Cartilage-Forming Tumors

Clinical Features

Osteochondroma is a cartilaginous neoplasm composed of a bony projection covered by a cartilaginous cap arising on the surface of bone, containing a marrow cavity and cortices that are continuous with that of the underlying bone. It most commonly arises in the distal femoral, proximal tibial, or proximal humeral metaphysis of adolescents and young adults (peak incidence in the second decade of life). It is more frequently seen in men than in women (2:1). Osteochondromas may be an incidental finding on imaging or present as a symptomatic, slow-growing mass of long duration. It may cause pain due to compression of nearby structures, associated bursitis or, less commonly, from fracture of the stalk. A recent and rapid growth of a preexisting osteochondroma, especially in a proximal skeletal location, raises the suspicion for malignant transformation.

Most patients have a solitary osteochondroma. Less commonly, it occurs as multiple lesions in patients with the autosomal dominant disorder, multiple osteochondromas (also known as hereditary multiple exostoses or multiple hereditary osteochondromatosis). The rate of malignant transformation for solitary osteochondromas is approximately 0.4%–2%. This is in contrast with patients with multiple osteochondromas who have higher rates of malignant transformation (0.2%–5%). Rates as high as 25% have been reported in multiple osteochondromas; however, these data are from large referral centers and likely overestimate the true incidence secondary to selection bias.

Radiologic Features

An osteochondroma presents as an exophytic, sessile, or pedunculated projection of bone capped by cartilage ( Fig. 17.5 ). It demonstrates both medullary and cortical components contiguous with the medulla and cortex of the underlying bone. These features are seen in each exostosis in patients with multiple osteochondromas ( Figs. 17.6 and 17.7 ). Features that have been associated with malignant transformation include irregularity of the margin, inhomogeneous mineralization ( Fig. 17.8 ), an associated soft tissue mass, and a thick cartilaginous cap ( Fig. 17.9 ).

Pathologic Features

Gross Findings

Grossly osteochondroma shows a thin (1 to 2 cm), pale blue, smooth cartilage cap overlying cortical and medullary bone ( Fig. 17.10 ). The thickness of the cartilaginous cap decreases with age. It may be a single dome-shaped cap with a mushroom-like configuration or be composed of multiple excrescences, imparting a bosselated appearance ( Fig. 17.11 ). A pedunculated osteochondroma demonstrates a prominent stalk, whereas the sessile form shows a broad attachment to the underlying bone. Lesions that have ceased active growth will show increasing amounts of endochondral ossification ( Fig. 17.12 ) and may be composed principally of bone.

Microscopic Findings

In general, osteochondroma mimics an epiphyseal growth plate beginning with a thin (usually <1 cm) cartilaginous cap superficially, a zone of endochondral ossification followed by lamellar trabecular bone, and fatty or hematopoietic marrow ( Fig. 17.13 ). The chondrocytes may be somewhat haphazard superficially but are arranged in orderly columnar growth at the ossification zone ( Fig. 17.14 ). Scattered zones of residual cartilage may be seen among the trabeculae. The chondrocytes lack nuclear atypia ( Fig. 17.15 ). Occasionally, degenerative change and calcification are seen in the cartilage cap. In the growing skeleton, osteochondromas have conspicuous endochondral ossification ( Fig. 17.16 ) with prominent regions of primary spongiosa-like tissue ( Fig. 17.17 ).

Ancillary Studies

Differential Diagnosis

Rarely, secondary chondrosarcoma can arise from an osteochondroma. The distinction between these entities requires clinical, radiographic, and pathologic correlation. Clinical or radiographic evidence of recent growth, pain, the involvement of the proximal skeleton, irregular mineralization, or soft tissue destructive growth are concerning features. The thickness of the cartilage cap (usually <1 cm) has long been reported as an indicator of potential malignancy with malignancy suspected in cases with a cartilage cap of >2 cm in thickness. The best feature to confirm a diagnosis of malignancy is histologic evidence of permeative growth of bone or adjacent soft tissue. Other helpful microscopic features that suggest malignant change include prominent myxoid cystic change in the matrix, wide fibrous bands, increased cellularity, and loss of the normal columnar arrangement of chondrocytes at the ossification zone ( Fig. 17.18 ) along with mitotic activity and nuclear pleomorphism, if present. The diagnosis is difficult to make on biopsy specimens.

Parosteal osteosarcoma may also demonstrate a cap of cartilage with underlying bone. However, radiographically and grossly, the bone is not contiguous with the medullary cavity of the native bone. Most importantly, the intertrabecular space within parosteal osteosarcoma is occupied by a spindle cell proliferation with nuclear atypia rather than fat and hematopoietic marrow, as is typically seen in osteochondroma.

Bizarre parosteal osteochondromatous proliferation (Nora lesion) differs clinically in the age of presentation (adults) and location (hands and feet). Although the lesion is composed of a mixture of bone and cartilage, it is not contiguous with the underlying medullary cavity, nor does it show the organization of a growth plate.

Periosteal (surface) chondroma is composed exclusively of lobules of cartilage and lacks endochondral ossification and growth plate-like architecture.

Prognosis and Treatment

Surgical excision should be curative treatment for osteochondromas, though recurrence is possible if a portion of the cartilage cap or perichondrium is left behind.

Clinical Features

Bizarre parosteal osteochondromatous proliferation (BPOP) or Nora lesion is an uncommon osteochondromatous proliferation involving the surface of the bone, in particular the small bones of the hands and feet (75% of cases). Approximately 25% of cases involve the long bones. BPOP tends to affect adult patients in the third to fourth decades of life. There is no sex predilection, and the typical presentation is one of a localized swelling, with or without pain. Some examples demonstrate rapid growth.

Subungual exostosis is a distinct lesion with similar morphologic features but with different genetic abnormalities.

Radiographic Features

Radiographic studies show a well-marginated, mineralized mass attached to the surface of the affected bone with an intact underlying cortex ( Fig. 17.19 ).

Pathologic Features

Microscopic Findings

BPOP demonstrates a disorganized proliferation of fibrous tissue, cartilage, and bone. The surface is composed of fibrous tissue overlying a cartilaginous component that undergoes endochondral ossification. The cartilage may be hypercellular with enlarged and binucleated chondrocytes (atypical or “bizarre”) and undergoes endochondral ossification manifested by distinctive basophilic mineralization (so-called “blue bone”), which can remain after decalcification ( Fig. 17.20 ). Marked nuclear hyperchromasia and atypical mitotic figures are absent. Similar features are seen in subungual exostosis ( Fig. 17.21 ). The fibrous tissue ( Fig. 17.22 ) ranges in cellularity and may be hypercellular.

Ancillary Studies

Differential Diagnosis

Osteochondroma demonstrates cortical and medullary continuity with the involved bone, whereas the cortex is intact in BPOP and related lesions; radiographic features are helpful to distinguish these entities. Histologically, in osteochondroma, the cartilage demonstrates growth plate-like architecture, and the lesion lacks a fibrous tissue component.

A subungual exostosis is a morphologically similar process that generally lacks “blue bone.” It exclusively affects distal phalangeal bones, typically the great toe, and genetically harbors a distinct translocation, t(X;6).

Parosteal osteosarcoma is very rare in the small bones of the hands and feet. The spindle cells in parosteal osteosarcoma show cytologic atypia and are oriented around trabeculae of tumor bone. The amplification of MDM2 seen in parosteal osteosarcoma is absent in BPOP.

Prognosis and Treatment

Asymptomatic lesions can be observed. Most cases are treated by simple surgical excision. Approximately 50% of cases recur, likely due to incomplete removal. Multiple, non-destructive recurrences are possible. Metastasis does not occur.

Clinical Features

Periosteal chondroma (surface chondroma or juxtacortical chondroma) is a relatively uncommon benign cartilage tumor that arises on the surface of bone beneath the periosteum, frequently eroding the underlying cortex. It most commonly involves the small bones of the hands and feet or the long bones of the appendicular skeleton, in particular the proximal humerus. Periosteal chondroma affects all age groups with a peak in the second and third decades of life, demonstrates a male predilection (1.5:1), and is usually asymptomatic, solitary and found incidentally on radiographs obtained for other reasons. Patients can also present with a painful, palpable mass.

Radiographic Features

Periosteal chondromas are small (1 to 3 cm), radiolucent masses attached to the cortex with cortical erosion (saucerization) and underlying cortical sclerosis ( Fig. 17.23 ). Variable degrees of internal mineralization may be noted ( Fig. 17.24 ). Periosteal reaction with peripheral buttressing may be seen. MRI findings show a surface tumor that is bright on T2-weighted sequences ( Fig. 17.25 ), similar to other benign cartilage tumors.

Pathologic Features

Gross Findings

The mass is firm, translucent, and pale blue, indicative of hyaline cartilage. The interface between the tumor and the underlying cortex is sharp with a zone of sclerotic bone ( Figs. 17.26 and 17.27 ). The medullary cavity may be impinged upon my reactive endosteal bone but is not invaded.

Microscopic Findings

The tumor consists of lobules of hyaline cartilage of low-to-moderate cellularity ( Fig. 17.28 ). Focal myxoid matrix can be present. The tumor has a sharp interface with the underlying bone ( Fig. 17.29 ) and does not permeate Haversian canals or the medulla. Chondrocytes are generally bland but mild to moderate atypia, binucleated chondrocytes, and chondrocyte drop out ( Fig. 17.30 ) may be present. Rare examples can be hypercellular in comparison to enchondroma ( Fig. 17.31 ).

Ancillary Studies

Differential Diagnosis

Periosteal (juxtacortical) chondrosarcoma demonstrates some clinical and morphologic overlap. Like periosteal chondroma, periosteal chondrosarcomas arise from the surface of the bone and may erode the cortex but usually does not involve the medulla. However, these tumors are typically large (>5 cm) with poorly defined margins and demonstrate greater cellularity and cytologic atypia. Infiltration of underlying bone is diagnostic of periosteal chondrosarcoma.

Periosteal osteosarcoma is a surface lesion that contains cartilage. However, it also contains neoplastic bone or osteoid production, typically in the center of the cartilage lobules, and surrounding spindle cells with significant cytologic atypia.

Parosteal osteosarcoma may contain a cartilaginous cap that is disorganized and may undergo endochondral ossification. Like periosteal osteosarcoma, it additionally contains malignant spindle cells and neoplastic bone.

Prognosis and Treatment

Surgical removal is the treatment of choice by curettage or simple excision. Local recurrence is uncommon. Malignant transformation is exceedingly rare.

Clinical Features

Chondroblastoma is a rare (<1% of primary bone tumors) benign chondroid matrix-producing tumor that affects the epiphyses (or apophyses) of skeletally immature individuals, adolescents, and young adults with a slight male predominance (1.5:1). It may occasionally present in older individuals. The most common complaint is several months to years history of pain involving a joint that may be accompanied by gait abnormality, swelling or decreased mobility. Although most chondroblastomas occur in the epiphyses of long bones, the small bones of the hands and feet are involved in approximately 10% of cases. A range of other sites, including temporal bone, ribs, the base of the skull, iliac crest, acetabulum, patella, and calcaneus, have also been reported. Almost all chondroblastomas are unifocal.

Radiographic Features

The lesion is sharply marginated, predominantly lytic, and rimmed by sclerotic bone ( Fig. 17.32 ). In long bones, it involves the epiphysis but may cross the physis into the metaphysis. Approximately 25% of cases are apophyseal ( Fig. 17.33 ). Periosteal reaction is uncommon. Matrix calcifications within the tumor are seen in 25% of cases. MRI often demonstrates tumor-associated reactive changes.

Pathologic Features