Benign Bone Tumors and Nonneoplastic Conditions Simulating Bone Tumors

Osteoid osteoma

Osteoid osteoma is a benign neoplasm most often seen in young men. Most osteoid osteomas are found in the second or third decades of life, but an occasional lesion has been reported in older patients. Almost any bone can be involved, although there is a predilection for the lower extremity, with half the cases involving the femur or tibia. The tumor may be found in cortical or cancellous bone. Multicentric lesions have been reported. No malignant change has ever been documented. The typical patient with an osteoid osteoma has pain that is worse at night and is relieved by aspirin or other nonsteroidal antiinflammatory medications. Increased levels of cyclooxygenases and prostaglandins have been demonstrated in the lesions. This fact explains the cause of the intense pain as well as the dramatic pain relief that results from treatment with nonsteroidal antiinflammatory medication. When the lesion is near a joint, swelling, stiffness, and contracture may occur. When the lesion is in a vertebra, scoliosis may occur. Imaging studies are usually diagnostic. Biopsy is rarely required to confirm the diagnosis. The lesion consists of a small (<1.5 cm), central radiolucent nidus with surrounding bony sclerosis. Plain radiographs are often sufficient to make the diagnosis. Computed tomography (CT) is the best technique to identify the nidus and confirm the diagnosis. The lesions demonstrate marked increased uptake on technetium bone scans. Magnetic resonance imaging (MRI) studies usually demonstrate extensive surrounding edema.

The microscopic appearance consists of fibrovascular tissue with immature bony trabeculae that are rimmed by prominent osteoblasts. The histologic appearance is similar to that of an osteoblastoma, with the exception that osteoblastomas are larger. The lesion is usually surrounded by a sclerotic rim. There is no nuclear atypia. Osteoclasts and occasional giant cells can be seen. There are no aggressive features.

Multiple treatment options are available, including medical treatment, percutaneous radiofrequency ablation, and open surgical procedures. If the patient’s symptoms are adequately controlled and the patient is willing to undergo long-term medical management, antiinflammatory medication can be used as the definitive treatment. Patients treated in this manner usually experience spontaneous healing of the lesion within 3 to 4 years.

Most patients with lesions of the pelvis or long bones of the extremities can be treated with percutaneous radiofrequency ablation ( Fig. 25.1 ). This technique involves a CT-guided core needle biopsy after which a radiofrequency electrode is inserted through the cannula of the biopsy needle. The temperature at the tip is increased to 90°C for 6 minutes. Multiple authors have reported excellent results with this procedure. It is usually done as an outpatient procedure, and patients can usually return immediately to full activity. Recurrence rates are less than 10%. Great care must be exercised when performing radiofrequency ablation on vertebral lesions because of the risk of injury to the spinal cord or nerve roots. The procedure may not be indicated for lesions of the small bones of the hands or feet because of the risk of thermal injury to the skin.

Surgical management involves removal of the entire nidus. This can be accomplished by curettage or en bloc resection. The latter is associated with a low recurrence rate but is rarely indicated for lesions in the long bones because of an increased risk of postoperative pathologic fracture. More often, removal is done using the burr-down technique ( Fig. 25.2 ). This method consists of identifying the nidus intraoperatively with fluoroscopy and using a power burr to remove the sclerotic bone directly over the nidus. The nidus is removed using curets and sent for pathologic examination. The cavity is treated again with the power burr to ensure that the entire nidus has been removed. In this manner, only a minimal amount of surrounding reactive bone is removed, minimizing the risk of subsequent fracture. Recurrence rates with this technique are less than 10%.

A new noninvasive and radiation-free method of treatment is currently under investigation. Magnetic resonance–guided focused ultrasound (MRgFUS) ablation is a technique in which ultrasound waves are focused on the osteoid osteoma. When high-intensity waves are focused on a single point, considerable heat can be generated. MRI is used for both lesion localization and temperature monitoring. Preliminary studies have shown this technique to be safe and effective. Larger studies are needed to confirm preliminary findings.

Bone island

Bone islands, also called enostoses, are benign lesions of cancellous bone. They are usually asymptomatic and are discovered incidentally. Almost any bone can be involved, but the pelvis and the femur are the most common sites. It is unclear whether they represent a developmental abnormality or neoplastic process. Regardless, most remain quiescent. They are of interest primarily because other, more aggressive, lesions are occasionally in the differential diagnosis for patients with bone islands. Osteopoikilosis is a rare condition consisting of multiple small bone islands throughout the skeleton. Autosomal dominant and sporadic forms of the syndrome have been identified.

Bone islands can usually be diagnosed by plain radiographs. They are typically small, round or oval areas of homogeneous-increased density within the cancellous bone ( Fig. 25.3A ). Radiating spicules on the periphery of the bone islands merge with the native bone creating a brush-like border. No bony destruction or periosteal reaction is noted. They may show mildly increased uptake on bone scans; however, markedly positive scans should raise suspicion of more aggressive lesions. CT scans show thickened trabeculae, which merge with the surrounding bone. MRI usually shows well-defined lesions that are isointense to cortical bone and thus dark on T1- and T2-weighted images ( Fig. 25.3B and C ) with no surrounding edema. There are no aggressive imaging features.

The microscopic appearance reflects the imaging characteristics. Bone islands consist of mature bone with thickened trabeculae. At the periphery of the lesion, the lesional trabeculae merge with the normal bone. There is no sclerotic rim. Occasionally, woven bone is a minor part of the lesion.

Most patients with bone islands can be treated with observation with serial plain radiographs. As long as the lesions remain asymptomatic and do not grow, no further intervention is indicated. If a patient experiences pain, or if the lesion grows, biopsy is indicated to rule out more aggressive lesions, such as a sclerosing osteosarcoma, blastic metastasis, or sclerotic myeloma.

Chondroma

Chondromas are benign lesions of hyaline cartilage. They are common, and all age groups are affected. Although any bone can be involved, the phalanges of the hand are the most common location. They are the most common tumor of the small bones of the hands and feet. Chondromas are usually asymptomatic and are frequently discovered incidentally during an unrelated radiographic examination. They can also be discovered after a pathologic fracture. They usually arise in the medullary canal, where they are referred to as enchondromas . Rarely, they arise on the surface of the bone, where they are referred to as periosteal chondromas or juxtacortical chondromas .

Multiple enchondromatosis, also known as Ollier disease, is a rare condition in which many cartilaginous tumors appear in the large and small tubular bones and in the flat bones. It is caused by failure of normal endochondral ossification. The tumors are located in the epiphysis and the adjacent parts of the metaphysis and shaft, and many bones may be affected. Deformities resulting from the tumors include shortening caused by lack of epiphyseal growth, broadening of the metaphyses, and bowing of the long bones. Multiple lesions of the small bones of the hand may cause considerable disability. When associated with hemangiomas of the overlying soft tissues, the disease is known as Maffucci syndrome. The individual lesions are similar to solitary enchondromas but they have a definite tendency to become malignant. Approximately 25% of patients with Ollier disease are diagnosed with sarcomas by 40 years of age.

Radiographically, enchondromas are benign-appearing tumors with intralesional calcification ( Fig. 25.4A and B ). The calcification is irregular and has been described as “stippled,” “punctate,” or “popcorn.” In the small bones of the hands and feet there may be considerable erosion and expansion of the overlying cortex. In more proximal locations (e.g., the pelvis, proximal humerus, or proximal femur), deep endosteal erosion (two thirds of the thickness of the cortex) frequently indicates a chondrosarcoma. An associated soft-tissue mass is never present with an enchondroma and always indicates a chondrosarcoma. Juxtacortical chondromas are usually small (<3 cm), well-defined lesions that frequently appear to fit in a saucer-shaped defect on the surface of the bone ( Fig. 25.5 ). The underlying cortex appears sclerotic and the edges of the lesion appear to be buttressed by a thick rind of cortical bone. Plain radiographs are usually sufficient to diagnose a chondroma. If the diagnosis is in question, CT is best to evaluate endosteal erosion that could indicate a chondrosarcoma ( Fig. 25.4C ).

The microscopic appearance of a chondroma is that of mature hyaline cartilage. Proximally located enchondromas should appear bland and hypocellular. Any degree of hypercellularity or atypia in a proximally located cartilage tumor should raise suspicion of a chondrosarcoma. Enchondromas of the hand, juxtacortical chondromas, and lesions associated with multiple enchondromatosis may be relatively hypercellular with mild atypia, and still be benign. The differentiation of benign from malignant cartilaginous tumors is one of the most difficult problems in bone pathology. All available tissue must be examined, and even then the diagnosis may depend more on the clinical and radiographic features than on the microscopic changes.

Treatment of patients with solitary enchondromas usually consists of observation with serial radiographs. If the lesion remains radiographically stable and asymptomatic, no further intervention is indicated. If a lesion grows or if it becomes symptomatic, extended curettage is usually curative. Before recommending surgery for a symptomatic lesion, however, all efforts should be made to rule out other possible sources of the patient’s pain (e.g., rotator cuff tear in a patient with a proximal humeral enchondroma) ( Fig. 25.6 ). Recurrence rates are low. Treatment of patients with multiple enchondromatosis can be more difficult. Although the individual lesions are usually not treated, the more obvious deformities can be corrected by osteotomy. These patients must also be monitored indefinitely for malignant change.

Osteochondroma

Osteochondromas are common benign bone tumors. They are probably developmental malformations rather than true neoplasms and are thought to originate within the periosteum as small cartilaginous nodules. The lesions consist of a bony mass, often in the form of a stalk, produced by progressive endochondral ossification of a growing cartilaginous cap. In contrast to true neoplasms, their growth usually parallels that of the patient and usually ceases when skeletal maturity is reached. Most lesions are found during the period of rapid skeletal growth. Approximately 90% of patients only have a single lesion. Osteochondromas may occur on any bone preformed in cartilage but are usually found on the metaphysis of a long bone near the physis ( Fig. 25.7 ). They are seen most often on the distal femur, the proximal tibia, and the proximal humerus. They rarely develop in a joint. Trevor disease (dysplasia epiphysealis hemimelica) refers to an intra-articular epiphyseal osteochondroma. When multiple joints are involved, it is usually unilateral (hemimelica).

Many of these lesions cause no symptoms and are discovered incidentally. Some cause mechanical symptoms by irritating the surrounding structures and, rarely, one becomes painful due to a fracture. False aneurysms of major lower extremity vessels caused by pressure from osteochondromas have been reported. Also, neuropathies caused by pressure from contiguous osteochondromas have occurred; the physical finding is usually a palpable mass.

Multiple hereditary exostoses is an autosomal-dominant condition with variable penetrance. Most patients with this disorder have a mutation in one of two genes: EXT1, which is located on chromosome 8q24.11-q24.13, or EXT2, which is located on chromosome 11p11-12. In this disease, osteochondromas of many bones are caused by an anomaly of skeletal development. The most striking feature is the presence of many exostoses ( Fig. 25.8 ), but disturbances in growth also occur, such as abnormal tubulation of bones, producing broad and blunt metaphyses, and sometimes bowing of the radius and shortening of the ulna, producing ulnar deviation of the hand. The disease only occurs 5% to 10% as often as solitary osteochondroma and is more common in males. It is usually discovered at approximately the same age as the solitary lesion, but closer examination of children in families with the disease might lead to earlier discovery.

Osteochondromas are of two types: pedunculated and broad based or sessile. All gradations between these types also occur. Pedunculated tumors are more common, and any definite stalk is directed away from the physis adjacent to which it takes its origin. The projecting part of the lesion has cortical and cancellous components, both of which are continuous with corresponding components of the parent bone. The lesion is covered by a cartilaginous cap that is often irregular and cannot usually be seen on radiographs; occasionally, calcification within the cap may be seen. Typically, the cap is only a few millimeters thick in adults, although it may be 2 cm thick in a child. A bursa frequently overlies the tumor and may contain osteocartilaginous loose bodies. Plain radiographs are usually sufficient to make a diagnosis. Occasionally, a CT or MRI scan is needed to confirm the diagnosis.

Malignant degeneration is extremely rare. Large series have estimated the incidence of malignant degeneration to be approximately 1% for patients with a solitary osteochondroma and 5% for patients with multiple hereditary exostoses. However, these percentages were derived from pathologic data and thus there is inherent bias toward large, symptomatic lesions that subsequently underwent resection at a referral center. The true incidence of malignant degeneration is much lower than these figures suggest because the true prevalence of osteochondromas is unknown. Most patients are asymptomatic and never seek medical attention. Malignant transformation should be suspected when a previously quiescent lesion in an adult grows rapidly; it usually takes the form of a low-grade chondrosarcoma. In these cases, the cartilage cap is usually more than 2 cm thick. Malignant transformation is best evaluated by CT or MRI.

Surgery (en bloc resection) is indicated when the lesion is large enough to be unsightly or produces symptoms from pressure on surrounding structures or when imaging features suggest malignancy. On rare occasions, the diagnosis of a sessile lesion cannot be established by studying the radiographs, and biopsy is indicated. Recurrence is rare and is probably caused by failure to remove the entire cartilaginous cap. Patients with multiple hereditary exostoses may require osteotomies to correct deformity.

A similar lesion, subungual exostosis, may develop on a distal phalanx, especially of the great toe. Often there is a definite history of trauma. Excision is indicated when elevation of the nail produces pain. The history and location of the lesion distinguish it from a true osteochondroma.

Nonossifying fibroma

Nonossifying fibromas (also known as metaphyseal fibrous defects, fibrous cortical defects, and fibroxanthomas) are common developmental abnormalities and are believed to occur in 35% of children. Usually, they are found incidentally. Generally, these lesions occur in the metaphyseal region of long bones in individuals 2 to 20 years old. Although any bone may be involved, approximately 40% of these lesions are found in the distal femur, 40% in the tibia, and 10% in the fibula. On plain radiographs, a nonossifying fibroma appears as a well-defined lobulated lesion located eccentrically in the metaphysis ( Fig. 25.9 ). Multilocular appearance or ridges in the bony wall, sclerotic scalloped borders, and erosion of the cortex are frequent findings. There is no periosteal reaction in the absence of a pathologic fracture.

Histologically, the defect is filled with spindle-shaped cells distributed in a whorled or storiform pattern. There is fibroblastic proliferation with high cellularity. Giant cells and foam cells are almost always apparent.

Most nonossifying fibromas are asymptomatic and regress spontaneously in adulthood. Most pathologic fractures can be treated nonoperatively. Lesions may become symptomatic and require treatment if they become large or if they are subjected to repeated trauma. Some authors have recommended treatment for lesions that are larger than 50% of the diameter of the bone because of a theoretical increased risk of pathologic fracture, although this parameter is not universally accepted as an indication for surgery. Recurrence after curettage is rare ( Fig. 25.10 ).

Cortical desmoid

A cortical desmoid is an irregularity in the posteromedial aspect of the distal femoral metaphysis and is usually seen in boys 10 to 15 years old. It may be a reaction to muscle stress exerted by the adductor magnus. The lesion is best seen on an oblique radiograph made with the lower extremity externally rotated 20 to 45 degrees. Clinical symptoms, if any, include soft-tissue swelling and pain. Radiographs and MRI reveal erosion of the cortex with a sclerotic base ( Fig. 25.11 ). A biopsy is not warranted. Treatment usually consists of observation.

Benign fibrous histiocytoma

Benign fibrous histiocytoma is a rare entity that was first described by Dahlin in 1978. This lesion occurs most frequently in the soft tissues and is less common in bone. Although it is histologically similar to nonossifying fibroma, it is a much more aggressive tumor in its biologic behavior and radiographic characteristics ( Fig. 25.12 ). In contrast to nonossifying fibroma, which is usually an eccentric metaphyseal lesion, benign fibrous histiocytoma may occur in the diaphysis or epiphysis of long bones or in the pelvis. It is distinguished further by its occurrence in older patients between the ages of 30 and 40 years. Radiographically, benign fibrous histiocytoma is a well-defined, lytic, expanding lesion with little periosteal reaction. Bone scans are usually mildly positive. In contrast to nonossifying fibroma, this lesion is considered a true neoplasm. Because of its tendency for local recurrence, extended curettage or wide resection is recommended.