Benign Bone Tumors

Benign Bone Tumors

Topics covered in this section:
  • Unicameral Bone Cyst
  • Aneurysmal Bone Cyst
  • Osteochondroma and Multiple Hereditary Exostosis
  • Fibrous Cortical Defect and Non-Ossifying Fibroma
  • Osteoid Osteoma and Osteoblastoma
  • Chondroblastoma
  • Enchondroma


Unicameral Bone Cyst (UBC)

UBC is a benign fluid-filled cystic lesion, most commonly seen in the metaphyseal region of the long bones. While its etiology is unknown, it is believed that a venous blood flow obstruction leads to accumulation of fluid and cyst formation. UBC is usually diagnosed between ages 5 and 15, with a 3:1 male predominance.
 
Approximately 85% of the cases will present at time of a pathologic fracture. The proximal humerus and femur are the most common sites. UBC is a centrally located, well-defined, slightly expanded, lytic lesion with no periosteal reaction or soft-tissue mass. Pathologic fractures are usually incomplete and stable; they are characterized by the “fallen leaf” sign, where a small fragment of bone is “floating” in the fluid filled cavity.
 
Not all UBCs warrant treatment, as the natural history could evolve with healing over time. Lower extremity lesions, especially around the hip, often warrant intervention due to potential complications associated to the fracture. There are several treatment options including steroid or bone marrow injection, internal fixation, etc. The author’s preferred method is through a percutaneous aspiration and cystogram, followed by curettage, IM decompression and bone grafting. Although the healing rate at long-term follow-up (80%) is inferior to the initial results (~95% healing rate), ~100% healing can be achieved after repeat procedure.
 
Pathologic fractures tend to heal within 3-4 weeks, and while 10-15% of the cysts heal after a fracture, the majority persists. Recurrence is the most common complication of treating UBCs, especially among younger children (<10 yo) (~20% recurrence). Other complications include growth disturbance and angular deformities.
 

Aneurysmal Bone Cyst (ABC)

ABC is a benign but locally aggressive cystic lesion composed by blood filled spaces separated by connective tissue. Approximately 50% of cases occur in the 2nd decade, and while the metaphyseal region of long bones is the most common site, the posterior elements of the spine are often involved. There is no sex predilection and overall incidence is 0.15/ million.
 
ABCs can be primary lesions, or occur in association with other benign (e.g. chondroblastoma, giant cell tumor) or malignant (e.g. telangectatic osteogenic sarcoma) bone lesions. Most cases present with pain and swelling or mass. Pathologic fractures occur in up to 35% of cases. On imaging, this is an eccentric, septated, well-defined lytic, expansile (it may be expanded beyond the adjacent growth plate) lesion. Periosteal reaction may occur when there is cortical disruption. MRI is helpful for differential diagnosis and demonstrating the classic fluid levels (characteristic but not pathognomonic of ABC).
 
Percutaneous serial injections have been used with variable results. It is mostly indicated for axial lesions. Among some of the substances utilized are Ethibloc, histoacryl glue, alcohol, P32, doxycycline and others. Serial embolization is also an alternative. Surgical treatment includes open incisional biopsy and intraoperative frozen section for diagnostic confirmation, followed by intralesional tumor resection, high-speed burring, electrocauterization, +/- adjuvants such as phenol. The defect is grafted with allograft or synthetic bone filler.
 
Recurrences rates are around 15%. Juxta-articular and juxta-physeal ABCs can cause joint stiffness and growth arrest. Pathologic fractures will not lead to cyst resolution.
 

Osteochondroma and Multiple Hereditary Exostosis (MHE)

Osteochondroma is a benign osteo-cartilaginous tumor that arises at the physeal level and grows towards the metaphyseal area. It represents ~8% of all bone tumors and is one of the most common benign bone tumors (~35%). There is no sex predilection and diagnosis occurs during skeletal growth years. While most osteochondromas are an isolated lesion, approximately 1 in 50,000 children have multiple hereditary exostosis (MHE) which is an autosomal dominant syndrome characterized by the formation of multiple osteochondromas throughout the skeleton.
 
The radiographic appearance of an osteochondroma is pathognomonic; the exostosis shows continuation of the intramedullary bone into the lesion and continuous overlying cortices. The base of the lesion may be narrow (pedunculated) or broad (sessile). The bony part of the lesion is covered by a large cartilaginous cap better seen on MRI. Cartilaginous cap thicker than 2 cm after skeletal maturity may be concerning for malignant transformation.
 
Osteochondromas can lead to pain due to impingement on surrounding soft-tissue structures, at times (especially with MHE) lesions will lead to mechanical block of ROM, abnormal growth patterns with limb discrepancy and angular deformities. Symptomatic lesions are marginally excised. Inclusion of the cartilage cap and overlying periosteal in the resected specimen is important to reduce the chance of recurrence. Complex reconstructive surgeries are indicated on a case-by-case basis.
 
Most isolated asymptomatic osteochondromas should be left alone. The risk of malignant degeneration is likely less than 0.5%, but it is higher for patients with MHE (2-20%), interval growth or new pain after skeletal maturity are red-flag signs.
 

Fibrous Cortical Defect (FCD) and Non-Ossifying Fibroma (NOF)

FCD and NOF are benign fibrous-osseous lesions of unknown origin. It is debatable as to whether these are developmental or neoplastic processes. The exact prevalence of these lesions is unknown, but estimates are that they can be found in approximately 20% of all growing children. In approximately 20% of the cases, multiple lesions are found.
 
The appearance of these lesions on plain radiographs is typical; they are cortical-based, uni or multi-loculated, lytic lesions, extending into the intra-medullar cavity, surrounded by a sharp sclerotic border. While most are diagnosed incidentally, path fractures may occur. FCD are smaller (1 to 2 cm), while NOFS can be large (>4cm).
 
The natural history is of spontaneous healing (ossification) with skeletal maturity. Therefore, conservative treatment/ observation is usually all it’s needed. Larger lesions, particularly in weight-bearing bones and/or associated to pathologic fractures (acute or stress related) may warrant surgical treatment with curettage and bone grafting. It is important to disrupt the “halo of sclerosis” and access the intramedullary canal to avoid recurrence.
 

Osteoid Osteoma (OO) and Osteoblastoma (OB)

OO and OB are related lesion, essentially undistinguishable at a histological level. They are rare, benign, bone-forming tumors, with peak incidence in the 2nd decade.
 
The lesion is a small lytic central nidus, surrounded by significant amount of new bone formation. OO is smaller than <2 cm, while OB is larger. Radiographs will often fail to identify the nidus. Computed tomography (CT) is the gold standard for visualization. Bone scan is sometimes helpful to localize the lesion (particularly for spinal OO). MRI will often miss identifying the nidus due to significant amount of surrounding edema.
 
OO tend to present with a classic history of nighttime pain (waking up the child from sound sleep) that is readily alleviated with aspirin or NSAIDS. OB symptoms are not as typical nor tend to respond to NSAIDS as quickly. Both lesions are more common in the long bones, but they also have a tendency to involve the posterior elements of the spine and lead to painful atypical scoliosis.
 
Although OO may burn-out over time, they tend to be quite painful and may last for several years, so the current standard of care for OO is image-guided ablation techniques (radiofrequency, ultrasound, others) which are safe and have very low recurrence rate (<10%). Due to their size, OB are more often treated with formal curettage and grafting, however more recently, image guided ablation with multiple points of insertion has been shown to be successful.
 

Chondroblastoma

Chondroblastoma or “Codman’s tumor” is a benign, at times locally aggressive tumor that represents approximately 5% of all benign bone tumors. It involves the epiphysis of long bones, while the physis is still open. The majority of patients are in the 2nd decade, with a 2:1 predilection for boys. The most common locations are around the knee, hip and shoulder.
 
On imaging this is a well-defined lytic lesion within the epiphysis. At times, a sharp sclerotic margin is visualized, as well as intralesional calcifications. MRI demonstrates significant edema surrounding the lesion.  There is a reported 1-2% chance of lung metastasis.
 
Chondroblastoma causes pain around the joint, often worsening with activities. The non-specific nature of this presentation may lead to delayed diagnosis. The treatment is surgical with curettage +/- adjuvants and bone grafting; a transepiphyseal approach is preferred to avoid disturbing the adjacent physis and articular cartilage.
  

Enchondroma

Enchondroma is the most common benign bone tumor seen in the small tubular bones of the hands and feet, especially in the adult population. Although isolated enchondromas may be seen in the growing child, most of these lesions will present as part of the multiple enchondromatosis complex, or Ollier’s disease. Children tend to have one side of the body more involved than the other and can present with pain, swelling, fracture, and angular deformities1.
 
On radiographs it presents as a well-defined, lytic, intra-medullary lesion leading to cortical thinning, scalloping and expansion. There may be intralesional calcifications in a “popcorn” pattern.
 
Most isolated lesions will be small and not warrant surgical treatment. Sometimes biopsy is needed for diagnostic confirmation. For large (>3cm) and painful lesions, curettage and bone grafting is recommended. Children with Ollier’s disease may develop angular deformities, limb length discrepancy, pain and also have a higher risk of developing malignant degeneration of the primary lesions.
 
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