Hamartoma

Bone-forming tumors

Bone islands usually occur in intramedullary spongy bone of the pelvis, femur, humerus, and ribs; they are uncommon in the thoracic or lumbar vertebrae of the spine. Bone islands are small, white-yellow, circular masses. Compact bone is seen in the haversian systems. Bony spicules arise from these masses and create a spokelike appearance in the trabecular bone. The usual size is 1-2 mm, but lesions as large as 10 cm have been documented. Lesions larger than 2 cm are termed giant bone islands.

Osteopoikilosis occurs most commonly in the small bones of the hands and feet, as well as at the ends of the long bones of the hands ^[7] ; it is rarely found in the skull, facial bones, ribs, or spine. The histologic appearance of osteopoikilosis is similar to that of bone islands.

The lesions of melorheostosis are commonly progressive and spread in a linear fashion. If they cross an open epiphyseal plate or a joint space, they can mimic osteochondromatosis. The progressive nature of these lesions is marked by increasing pain and deformity.

The pathophysiology of osteopathia striata is currently unknown.

Cartilage-forming tumors

Osteochondroma and multiple osteochondromas are large masses that may be sessile or (occasionally) pedunculated. Their size ranges from 1 to 10 cm, and their surface is smooth or lobulated and often covered by a thin fibrous layer. The bony protuberance has a thin cortex and a medullary cavity that is continuous with the affected bone. The osteochondroma is derived from abnormal cartilaginous epiphyseal growth plate tissue. It remains in subperiosteal and either disappears by means of remodeling or proliferates as an early osteochondroma.

The growth of osteochondromas terminates at puberty with the fusion of the epiphyseal plate. Osteochondromas that continue to grow after puberty should raise the possibility of malignant degeneration. Multiple osteochondromas have a relatively high (1-25%) incidence of malignant transformation.

There are many theories regarding the pathophysiology of epiphyseal osteochondromas. The most common are abnormal regulation of cartilage proliferation, a congenital error of epiphyseal development, and a fetal developmental abnormality in the apical cap of the limb.

Enchondromatosis is believed to arise from remnants of the epiphyseal cartilage. Growing slowly, these lesions erode the adjacent cortex and cause the contours of the bone to expand. Rarely do they rupture into the soft tissue. Foci of calcification or ossification may be present and punctuate; this is the classic osteolytic appearance seen on radiographs. Multiple large and eccentric cartilaginous tumors can appear in any part of the bone.

Fiber-forming tumors

Nonossifying fibromas are common, benign, and well-delineated solitary lesions. They exist in the metaphysis of long bones. The tumors are thought to arise from the continued growth of fibrous cortical defects that extrude into the medullary cavity. The tibia and the femur are the bones most commonly involved. The lesions commonly regress and can result in new bone growth in areas of involution.

Fibrous dysplasia usually manifests as solitary, slow-growing hamartomatous lesions. The overgrowth of fibrous bone leaves areas of bony weakness, which results in deformity through repetitive minor fractures. Repeated fracture of the femur results in the characteristic shepherd’s-crook deformity at the upper end of the bone. ^[8]

A study of two groups of female patients with fibrous dysplasia, one in the Netherlands and one in the United States, found that this condition was associated with an increased risk of breast cancer at a young age. ^[9]

Benign non-matrix-forming tumors

Although hemangiomas of bone are the most frequently occurring tumors in the vertebral body, they are usually incidental findings. Vertebral hemangiomas are located mostly at the thoracic level. Medullary hemangiomas are rare. When they occur, they are found in the metaphyseal region. Periosteal hemangiomas are also rare and involve the midshaft of long tubular bones. Finally, cortical hemangiomas of the long bones are rare, and most are in the tibial shaft.

The lesions are well-demarcated dark-red cavities with hamartomatous proliferation of the vascular tissue in the endothelial lining of the cavity. These lytic bone lesions are usually limited to the vertebral bodies, but they can cross the joint spaces. Patients can develop symptoms as a consequence of nerve compression or collapse of a vertebral body.

Skeletal hemangiomatosis is rare, and patients usually present in the first two decades of life; by comparison, lymphangiomatosis is most commonly identified in early childhood. The lytic defects of skeletal hemangiomatosis can occur anywhere in the bone and give a honeycomb pattern. They can increase in size or regress, with eventual obliteration of the cystic cavity due to sclerosis.

Lymphangiomatosis involves the formation of cystic spaces, which are lined with a single layer of epithelial cells and filled with proteinaceous fluid. A combination of hemangiomas and lymphangiomas may also occur. The distinction is often made at the time of surgery. Skeletal hemangiomatosis and lymphangiomatosis are commonly self-limiting. Cases associated with involvement of additional soft tissue or viscera worsen the patient’s prognosis.

For additional, more detailed descriptions of most of the hamartomas described in this article, see the following:

• Benign and Malignant Soft Tissue Tumors
• Bone Island
• Cartilage Tumors, Low Grade
• Connective Tissue Nevus
• Cowden Disease (Multiple Hamartoma Syndrome)
• Dermatofibrosis Lenticularis (Buschke-Ollendorf Syndrome)
• Enchondroma and Enchondromatosis
• Fibrous Cortical Defect and Nonossifying Fibroma
• Fibrous Dysplasia
• Hamartoma, Lung
• Hand, Tumors: Benign
• Hemangioma
• Hemangioma, Bone
• Lymphangioleiomyomatosis
• Maffucci Syndrome
• Neurofibromatosis
• Nevi, Melanocytic
• Osteochondroma and Osteochondromatosis
• Osteofibrous Dysplasia
• Peutz-Jeghers Syndrome
• Solitary osteochondroma

Bone-forming tumors

Bone islands are developmental in origin. No associated risk factors are reported.

Osteopoikilosis has an autosomal dominant pattern of inheritance. Associated conditions include Buschke-Ollendorff syndrome and other forms of sclerosing bone dysplasia (eg, melorheostosis and osteopathia striata).

The etiology of melorheostosis is unknown.

Osteopathia striata has an autosomal dominant pattern of inheritance when associated with sclerosis of the skull base. ^[10]

Cartilage-forming tumors

The etiology of osteochondroma is unknown. Evidence suggests an abnormality in the long arm of chromosome 8. After irradiation in childhood, the risk of the development of one or more of these tumors increases 10-12%.

Multiple osteochondromas have an autosomal dominant pattern of inheritance. Three gene mutations have been identified in association with tumor suppressors EXT1 and EXT2. This disease is associated with other genetic syndromes, including Langer-Giedion syndrome and DEFECT 11 syndrome (also referred to as proximal 11p deletion syndrome or Potocki-Shaffer syndrome).

Epiphyseal osteochondroma is a rare developmental abnormality, and its etiology is unknown.

Enchondromatosis is a sporadic condition with no reported familial association. It is usually diagnosed before the age of 10 years. It is associated with Maffucci syndrome (enchondromas and hemangiomas) and metachondromatosis (enchondromas and hemangiomas); the former is nonhereditary, whereas the latter has autosomal dominant transmission. Enchondromatosis is associated with tumor-suppressor genes in the malignant transformation to chondrosarcomas. Notable alterations are inactivation of 9p21 and 13q14 and overexpression of p53. ^[11]

Fiber-forming tumors

Nonossifying fibroma is a nonaggressive developmental abnormality with no known genetic predisposition. It often involves the metaphysis of long, growing bones, and it usually regresses spontaneously. Nonossifying fibroma can be an incidental finding on radiographs, or it may be secondary to a pathologic fracture. The observation that most lesions seem to occur near points of tendinous attachment suggests that the disease may be associated with stress or trauma.

With fibrous dysplasia, a gene mutation is involved that results in the overgrowth of fibrous bone. The bone is weak, and this weakness predisposes the patient to pathologic fractures. No environmental or dietary factors have been associated with fibrous dysplasia, and the cause of the gene mutation is unknown. The condition begins before birth and is most commonly seen in childhood. The femur, tibia, skull, and ribs are the bones most commonly affected.

Fibrous dysplasia is usually monostotic and asymptomatic. When it is polyostotic, more than 60% of patients present before the age of 10 years. About 3% of cases are associated with McCune-Albright syndrome, and 50% are associated with abnormal cutaneous pigmentation. Other diseases that may be associated with fibrous dysplasia are desmoplastic fibroma, eosinophilic granuloma, giant cell reparative granuloma, and secondary aneurysmal bone cysts.

Benign non-matrix-forming tumors

With hemangiomas of bone, asymptomatic lesions are usually discovered incidentally. No familial tendency is reported.

Skeletal hemangiomatosis or lymphangiomatosis is usually an incidental finding on radiographs. No familial tendency is reported.

The figures given below are for hamartoma frequencies in the United States. In most cases, international frequencies are similar to those in the United States, unless stated otherwise.

Bone-forming tumors

Bone islands are found in approximately 1% of the population and occur in adults more often than children. There is no sexual predilection, and no risk factors that predispose to the development of bone islands are known. This profile differs from that of osteoid osteomas, which are most commonly seen in male individuals and which make up 10-12% of all benign bone tumors. Likewise, osteosarcomas most commonly affect male persons; however, they are most frequent between the ages of 10 and 20 years. ^[12]

Approximately 1000 cases of osteosarcoma are diagnosed in the United States each year, and about half of these are in children and teens. ^[13] The incidence of osteosarcoma in persons younger than 20 years is 5.7 per 1,000,000. ^[14] Osteosarcoma is the most common primary malignancy of bone in children. ^[15]

Osteopoikilosis demonstrates no sexual predilection, and it can appear at any age. The lesions are associated with a predisposition to form keloids, sclerodermalike lesions, and other subcutaneous nodules in 25% of patients.

Melorheostosis is most common in the young and has no sexual predilection.

Osteopathia striata is a rare disease, one that is often associated with other bony hamartomatous tumors. Its incidence and prevalence are currently unknown.

Cartilage-forming tumors

Osteochondroma is most commonly seen in the second and third decades of life. Because most lesions are asymptomatic, the true prevalence is unknown. No sexual predilection has been established, though some authors report a 2:1 male-to-female preponderance. ^[16]

Multiple osteochondromas are commonly observed in childhood, with 40-50% of the lesions found in the distal femur and proximal tibia. This disease has an autosomal dominant mode of inheritance. It is not sex-specific, but the proportion is increased among White persons. ^[17]

The incidence of epiphyseal osteochondroma is higher among males than among females, with a 3:1 ratio. When this disease occurs, it affects the medial aspect more often than the lateral aspect. The total prevalence has been estimated at one case per million population. ^[18]

Enchondromas account for 12-14% of benign bone neoplasms and 3-10% of neoplasms in general. ^[6] Malignant transformation occurs in 30% of patients, and 25% present with a sarcoma at the age of 40 years. The tumor most often associated with enchondromatosis is a chondrosarcoma (grade 1, grade 2, or dedifferentiated). Evidence also suggests an association with osteosarcomas and chordomas. When enchondromatosis is associated with systemic syndromes, the frequency of sarcoma reaches 30-50%. ^[19]

Fiber-forming tumors

The exact incidence of nonossifying fibroma is unknown, but reports have suggested that 30-40% of all children may have one or more of these lesions at any one time. Although precise figures are unavailable, an increased incidence has been reported in children, adolescents, and young adults aged 2-18; the male-to-female ratio is 2:1. The most commonly affected sites are in the lower extremity (femur, 40%; tibia, 40%; and fibula, 10%); the upper extremity is rarely involved.

The exact incidence of fibrous dysplasia is currently unknown. This lesion is thought to account for 7% of all benign bone tumors. It occurs equally in male and female individuals. Approximately 75% of all cases are diagnosed before the age of 30 years. ^[20]

Benign non-matrix-forming tumors

Different histologic types of hemangioma of bone exist. Vertebral hemangiomas account for 10-12% of all bone hemangiomas found at autopsy. About 75% of hemangiomas involve the vertebra and skull, 10% involve the long bones, and 5% involve the ribs. The male-to-female ratio is 2:1.

Hemangiomas of bone account for 1% of all spinal neoplasms and represent 2-3% of all spinal tumors. ^[21] They are the most common benign tumor of the spinal column. The lesions occur most commonly in the lower thoracic vertebrae, less frequently in the lumbar spine, and infrequently in the cervical spine and sacrum.

Skeletal hemangiomatosis and lymphangiomatosis are usually discovered in childhood. The prevalence and incidence is currently unknown, because the diagnosis is often serendipitous. Solitary lesions are rare, and many cases of lymphangiomatosis of the bone are associated with visceral and soft tissue involvement. About 60% of all cases of skeletal hemangiomatosis involve other viscera, commonly the spleen. However, evidence in the literature also describes involvement of the liver, lungs, and soft tissue.