopathological review of 64 cases with emphasis on diagnostic pitfalls. Int J Surg Pathol 11(3):187-195, 2003.

Dupree WB, Enzinger FM. Fibro-osseous tumor of the digits. Cancer 58: 2103-2109, 1986.

Fletcher CDM, Unni KK, Mertens F. World Health Organization Classification of Tumors. Pathology and Genetics of Tumours of Soft Tissue and Bone. IARC Press: Lyon 2002.

Nishio J, Iwasaki H, Soejima O, Naito M, Kikuchi, M. Rapidly growing fibro-osseous pseudotumor of the digits mimicking extraskeletal osteosarcoma. J Orthop Sci 7(3):410-413, 2002.

Sleater J, Mullins D, Chun K, Hendricks J. Fibro-osseous pseudotumor of the digit: a comparison to myositis ossificans by light microscopy and immunohistochemical methods. J Cutan Pathol 23(4): 373-374, 1996.

Weiss SW, Enzinger FM. Soft Tissue Tumors, 4th Ed. Mosby Inc.: St. Louis, MO, 2001.

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