Nora's lesion: bizarre parosteal osteochondromatous proliferation of the tibia

Abstract

Bizarre parosteal osteochondromatous proliferation is a rare benign condition of locally aggressive and often recurrent osteochondromatous exostosis arising from the bony cortex. We present a case of a patient who presented with this lesion in her tibia, focusing on imaging findings. Because of the lack of information on the disease's natural history, etiology, and clinical course, a multidisciplinary approach is needed for diagnosis and treatment.

PATIENT DESCRIPTION

A 37-year-old woman presented to an outside facility for a mass of the right tibia that she discovered about a year earlier while shaving her legs. In the preceding month, the patient noted an enlargement of the mass with associated pain. She did not recall a specific inciting event such as trauma and denied fevers, weight loss, or a family history of bone tumors. The pain worsened at night and lessened with wrapping of the leg. The mass, located in the posteromedial right tibia, was tender, firm, and nonmobile. There was no associated swelling, redness, skin ulceration, or ecchymosis. Lower-extremity strength and range of motion were normal. Lower-extremity radiographs showed a nonspecific 3.8-cm area of amorphous calcium or bone without definite medullary communication (Figure 1). Lower-extremity magnetic resonance imaging (MRI) (Figures 2 and 3) demonstrated an expansile, “aggressive”-appearing lesion from the posteromedial right tibia with probable osteoid matrix. Both intramedullary and soft tissue edema was present without nerve or vascular encasement. Incisional biopsy produced a 5 × 3 × 1.6 cm mass described as extremely hard, gray-white, with a nearly homogeneous bony cut surface. The final microscopic diagnosis was benign bizarre parosteal osteochondromatous proliferation (BPOP).

Figure 1.

Radiograph of the right tibia and fibula: (a) anteroposterior, (b) lateral.

Figure 2.

Two series of coronal MR images (each from left to right): T1, T2 with fat saturation, and T1 with fat saturation after intravenous gadolinium contrast.

Figure 3.

MR image through palpable mass: (a) axial T1 weighted; (b) axial T2 weighted, with fat saturation; (c) axial T1 weighted, with fat saturation, and (d) axial T1 weighted, with fat saturation and intravenous gadolinium contrast.

DISCUSSION

BPOP, or Nora's lesion, so named for the lead physician who described the process in 1983, is a rare benign condition of locally aggressive and often recurrent osteochondromatous exostosis arising from the bony cortex. Nora's lesion can be a challenge to clinicians, radiologists, and surgeons alike. This rather rare entity often presents as a painful growth from the small bones of the hands and feet (1), among other more rare sites such as the long bones (2), as in the case of our patient, or maxilla, skull, or sesamoid bones (3, 4). The lesion may demonstrate a local aggressive nature and must be differentiated from more ominous lesions such as parosteal osteosarcoma or chondrosarcoma. Because the mass may demonstrate local recurrence in >50% of cases (5), the balance between complete wide excision with the inherent increased risk of morbidity and more local excision with the risk of recurrence must be weighed in this benign lesion (6).

BPOP has been reported throughout a wide range of age groups (average 34), with no sex predilection (7). The mass can either present symptomatically or be discovered incidentally, and up to half of patients report prior trauma at the site of occurrence, although definitive inciting factors have yet to be determined.

The major challenge for radiologists is differentiating this lesion from radiologically similar conditions, both benign and malignant. The classic imaging appearance of the lesion is an exophytic bony exostosis that is contiguous with, but not disruptive of, the bony cortex on radiographs. MRI can demonstrate more specific attributes of the lesion, classically showing a cortically based lesion without intramedullary extension. For example, Rybak et al (8) demonstrated corticomedullary continuation, a radiologic feature that has long been a mainstay in its differentiation from osteochondroma. Imaging features, however, may be nonspecific, necessitating histology for definitive diagnosis. Meneses et al described histologic findings of hypercellular cartilage with maturation to trabecular bone along with frequent spindle-shaped chondrocytes interspersed throughout the intertrabecular spaces. A distinct blue tinctorial appearance to the bone may be helpful in distinguishing the lesion from parosteal osteosarcoma (5).

The etiology, natural history, and clinical course of BPOP are not completely understood. Because of this, the lesion is best managed in a multidisciplinary fashion, involving clinicians, radiologists, and pathologists. This case was indeed presented in the multidisciplinary bone and soft tissue tumor conference at Baylor University Medical Center at Dallas.

Differential diagnostic considerations for this lesion include myositis ossificans, which generally occurs adjacent to larger muscles after trauma and has a centripetal pattern of ossification. Turret exostosis and florid periostitis are related reactive processes of the bone that can have similar characteristics to BPOP radiographically, and these three entities have been proposed to reflect a spectrum of disease as opposed to separate entities by both histologic and radiologic criteria (9, 10). Parosteal osteosarcoma, the favored radiologic diagnosis in our case, only rarely involves the small bones of the hands or feet, where BPOP tends to occur. While BPOP should be considered in cortically based masses such as this, the oftentimes aggressive, nonspecific imaging appearance can preclude definitive radiologic diagnosis. A multidisciplinary approach to diagnosis and subsequent therapy will likely result in the best outcome for the patient.