2.5 Benign bone tumours

Unicameral bone cyst, (solitary bone cyst, simple bone cyst) (UBC) is a benign lesion characterized by a fluid-filled intramedullary cavity lined by a thin soft tissue membrane. It comprises between 3–5% of all primary bone tumours, with a 2:1 male predominance. Most UBCs occur in the proximal humerus and femur in young people aged 5–15 years, and in the ilium and calcaneum in older patients. The exact pathogenesis is still unclear.

Unicameral bone cysts are often asymptomatic, and discovered following an associated fracture. Other symptoms include localized tenderness, joint stiffness, swelling, and deformity or growth retardation if the cyst disrupts the epiphysis.

Plain radiographs are the investigation of choice for UBC (Figure 2.5.1). Typically they appear as a purely lytic lesion with a well-defined border and narrow zone of transition. They tend to occur centrally with some mild expansion of the bone but not breaching the cortex. The cortex is thinned and there is no periosteal reaction unless it is associated with a fracture. It is a unilocular lesion, but often appears multiloculated due to the prominent ridges on the cortical wall.

When a thinned cortical area fractures, the periosteum often remains intact, and the small fracture fragment can be seen within the base of the lesion (the fallen fragment sign) which is pathognomonic for a UBC with a fracture.

In young children, a UBC starts at the metaphysis adjacent to the epiphysis. As the bone grows, the lesion moves away towards the diaphysis. The lesion is classified as active when it is still within 1cm from the epiphysis, and latent when it is near the diaphysis.

Bone scan is often normal, and computed tomography (CT) shows only a fluid level. Intracystic septa can be demonstrated if radio opaque contrast is injected within the lesion. Magnetic resonance imaging (MRI) shows a bright fluid signal in T2-weighted images, and low signal in T1-weighted images.

Occasionally, a biopsy is taken to differentiate between UBC, aneurysmal bone cyst, and fibrous dysplasia. Grossly, the cortex is thin; the fluid is clear or yellow, with a fibrous membrane which lines the inner wall. Associated bleeding can be seen within the cavity if there is a fracture. Histological examination shows that the membrane consists of fibroblasts, with a cementum-like structure deep to it. This consists of strong eosinophilic and calcified material, surrounded by immature bone fragment, osteoclast-like giant cells, mesenchymal cells, and lymphocytes, but there would be no cellular atypia.

Treatment is often non-operative. Small lesion in non-weight-bearing bone can be monitored regularly until skeletal maturity, but large cysts, or cysts at sites where there is high risk of fracture, should be treated. Operative management includes aspiration of the cyst followed by injection with either steroid or bone marrow, with recent studies suggesting that steroids may provide superior healing rates. Repeat injection may be required if there is no evidence of healing after 2 months. If it is not healing despite several injections, especially in the weight-bearing skeleton, curettage and bone grafting with or without internal fixation can be of benefit. Pathological fracture through the cyst often causes healing of the cyst after the fracture is united, probably due to an antiprostaglandin effect or decompression of the pressure of the cyst following the fracture.

More recently, injection of demineralized bone matrix and bone marrow had been shown to have good effect. In general, most patients will achieve an excellent outcome following any of the treatments.

Aneurysmal bone cyst (ABC) is a locally destructive, expansile benign lesion. It is characterized by multiple loculations filled with blood and separated by fibrous septa. It accounts for about 2% of all primary bone tumours, and there is a slight female dominance of 3:2. It can arise as a primary lesion, or as secondary process associated with other lesions such as giant cell tumour, chondroblastoma, UBC, osteoblastoma, fibrous dysplasia, or even osteosarcoma.

ABC usually presents in the metaphysis of long bones, especially the proximal humerus, distal femur, and proximal tibia. It can also present in the ilium and in the posterior column of the vertebrae (accounts for 15–25%). Seventy-five per cent of patients present under the age of 20. It is postulated that ABCs arise from an increase of venous pressure secondary to some local circulatory compromise causing local haemorrhage.

Most cases present with mild pain and swelling, but sometimes it can progress quite rapidly, mimicking malignancy. Spinal lesions can present with neurological symptoms. Pathological fracture, however, is not common (8–20%).

ABC appears as a destructive cystic lesion with bony resorption on plain radiographs (Figure 2.5.2). It is usually eccentric, with a narrow zone of transition and associated with cortical expansion. The growth plate is not normally breached, unless the ABC is secondary to a chondroblastoma or giant cell tumour.

Radiographic appearance is often seen in four phases:

Bone scan shows a diffuse increased uptake in the periphery with a decreased central uptake. Angiography shows accumulation of contrast in the cavity, with hypervascularity in the periphery. CT and MRI can demonstrate the cavities and fluid level as well as delineating the lesion from the surrounding structures.

Macroscopically, ABC is a cystic lesion with septated cavities filled with blood. Microscopically, evaluation of the curetted tissue shows haemorrhagic tissue separated by cellular stroma. This stroma contains fibroblast, histiocytes, haemosiderin-packed macrophages, and osteoclast-like giant cells. There are numerous mitotic figures without atypical forms present in this stroma, and the vessels within them are quite dilated, probably contributing to the bleeding in the cavities. A ‘highly mineralized matrix with a chondral aura’ lining the vascular area can be a diagnostic characteristic for ABC.

A solid variant of ABC has also been described. It has similar clinical and radiographic features to the cystic from, but lacking the cavernous spaces. The highly proliferative stromal cells appear very similar to giant cell tumour, which gives an impression that it may be a secondary ABC.

Spontaneous healing is rare, and treatment is usually surgical. Curettage with or without bone grafting has been the acceptable method, together with various adjuvant therapies like phenol, liquid nitrogen and high-speed burring of the cavity wall, to reduce the recurrence rate. Recurrence rate of ABC following curettage with or without grafting has been shown to be about 20–30%, and reduced to about 10% with adjuvant therapy. Selective embolization of the feeding vessels prior to surgery can help to significantly reduce blood loss and should be used in large lesions. Resection of the expanded extremity of lesion, such as fibula or metatarsal, is another option.

Newer methods of minimally invasive introduction of demineralized bone and autologous bone marrow into the cysts, with no curettage has been used with some success. Percutaneous sclerotherapy using polidocanol has also been tried with some good results. Both of these methods have the advantage of avoiding open curettage, and hence reducing the risk of surgical morbidity. However, this does not provide tissue sample for definitive diagnosis and still has the potential for further cyst progression.

Fibrous cortical defects and non-ossifying fibroma are considered as abnormal developmental proliferations of fibrous tissue and histiocytes. Non-ossifying fibroma is larger, slightly different in radiological appearance, but is histologically identical. They are often termed as ‘metaphyseal fibrous defects’. These lesions are common, and are present in over one-third of children under 14 years. Most are incidental findings. There is a slight male dominance of about 2:1.

They commonly present in the metaphysis of lower limbs, such as distal femur, proximal/distal tibia, and fibula. Occasionally they can present in the humerus, clavicle, or ilium. Most of the lesions are solitary, but rarely can be multiple. A Jaffe–Campanacci syndrome had been described with multiple lesions, mental retardation, precocious puberty, congenital blindness, and kyphoscoliosis. The aetiology of these lesions is largely unclear.

Most of these lesions are completely asymptomatic. Large lesions can present with swelling, mild pain, or pathological fracture. There is no evidence to suggest that they can undergo malignant transformation.

On plain radiographs, fibrous cortical defects are usually in the metaphysis or diaphysis of long bones, they appear as eccentric radiolucent areas up to 2cm with a smooth margin and a thin sclerotic rim (Figure 2.5.3). The cortex can sometimes be eroded, but no periosteal reaction develops. Non-ossifying fibroma presents with similar features, but much larger in size, reaching to about 7cm. They can cause cortical expansion, and sometimes involve the whole width of thin bones, such as the fibula. Both lesions have their long axis parallel to the axis of the long bone, and are often found near the insertion of tendons, which suggests that stress or avulsion injury may contribute to the cause. With bone growth, they migrate from metaphysis to diaphysis. But when healing occurs, the sclerosis starts from the diaphyseal end towards the metaphysis.

The diagnosis of these lesions can usually be made by plain radiographs; bone scans only show increased uptake in the active healing phase or in pathological fractures. CT and MRI are not usually indicated.

It is rare that any sample will reach the pathologist as no surgical treatment or biopsy is required. For those curetted samples which reach the pathology lab, the proliferative spindle cell stroma is intermixed with fibroblasts, collagen fibres, and histiocytes. Multinucleated giant cells are often present, and there is no cellular atypia. There is no reactive bone formation unless in the case of pathological fractures.

Most of the fibrous cortical defects and non-ossifying fibromas regress and heal spontaneously, and no treatment is necessary. For those who present with pathological fracture, closed treatment is recommended to allow spontaneous union. The lesions that are at high risk of fracturing can be treated with close monitoring and modification of activity until they regress. Surgical treatment comprises curettage and bone grafting; more recently, demineralized bone matrix with bone marrow has been used with good results. Internal fixation is occasionally necessary if the pathological fracture is unstable or irreducible.

Bone island, or enostosis, is a benign solitary area of mature lamellar bone within the cancellous bone. Found in about 1% of the population, predominantly adults, they have no gender predilection. They are predominantly found in adults, and rarely in children. In long bones they usually present in the epiphysis or metaphysis. The size of the bone island varies between 2–20mm, but occasionally they can be as large as 10cm. Any lesion larger than 2cm is termed ‘giant bone island’.

Bone islands are asymptomatic, usually found incidentally, and should not cause pain. They can grow very slowly with time, enlarging about 30% over 20 years. Any bone island that is painful or grows more rapidly in size would suggest more aggressive disease.

Osteopoikilosis is a rare condition consisting of multiple periarticular lesions, identical to bone island, throughout the skeleton.

Typically bone island can be diagnosed with plain radiographs, with the characteristic round or oval shape of high density within the cancellous bone (Figure 2.5.4). It is homogenous and blends into the native bone, giving a ‘brush border’-like appearance. There is no bone destruction or periosteal reaction in the surrounding area.

Bone island is usually cold on bone scanning, but occasionally it can have increased uptake in the giant form. CT can demonstrate the benign morphology if plain radiographs are not enough to make the diagnosis.

Bone island rarely needs biopsy, but histologically it is an area of compact, mature lamellar bone with a well-developed Haversian system. Occasionally there is some woven bone present, with some increased osteoblastic activity and blood flow, indicating a degree of remodelling.

Repeat radiographs can be used to monitor if the lesion is growing in size. If it enlarges more than 25% in 6 months, or 50% in 12 months, especially in the large ones (greater than 2cm), then it should be biopsied to exclude more aggressive lesions. Otherwise no treatment is required.

Fibrous dysplasia is a slow-growing lesion where normal bone marrow is replaced by metaplastic woven bone and fibrous tissue. It may involve one (monostotic, 85% of the cases) or multiple bones (polyostotic 15%). They can be clustered around a single limb of the upper or lower extremity, or involve the whole skeleton. Certain syndromes, such as McCune–Albright disease and Mazabraud disease, have been associated with this lesion.

Fibrous dysplasia accounts for about 7% of all bone tumour or tumour-like conditions, and has no gender predilection. It can present at any age, but most lesions are found before 30 years of age as they are more active during growth. The most common locations are long bones, ribs, craniofacial, and pelvis.

A mutation of the Gsα gene has been found to be associated with fibrous dysplasia. It was first discovered in patients with McCune–Albright syndrome, and recent studies had shown that isolated fibrous dysplasia also has the mutated Gsα gene present. This gene mutation can lead to activation of adenylate cyclase within the cell, which increases cell proliferation but inappropriate differentiation, resulting in excessive production of disorganized fibrotic bone matrix.

The majority of monostotic lesions are asymptomatic, and are found incidentally during the investigation of other problems in the same region. Bone pain, stress fractures, and pathological fractures are the usual modes of presentation, especially on weight-bearing bones such as the femur.

Fibrous dysplasia can also present with a gradual deformity of the limb. The severity of the deformity depends on the age of the patient, the extent, the site of the lesion, or whether it is monostotic or polyostotic. The classic ‘shepherd’s crook deformity’ of the proximal femur is characteristic of fibrous dysplasia. It is postulated that the deformity is secondary to the intermittent stress fractures through the dysplastic bone, and the bone then deforms with normal mechanical load. Other presenting symptoms and signs include limping, leg length discrepancy, tibial bowing, and exophthalmos associated with skull lesions.

Fibrous dysplasia is known to be associated with other conditions in certain syndromes. In McCune–Albright syndrome, polyostotic fibrous dysplasia is associated with precocious puberty and pigmented skin lesions. In Mazabraud syndrome, polyostotic lesions are associated with intramuscular myxoma. Oncogenic osteomalacia is a rare condition that is sometimes found in patients with fibrous dysplasia, symptoms of which are fatigue, joint pain, weakness, and fractures with an abnormal blood picture (hypophosphataemia, normocalcium, and increased alkaline phosphatase). Malignant transformation can occur in about 0.5%, but much higher (4%) in McCune–Albright syndrome.

On plain radiographs, fibrous dysplasia appears as a well-circumscribed diaphyseal lesion with a ‘ground-glass’ appearance (Figure 2.5.5). This is similar to cancellous bone, but more homogenous with no trabecular pattern. The lesions are usually in the centre of the medullary canal and are surrounded by a rim of reactive bone which fades into the surrounding cancellous bone. As the lesion replaces normal cortical and cancellous bone, the distinction becomes obscured. The bone diameter can expand as the lesion enlarges with its reactive rim of bone, and the cortex is slowly resorbed at variable rates, producing endosteal scalloping. There is, however, no periosteal reaction. As the lesion becomes quiescent after skeletal maturity, the rim of reactive bone is thicker and the lesion itself has a higher density.

Other variations of the classical appearance of fibrous dysplasia have also been described, with a cystic form (a radiolucent area with a sclerotic rim), pagetoid form with dense trabeculae-like pattern, or intralesional calcification indicating the presence of cartilage. Of note, the polyostotic form has similar appearance and variations as the solitary form.

Radionuclide bone scintigraphy can be used to show the extent of the disease. Isotope uptake is high in active lesions in young people, but reduces as the lesions mature. CT is the best investigation to show the characteristic of fibrous dysplasia, where the native cortex, reactive bone, and endosteal scalloping can be demonstrated clearly. The poorly mineralized area of abnormal bone also appears classical with CT. MRI can demonstrate the shape and size of the lesion, and returns a low signal intensity in T1- and T2-weighted images as they tend to have a lower water content. In the case of fracture with haemorrhage, or cystic changes, a more hyperintense signal on T2-weighted images is seen.

The gross appearance of the lesion shows a yellowish gritty fibrous tissue with some trabeculae of bone scattered throughout. The surrounding rim of bone is usually intact. Microscopically, the thin trabeculae of bone are surrounded by a fibrous stroma of spindle shape cells. The pattern of the trabeculae is haphazard. There is minimal osteoblastic rimming, unless it is associated with fracture, and there is no cellular atypia or pleomorphism.

Asymptomatic lesions do not require any surgical treatment, but they will need to be monitored to assess for progression, risk of pathological fracture, deformity, or, rarely, malignant changes. Bisphosphonate therapy, especially pamidronate, has been shown to have some good effect in reducing bone pain, and also induce healing of the lesion with thickening of the cortex and ossification of the lesion.

Surgical treatment of fibrous dysplasia is indicated for symptom control, correction of deformity, fixation of fractures, and non-union of fractures. Curettage and bone grafting had been used to fill up the defects. In weight-bearing bones with a large defect, internal fixation with or without cortical grafting had also been shown to have good results. In severe bony deformity, osteotomy and internal fixation may be required to straighten and strengthen the skeleton. External fixation using monoaxial or circular frame has also reported good results, especially in severe deformity or shortening. Utilizing the circular frame technique, distraction osteogenesis has been shown to achieve good length correction. Autologous fibula strut grafting can also be used and can achieve good results.

Pathological fractures in non-weight-bearing bones can usually be treated non-operatively, but in the weight-bearing lower limb may require fixation. Patients with large lesions, or severe polyostotic disease, often require multiple surgeries for progressive deformities.

Osteochondromas are benign cartilaginous neoplasms found in any bone that undergoes enchondral bone formation. They account for 35% of benign bone tumours and 9% of all bone tumours. There is a higher incidence in male patients (3:1) and generally affects patients younger than 20 years old. The most common locations for solitary osteochondromas are the distal femur, proximal tibia, proximal humerus, and pelvis.

The majority of patients are asymptomatic and the lesions are noted as incidental findings. Pain is usually caused by direct mass effect on the overlying soft tissue where it can cause irritation to surrounding tendons, muscle, or neurovascular structures and a bursa may form over the lesion. Pain can also indicate malignant transformation. The lesions tend to stop growing when a patient reaches skeletal maturity; increase in size of the lesion in adults is a sign of possible malignant transformation.

Hereditary multiple exostoses (HME) is an autosomal dominant condition that affects one in 50 000; it results in formation of multiple osteochondromas. If, in an affected family, an osteochondroma has not formed in a patient older than 12 the condition is unlikely to manifest itself. The condition is associated with short stature and long bone deformities particularly affecting the forearm and knee. Operative treatment is advocated to prevent and reduce progression of deformity and functional impairment.

Plain radiography is the modality of choice for identifying osteochondromas (Figure 2.5.6). They have the appearance of a pedunculated or sessile lesion with well-defined margins that blends imperceptibly with host bone. They usually point away from the growth plate, towards the diaphysis and are typically located at the metaphysis. Size varies from 2–15cm. Signs of malignant change can be fuzzy margins of the cartilage cap and the presence of lucent zones within the lesion. MRI can be useful in assessing for malignant change; in particular, the cartilage cap can be observed for irregularity and size. Normally the cap thickness decreases with age, therefore in adults with a cap greater than 1cm and pain, malignant transformation should be suspected.

Macroscopically, osteochondromas resemble cauliflowers, with a stalk that is continuous with the intramedullary bone topped by a cartilage cap. This cap is of varying thickness but reduces with age. Microscopically, osteochondromas derive from aberrant cartilaginous growth plate tissue which proliferates separate to the growth plate. This aberrant tissue remains subperiosteally where it may disappear or proliferate as an osteochondroma perpendicular to the growth plate. The cartilage cap is organized into a structure similar to the epiphysis and during skeletal growth the base of the cap undergoes enchondral ossification. Signs of malignant progression are an irregular surface to the cap with differing sizes of cartilage lobules and invasion into surrounding soft tissue. Malignant progression is usually to a low-grade chondrosarcoma.

Treatment for an asymptomatic lesion is non-operative, the patient should be informed to return to clinic if the lesion becomes larger or painful. Resection of osteochondromas should be performed if it is symptomatic, displays suspicious features of malignant transformation, causes functional impairment, cosmetic deformity, or damages surrounding structures. During resection all of the cartilage cap and perichondrium should be removed to prevent recurrence.

Malignant transformation of solitary osteochondromas is rare and occurs in less than 1% of patients. However, patients with multiple lesions have a higher rate of malignant transformation of 5%. The local recurrence rate after resection of osteochondroma is less than 2%.

Enchondromas are benign cartilaginous neoplasms, and usually present as solitary lesions in intramedullary bone. Enchondromas account for 12% of benign bone neoplasms. Solitary enchondromas are more common in patients aged 20–40 years and are often incidental findings; multiple enchondromas tend to affect 0–10 year olds. There is no sex predilection. Common sites for solitary lesions are the hands (especially the proximal phalanx) and feet, followed by humerus, femur, pelvis, and tibia.

Enchondromas are often incidental findings as they are generally asymptomatic; a growing lesion in the skeletally mature patient or pain in the absence of a fracture are generally highly suspicious for malignancy and should be further investigated.

Multiple enchondromas can also occur in three distinct disorders:

Plain films reveal a long, oval, lytic lesion in the intramedullary canal; calcification within the lesion is common, often appearing as rings and arcs (Figure 2.5.7 and 2.5.8). Enchondromas tend to occupy the diaphyseal region in the short tubular bones and the metaphyseal region in longer bones. It can be difficult to distinguish between enchondroma and low-grade chondrosarcoma on plain x-rays; cortical breakthrough, soft tissue mass, deep endosteal scalloping of the cortex, location in the axial skeleton, and size greater than 5cm generally guide towards chondrosarcoma. CT scan is helpful to determine any endosteal cortical erosion; this modality is used at skeletal maturity to provide a baseline for future comparison.

Macroscopically, enchondromas are well-circumscribed lesions with discrete cartilage lobules measuring up to 1cm. Again they are difficult to tell apart from low-grade chondrosarcomas. Cytology is not helpful in determining between the two. Microscopically, enchondromas consist of multiple hyaline cartilage nodules separated by normal marrow with lamellar bone encompassing it, whereas low-grade chondrosarcoma consists of a mass of cartilage commonly permeating the marrow and trapping lamellar bone on all sides.

Solitary enchondromas do not require surgical intervention but should be followed-up to monitor for malignant progression. Enlarging lesions in the skeletally mature or symptomatic patients should have intralesional curettage. Where malignant transformation is suspected, surgical treatment will vary from curettage to en bloc resection depending on the histological grade. Patients with multiple enchondromatosis should be monitored closely with regular follow-up clinics observing for symptomatic changes and cortical destruction in the lesions. Like solitary lesions, treatment depends on the histological grade of the tumour. Adjuvant therapy is not necessary.

Malignant transformation of solitary enchondromas is rare (less than 1%). In patients who have had intralesional curettage for solitary lesions, recurrence rates are less than 5%. In multiple enchondromatosis, malignant transformation can occur in up to 25% of patients.

Langerhans cell histiocytosis (LCH) is a very rare disorder characterized by proliferation of Langerhans cells and mature eosinophils in any tissue of the body. The most commonly affected sites are the skeleton (70%), skin, and lymph nodes; occasionally the liver, spleen, lungs and bone marrow are involved. LCH affects males more than females (2:1) and is more common in children younger than 15 years old; it has an unknown aetiology.

LCH has been divided into three distinct groups ranging from mildest to more severe:

Imaging studies alone are rarely enough to be diagnostic, and frequently biopsy is needed. Lesions can vary from difuse osteopenia to focal, sharply-defined lesions. The classic radiographic findings of vertebra plana are enough to make a presumptive diagnosis without need for biopsy (Figure 2.5.9). They include: vertebral collapse, maintenance of disc spaces, lack of extraspinal spread, and lack of soft tissue mass. Respiratory involvement can be evident on plain films acutely as interstitial infiltrate in the mid zones and lung bases; older lesions show a honeycomb appearance. Patients with an apparent solitary lesion should be screened to rule out any further lesions with a bone scan.

Microscopic features can vary with different proportions of eosinophils and histiocytes from one lesion to another. Criteria to form a diagnosis are at least two of either: proliferation of eosinophilic histiocytes on light microscopy, demonstration of X bodies by electron microscopy, positive OKT6 immunological marker, or immunohistochemical reactivity for S100 protein.

Solitary bone lesions can resolve spontaneously or after biopsy. Other treatment modalities that are used include injection of steroid and curettage with or without internal fixation with grafting. Cutaneous lesions respond well to PUVA (psoralen plus ultraviolet) treatment. Treatment of more widespread disease is with chemotherapy, a regimen of moderate dose methotrexate, prednisolone, and vinblastine is used.

Patients should be followed up for at least 5 years as even in isolated LCH recurrences can occur in as many as 10%.

Signs of a poor prognosis include: young age at the onset of disease, involvement of multiple organ systems, rapidity of disease progression, and especially liver, respiratory, or haematological dysfunction.

It is a rare and distinct tumour, accounting for approximately 4% of primary benign bone tumours composed of immature chondroblasts within a scanty chondroid matrix. Chondroblastoma is located in epiphyseal secondary ossification centres. The tumour does not heal spontaneously, can behave aggressively, and has a low but real risk of metastasizing; surgical removal is required.

Most patients are males, with a peak incidence in the second decade of life. The commonest presenting symptom is pain with local tenderness and loss of function.

Examination usually reveals local tenderness, and can show reduced range of motion or fixed deformity in the joint adjacent to the lesion. Because of the lesions’ proximity to joints, patients are commonly misdiagnosed as having intra-articular pathology such as labral or meniscal tears. When the proximal tibia or distal femur is involved an effusion may be detected in the adjacent knee joint.

Chondroblastoma most commonly occurs in the distal and proximal femur, the proximal humerus, and the proximal tibia.

Investigation with x-ray radiography should be performed, and chondroblastoma has a distinctive radiological appearance as a well demarcated lytic lesion with a thin sclerotic margin of increased bone density, almost always confined within the bone cortex (Figure 2.5.10). Occasionally the tumour breaches the cortex or has ill-defined margins; cortical destruction may be seen in aggressive tumours. Tumours are confined in epiphysis in 40% of cases; in the remainder of cases the tumour extends to the adjacent metaphysis.

MR and CT may be used to view the extent of the tumour and support the differential diagnosis. Because of the risk of metastasis, screening chest radiographs/CT scan should be performed on presentation.

Central chondrosarcoma, chondroma, and low-grade chondrosarcoma resemble chondroblastoma and radiological and clinical examination does not give a definitive diagnosis; histological examination of surgical specimens is required.

Macroscopically, chondroblastoma is most often contained by the periosteum, forming a surrounding sclerotic rim. Tumours can invade the adjacent joint, often through intra-articular ligaments such as the ligamentum teres or cruciate ligament. Chondroblastomas are lobulated with pink haemorrhagic areas of soft tissue interspersed with grey-blue chondroid tissue. The chondroid tissue may or may not be calcified.

Microscopically, chondroblastoma has distinctive pathological features. The predominant cell type is round or polygonal chondroblasts with eosinophilic cytoplasm and grooved coffee-bean like or indented nuclei, together with multinucleated osteoclast-like giant cells.

There is no accepted standard treatment for chondroblastoma. Curettage, either alone or with cryotherapy, or packing the cavity with bone graft or polymethylmethacrylate has been described.

En bloc resection with reconstruction may be necessary when there is extensive aggressive disease and intralesional excision would leave a large bony defect.

Radiation is not used due to the tumour’s low sensitivity to radiation and possibility of inducing a secondary sarcoma.

Successful radiofrequency ablation of chondroblastoma under CT guidance in locations which are challenging to access surgically, such as the femoral head, provides an alternative therapeutic option, but care must be taken to avoid damage to adjacent hyaline cartilage.

Eighty to 90% of patients with chondroblastomas treated with curettage make a full recovery without recurrence or metastatic disease. A proportion of patients are left with residual pain and limited range of motion. Although a relatively high rate of recurrence has been reported, ranging from 8–21%, this may reflect inadequate treatment and re-curettage usually results in cure.

The rate of metastasis ranges from 0.8–6% and typically involves the lungs. Metastatic lesions have the same benign histology as the primary tumour. Malignant transformation, a controversial entity, is thought to occur in roughly 1% of chondroblastoma and is often resistant to resection. These patients have a poor prognosis.

Giant cell tumour of bone (GCTB) is an aggressive and potentially malignant tumour that has been described as the most challenging benign tumour of bone. Its natural history varies widely, ranging from local bony destruction, local recurrence, to distant metastasis and malignant transformation.

GCTB accounts for 5% of all primary bone tumours and occurs most frequently in the distal femur, proximal tibia, proximal femur, proximal humerus, and the distal radius. Fifty per cent of lesions occur around the knee. It is slightly more common in females (ratio 1.5:1), usually presenting in the third and fourth decade of life. GCTB is more common in Asia and China, accounting for up to 20% of bone tumours, where it is also more common in men than women.

GCTB is a mesenchymal tumour originating from a neoplastic mononuclear fibrotic stem cell which coordinates the recruitment and activation of osteoclast-like giant cells, resulting in bone destruction.

Patients most often present with pain, local swelling, and warmth. Pathological fracture is the initial presenting feature in 15% of cases and is inevitable if presentation is delayed. Symptoms may be present for several months before presentation, by which time in one-third of cases the tumour exceeds half the bone diameter, has destroyed the cortex and reached the subchondral region.

The radiographic appearance of GCTB is of a purely lytic radiolucent lesion without matrix calcification located eccentrically in the epiphyseal region of the bone but often extending to the metaphysis (Figure 2.5.11). Most lesions have a blurred interface with surrounding bone. Cortical breakthrough and periosteal reaction may be seen. MRI has the benefit of illustrating soft tissue involvement. CT scanning can demonstrate mineralization and bony absence in the lytic region.

Because of the risk of metastasis, chest x-ray should be performed on diagnosis.

Grossly, GCTB is a lytic lesion, with the affected part of the bone expanded and the cortex thinned, or bridged in advanced cases. GCTB is typically red-brown due to haemorrhage, spongy, and friable. Cystic cavities, which may be filled with blood, are often seen. Microscopically GCT results from neoplastic growth of undifferentiated mesenchymal cells of bone. There are two predominant cell types, multinucleated giant cells and mononuclear stromal cells, typically uniformly arranged.

The histological composition of GCTB is complex and shares similarities with other bone lesions which contain multinucleate osteoclast-like giant cells; aneurysmal bone cyst, chondroblastoma, giant-cell rich variants of osteosarcoma, and osteoid osteoma.

The surgeon must decide on three variables for treatment of GCTB: 1) whether to use intra-lesional curettage or en bloc resection; 2) whether to use adjuvant therapy, such as phenol; and 3) what material to use to fill the defect.

The morbidity of en bloc resection makes it inappropriate for most GCTB, which are most often in the epimetaphyseal region, where en bloc resection would involve the articular surface. Therefore, the standard treatment for most cases of GCTB is intralesional curettage with/without adjuvant therapy and filling with bone cement (polymethylmethacrylate, PMMA). Adjuvant therapy, such as phenol, hydrogen peroxide, and cryotherapy, aims to remove microscopic tumour that may remain after curettage through their thermal (liquid nitrogen, PMMA) or chemical (hydrogen peroxide, phenol) toxicity.

The increased understanding of the molecular pathogenesis of GCTB as an osteoclastic resorptive lesion has led to the trial use of bisphosphonates, which are antiosteoclastic, as an adjuvant therapy, with promising results.

En bloc resection is generally reserved for large, aggressive GCTB which have destroyed the cortex usually with a fracture. Tumours of the proximal femur and distal radius have a higher risk of local recurrence due to the difficult anatomy

Radiation is an alternative when tumour resection is not possible or incomplete, e.g. in the pelvis, sacrum, or vertebra, and resection will result in neurological or functional morbidity. However, it is important to remember that radiotherapy increases the risk of malignant transformation.

The overall outcome for GCBT in large series is a disease-free survival of greater than 85%. Although recurrence is not fatal in most cases, the morbidity after multiple procedures and rate of secondary arthritis can be high.

More mitotically active lesions are more likely to recur. Recurrent lesions are more likely to develop possibly life-threatening metastasis and patients with recurrences should, as part of the staging process, have a CT scan of the chest.

Malignant transformation is seen in less than 10% of cases but has a poor survival. Pulmonary metastasis is rare, affecting 2–3% of patients with benign GCBT. The pathology of tumours that metastasize is identical to those that do not. Survival is not compatible with persistent pulmonary lesions and nodules should be removed surgically. When this is not possible, whole lung radiotherapy is recommended. Malignant lesions should be treated like sarcomas.

Follow-up with radiographs of the primary site and chest should be for at least 5 years, as cases of recurrence and metastasis have been reported many years after treatment.

Osteoblastoma accounts for less than 1% of all primary bone tumours. It is benign, although rare variants, termed malignant osteoblastoma or aggressive osteoblastoma exist. The term osteoblastoma is reserved for tumours greater than 1.5cm in diameter; those smaller than 1.5cm diameter are termed osteoid osteoma. Males are twice as likely to be affected as females and 80–90% of patients are under the age of 30.

Osteoblastoma can affect any bone but the most common site is in the posterior elements of the spine, which accounts for approximately one-third of all osteoblastomas. Long tubular bones are the second most common site (34% of cases, most often the femur, tibia, and humerus). Tumours in long tubular bones usually occur in the diaphysis or metaphysis, and rarely in the epiphysis. Tumours in the spine almost always involve the posterior elements, and may involve the vertebral body as well.

Almost all osteoblastomas present with pain, which is generally mild but progressive. Other common complaints are gait disturbance, swelling, warmth and tenderness. The pain may not respond to non-steroidal anti-inflammatory drugs, unlike osteoid osteoma. When found in the spine, paresthesiae, paraparesis, or scoliosis may be the presenting complaint. Presentation is on average 6 months to 2 years after onset of symptoms. Less often, osteoblastoma presents with atypical clinical features, such as weight loss, night pain, epistaxis, tooth impaction, and aspirin overuse.

Radiological features are non-specific and include osteosclerosis, osteolysis, cortical thinning, and expansion of bone.

Osteoblastomas in the spine are well-defined expansile osteolytic lesions that are variably calcified or ossified, most often arising from the posterior elements (Figure 2.5.12). Long bone osteoblastoma on radiographs show cortical expansion, sometimes extending to cortical destruction, and often have a periosteal reaction. CT scanning is the most useful scanning modality for osteoblastoma, although it does not provide a definitive diagnosis.

Osteoid osteomas on plain radiographs can be classified as cortical, subperiosteal, or medullary in location. It is generally a round or oval well-circumscribed lesion with a radiolucent nidus.

Definitive diagnosis of osteoblastoma on radiographs alone is rarely possible; it is more likely that osteoblastoma will be included in a list of differential diagnosis, including chondroblastoma, osteosarcoma, ABC, chondrosarcoma, osteomyelitis, giant cell tumour.

Grossly, lesions are gritty, friable, and red to tan in colour. The tumour–bone interface is generally sharp, with a rim of reactive sclerosis. Microscopically the classic description of osteoblastoma is of long interanastomosing trabeculae of osteoid or woven bone undergoing varying degrees of calcification, some trabeculae are calcified, others are pink immature osteoid. Trabeculae are surrounded by a single row of osteoblasts. The intertrabecular stroma is a loose fibrovascular tissue with capillary proliferation, a few osteoclast-like giant cells, and spindle cells. Mitosis is seen in less than 10% of cells. Osteoblastomas are known for their range of histological appearances sometimes making differentiation between osteoblastomas and osteosarcomas difficult.

Osteoblastomas should be removed to prevent or arrest bone destruction and remove pain symptoms. Treatment is surgical and includes curettage ± grafting. For small osteoid osteomas, radiofrequency ablation has been used successfully. For larger lesions wide resection is indicated; this can be difficult with spinal lesions and therefore aggressive intralesional excision with fusion and internal fixation may be used instead. Following complete removal of the tumour recurrence is uncommon.

Periosteal chondroma is a slow-growing benign lesion found within or underneath the periosteum of tubular bones. It is usually situated in the proximal end over the metaphysis, but may occur anywhere along the diaphysis. The most common sites are the proximal humerus, proximal femur, and small tubular bones of the hands and feet, though the tumour can occur in almost any bone.

It is uncommon, representing less than 1% of all bone tumours, but is three times more common than periosteal chondrosarcoma. It is most common in men in the second decade of life, though can affect adults and children. Very occasionally multiple lesions are seen.

Pain and palpable swelling with local tenderness are the most common presenting symptoms, often with reduced function at the adjacent joint, although many lesions are non-painful and found incidentally on imaging performed for other reasons. One-third of cases are associated with a soft tissue mass, palpable swellings are non-tender and fixed to underlying bone. In contrast to enchondroma and osteochondroma, in which growth after skeletal maturity is cause for concern, continued growth of periosteal chondroma in adults occurs without malignant transformation and is not a worrying feature.

On plain radiographs periosteal chondroma is seen as a sharply marginated, radiolucent, shallow, semi-lunar cortical concavity adjacent to the cortex, with scalloping as it erodes the cortex, and overhanging edges (Figure 2.5.13). There is variable calcification or the cartilaginous matrix, seen in roughly 50% of tumours and a sclerotic reaction of the cortex, which remains intact underlying the lesion. Focal calcification or ossification is seen in approximately one-third of tumours. Periosteal new bone forms buttresses overhanging the edges of the tumour. Lesions can have a blurred indistinct margin, or be surrounded by a thin cortical shell. Pathological fractures have not been reported. MRI is useful in the diagnosis of periosteal chondroma, showing a lobulated lesion bordered by a hypointense rim, indicating an intact periosteum.

Radiographs are helpful in differentiating from osteochondroma, periosteal osteosarcoma, and periosteal chondrosarcoma. Periosteal chondrosarcoma is round and shows popcorn calcification, is usually larger than 5cm, does not have a sclerotic reactive bone rim on its cortical side, and is primarily exophytic. Osteochondroma does not have a rim of bone separating it from the medullary cavity. Osteosarcoma shows perpendicular spicules of calcification, not seen in periosteal chondroma. Other benign conditions that can involve the periosteum should also be considered.

Grossly periosteal chondroma is covered by a thin shell of periosteum, is embedded in, and erodes the cortex without invading the medulla cavity. The lesion is composed of white-grey or blue, firm cartilagenous tissue. Tumours range from 1–10cm, but are usually less than 6cm in greatest dimension.

Histological examination shows a benign appearing lesion composed of hyaline matrix arranged in a lobular pattern with no atypical cellular elements. The matrix is separated by fibrous connective tissue or lamellar bone. Invasion of the surrounding soft tissues is not seen.

Unusual and atypical findings can result in misdiagnosis so careful clinical and radiological examination is required to avoid a wrong diagnosis.

Asymptomatic latent lesions may be managed by observation—malignant transformation has never been reported. Painful lesions may be treated with by en bloc or marginal excision, or interlesional curettage. The most effective treatment is en bloc resection, which prevents recurrence. Where the functional morbidity from resection is high, curettage should be performed, although recurrences have been reported. Bone grafting may be used with a large defect. Very few incidences of recurrence are recorded, estimated as less than 5%.