HISTORY AND PHYSICAL EXAMINATION
An 11-year-old girl was referred for a painful, progressively enlarging mass in the distal left forearm of 2 months duration. The mass subsequently was associated with increasing pain. She denied constitutional symptoms, fever, anorexia, weight loss, and any traumatic antecedent to the affected area. The pain had progressed and started interfering with her activities of daily living.
Physical examination revealed a healthy appearing adolescent with a 3 × 3-cm tender firm mass attached to the distal ⅓ of the ulna. There were no overlying skin changes and the neurovascular function in the left upper extremity was found to be intact. She had no palpable axillary lymphadenopathy.
Radiographs (Fig 1), computed tomography (CT) scans (Fig 2), magnetic resonance (MR) images (Figs 3, 4), and bone scans (Fig 5) were obtained. Based on the history, physical examination, laboratory tests, and imaging studies, what is the differential diagnosis?
Radiographs (Fig 1) show a well-defined mineralized soft tissue mass that partially overlies the distal ulna in all views. Computed tomography (Fig 2) shows that the mass is juxtacortical and almost completely circumscribes the distal ulnar diaphysis. The mass is ossified with a peripheral rim of cortical bone and a central, less-mineralized portion representing medullary bone. There appears to be a cleavage plane between the mass and the underlying ulna except laterally where the two cannot be separated.
Bone scans show diffusely increased radiotracer uptake in the distal ulna overlying the lesion. There are no other areas of abnormal radiotracer uptake in the skeleton. On magnetic resonance imaging, the mineralized areas show isodense to hypointense signals relative to skeletal muscle on all pulse sequences. On the T1-weighted image (Fig 3) there are few small focal areas of signals isointense to ulnar marrow fat in the central portion of the lesion. The mass is surrounded by a thin layer of very high signal on the short tau inversion recovery (STIR) sequence (Fig 4).
Myositis ossificansParosteal osteosarcomaBizarre parosteal osteochondromatous proliferation (BPOP, also called Nora's lesion)
Periosteal chondromaJuxtacortical chondrosarcoma
Based on the history, physical findings, laboratory tests, radiographic studies, and histologic picture, what is the diagnosis and how should this lesion be treated?
The lesion had a poorly organized cartilaginous cap (Fig 6) with disorganized columns of cellular cartilage (Fig 7). Immature (blue bone) and mature endochondral bone were seen underlying the fibrocartilaginous cap. The bony trabeculae had prominent but benign, osteoblastic rimming (Fig 8). The stroma consisted of benign spindle cells and granulation-like tissue. There was no evidence of atypia or mitoses.
Bizarre parosteal osteochondromatous proliferation (Nora's lesion)
DISCUSSION AND TREATMENT
Bizarre parosteal osteochondromatous proliferation (BPOP) was described by Nora et al in 1983.5 The etiology is unknown, though trauma has been cited as an antecedent event in some cases. Florid reactive periostitis may be a precursor and may eventually progress to BPOP.8 This is a rare, benign surface lesion of bone usually seen in the short tubular bones of the hands and feet, mainly the proximal phalanges and metacarpals and meta-tarsals. These lesions are commonly seen in young adults, usually in the third to fourth decade (range, 8-73 years) and are slightly more common in women.2 Recently, a balanced translocation, t(1;17) (q32;q31), has been identified in patients with BPOP. The consequences of this rearrangement, however, are not clear.4
The features present in this mass that favor osteochondroma included the inner cancellous bone and outer cortical bone surrounded by a peripheral cartilage cap with the mass arising from the bone surface. Histologically, osteochondromas also have a cartilaginous cap, though the cartilage cap is uniform and better organized into longitudinal columns of cartilage arranged in a parallel fashion. However, the absence of cortical and medullary continuity between the mass and the adjacent bone excludes the diagnosis of an osteochondroma.
Mature myositis ossificans consists of a peripheral rim of cortical bone and inner cancellous bone. The pattern of zonation in our patient was different from that in myositis ossificans where there is a central core of proliferating mesenchymal cells and an outer rim of mature bone. Peripheral high T2-weighted signal and enhancement from edema secondary to inflammation is present in the earlier stages. It may contact bone when extending to the muscle insertion. Preceding history of trauma is common, though frequently unrecognized.
Periosteal chondromas typically show external cortical saucerization and a thick periosteal reaction (buttressing) with or without mineralized chondroid matrix. They present as soft tissue masses, usually without buttressing, at the bone interface and with different degrees of mineralized chondroid matrix. Histologically, periosteal chondromas consist of cellular cartilage with mild atypia, but do not have any immature bone.
Juxtacortical chondrosarcomas usually are large lesions, most often seen in the third and fourth decades. The distal femur is the most common location, and histologically they show more atypia than periosteal chondromas. The location in the ulna and the young age of our patient make this diagnosis unlikely.
Periosteal osteosarcoma is most commonly seen in the lower extremities with less than 10% of cases involving the upper extremity (humerus and ulna). It shows external cortical saucerization with aggressive periosteal new bone formation and may show osteoid matrix, none of which were seen in our patient. Histologically, they are characterized by neoplastic new bone formation and atypical high-grade chondroid foci.
Parosteal osteosarcoma is the most common surface osteosarcoma and most commonly seen in slightly older patients (third and fourth decades). The posterior cortex of the distal femur is a favored location but involvement of other bones including the ulna has been reported.2,6 It presents as a juxtacortical mineralized mass, which usually has contact with the cortical bone at one location, often showing a cleavage plane with the adjacent bone. However, osteoid matrix is typically present without the osteo-blasts, layered organization of cancellous bone, cortical bone, and peripheral cartilage seen here, which suggests a benign process. On histology, lack of fibrocytic atypia and osteoid production help exclude parosteal osteosarcoma. The distinction between BPOP and parosteal osteosarcoma can be hard to make on the basis of histology alone and site, location, and age frequently aid in differentiating between these two lesions.
Florid reactive periostitis (previously referred to as parosteal fasciitis) usually presents with intact adjacent bone and adjacent soft tissue swelling out of proportion to the extent of periosteal reaction with rapid progression over time of periosteal bone production. It is thought to progress into BPOP.
Also known as Nora's lesion, BPOP is a reactive lesion consisting of a cartilaginous cap (unlike florid reactive periostitis) covering a bony stalk with areas of ossification attached to the cortex by a broad base and typically lacking cortical erosion or continuity with the medullary canal. Our patient had a mineralized, peripherally ossified mass with central areas showing some degree of fatty marrow. The linear area of high STIR signal surrounding the mineralized mass was consistent with hyaline cartilage in the cap. Bizarre parosteal osteochondromatous proliferation has been considered a continuum with florid reactive periostitis. The hand is the most common location, but it can also occur in other long bones.
Our patient was unusual because of the location of BPOP in the ulna. The hand and foot are involved in up to 72% cases, with the hand being the most common location (50%). All the cases reported in the original description by Nora et al involved the hands and feet. Since then, BPOP has been described in various locations including the skull, maxilla, and long bones of the extremities.1,3,7 Meneses et al3 reported 65 cases of this unusual condition, five from the Mayo Clinic and 60 consultation cases. Seventeen of these 65 cases (26%) involved long bones, with the ulna being the most common long bone involved (six of 17 cases).
Metastases have never been documented. These lesions do have a tendency to recur locally with reported recurrence rates up to 55%.3 The time to first recurrence has ranged from 2 months to 2 years. Secondary recurrences, which may be seen years later, have been reported in 22% cases.5 In view of the high local recurrence rate with marginal excision, we resected the lesion with a wide margin and reconstructed the ulna with an intercalary (fibular) allograft, stabilizing it with a dynamic compression plate and screws.
Bizarre parosteal osteochondromatous proliferation is a rare, benign surface lesion of bone of unknown etiology. Long bone involvement is unusual. Histologically, it is characterized by fibroosseous proliferation (bland fibrous tissue without atypia and immature woven bone) with haphazardly arranged chondrocytes in a cartilage cap. Clinically, it can be mistaken for a malignant process, most commonly a parosteal osteosarcoma. Differentiation between these lesions may be difficult even after a biopsy. Characteristic clinical, radiographic, and histologic findings help establish the diagnosis. Local recurrence rates are high and wide resection, when feasible, is therefore the initial treatment of choice.
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