Dr. Ernest "Chappie" Conrad  and Patrick Parenzin, PA-C , share a case in which clinical and radiographic considerations help distinguish a benign bone tumor from a malignancy.

History and Physical Exam

Distal tibia, AP

Image 1: Distal tibia, AP

A 17-year-old boy presents to the clinic due to increasing pain in the left lower leg for four weeks. He is a cross-country runner averaging 20 to 30 miles per week. Pain was of gradual onset in the distal tibia. Initially, pain was only with running and now occurs intermittently at rest after activity. He has been unable to run long distances for the past week due to the pain. He has no night pain and denies swelling, redness or warmth of the lower leg. He is afebrile, without report of chills or night sweats, and denies recent weight loss as well as changes in his appetite and energy levels.

Distal tibia, lateral

Image 2: Distal tibia, lateral

Physical exam shows a healthy teenage male with a nonantalgic gait. There is no swelling, redness or warmth of the lower leg. He is tender over the distal tibia and has pain with vibration from a tuning fork. (The tuning fork test is used to aid in the diagnosis of occult fracture and is positive when there is an increase in pain when the vibrating tuning fork is placed in contact with the affected bone.) He has full range of motion of the left ankle and knee.

Diagnostic Imaging

Initial films show a lucent, eccentric lesion with distinctive, scalloped sclerotic borders in the metaphyseal-diaphyseal region of the left distal tibia involving nearly 100% of the diameter of the bone.

Differential Diagnosis

 

  1. Nonossifying fibroma (NOF)
  2. Simple bone cyst/unicameral bone cyst (UBC)
  3. Aneurysmal bone cyst (ABC)
  4. Enchondroma
  5. Fibrous dysplasia
  6. Infection
  7. Osteoblastoma
  8. Osteosarcoma

Evaluation

Evaluating a bony lesion in a child should include several considerations:

  1. The age of the child: The likelihood of specific tumors (both benign and malignant) varies by age.
    • The most common bony lesions in children are the fibrous cortical defect and the related, larger, nonossifying fibroma. 1 These occur most commonly in the second decade of life (age 11 to 20 years). 2  
  2. Symptoms: Pain can be present in both benign and malignant bone tumors. Night pain is often seen with malignancy and should always prompt further evaluation. The sudden onset of pain may indicate pathologic fracture. Lesions located in the diaphysis (mid-shaft) with cortical erosion are more likely to cause pathologic fractures. (A pathologic fracture is a fracture through bone that has been weakened by a disease process such as infection or tumor).
    • In our patient, the distal tibia pain is primarily with activity and he denies night waking due to pain.
  3. X-ray evaluation: 3  
    • Location: The lesion's location in the bone (metaphysis, diaphysis or epiphysis) often helps to narrow the differential diagnosis.
      • The location of this particular lesion is not as helpful, as it is located in the metaphyseal-diaphyseal region. From our previously established differential, the NOF, bone cysts and infection are common in the distal tibial metaphysis, but NOF and cysts can also occur in the diaphysis, as can osteosarcoma.
    • Margin: What do the edges of the bone look like? Benign lesions usually have sclerotic margins, which demonstrate the host bone's ability to contain the tumor. By contrast, malignant tumors and ABCs are usually fast growing, not allowing the host bone time to react to the tumor by laying down new bone and essentially "walling it off." In these more aggressive lesions, there is often no distinctive bony margin.
      • This lesion has eccentric, sclerotic margins, making infection and osteosarcoma less likely. At this point, the differential has been successfully narrowed to two main diagnoses: the NOF and simple bone cyst. The sclerotic, scalloped margins of this lesion are typical of an NOF.
    • Density: What are the tumor's contents (i.e., calcified, osteoblastic, lucent, empty)?
      • This lesion is lucent, but demonstrates signs of "filling in" with bone (ossifying). This makes a simple bone cyst less likely as the diagnosis.
  4. Summary: An adolescent (second decade of life) patient with a distal tibial, metaphyseal-diaphyseal, lucent lesion involving 90% to 100% of the diameter of the bone with eccentric, distinctive sclerotic margins and increasing pain with activity, no pain at rest and reproducible pain with vibration from a tuning fork.

Diagnosis

Nonossifying fibroma with pathologic stress fracture.

Discussion

The NOF is a failure of ossification in periosteal-cortical bone development. The lesion is filled with fibrous tissue that has a 30 to 50% chance of ossifying over time. NOFs are frequently multiple, small and found incidentally in children under the age of 10 years. These smaller lesions are more commonly known as "fibrous cortical defects."

While NOF is the most common tumor in children, is generally found incidentally and is histologically benign, it can result in pathologic fracture. This is often seen when the size of the lesion is greater than 3 to 4 cm, involves greater than 50% of the diameter of the involved bone and/or involves diaphyseal cortical bone. Pathologic stress fractures in the distal tibia can initially be treated with cast immobilization for four to six weeks with a slow return to activities. Following with plain radiographs is all that is required, as further imaging (magnetic resonance imaging [MRI] or computed tomography [CT]) does not change the management.

Curettage and grafting is often indicated for large (greater than three cm) painful lesions, after a pathologic fracture or with a pathologic stress fracture that has failed multiple attempts at immobilization. CT is the imaging modality of choice for surgical planning purposes.

Most bone lesions, especially those that cause pain, require a six-to-eight-week follow-up to assure they do not enlarge. For large, asymptomatic NOFs, yearly radiographs are recommended.

Outcome

The patient was immobilized for four weeks in a short leg cast and then gradually returned to full weight bearing as tolerated. He progressively increased his mileage and was asymptomatic three months after coming out of the cast.

Further Discussion: Benign vs. Malignant X-ray Examples

As discussed earlier, there are general features of bony lesions that can assist one in formulating a differential diagnosis. The following are two examples highlighting other bone tumors for comparison.

Example 1: Osteosarcoma

Osteosarcoma, distal tibia, AP

Image 3: Osteosarcoma, distal tibia, AP

Osteosarcoma is the most common malignant bone tumor, with a peak incidence of 14 years. Osteosarcoma has features that separate it radiographically from most benign lesions, including:

Location

Osteogenic sarcoma (another term for osteosarcoma) is found most commonly in the distal femur and proximal tibia, but can be found in other bones, as seen here in the distal tibia. Osteogenic sarcoma is generally a metaphyseal lesion and can extend into the diaphysis.

Margin

No distinctive margins with substantial periosteal reaction.

Osteosarcoma, distal tibia, lateral radiograph

Image 4: Osteosarcoma, distal tibia, lateral radiograph

Density

Both osteolytic (dissolution of bone) and osteoblastic (bone forming) characteristics are seen, raising concern for malignancy. This is especially true with large osteoblastic lesions, as seen here.

Example 2: Simple Bone Cyst/Unicameral Bone Cyst

Simple bone cyst, proximal humerus, AP radiograph

Image 5: Simple bone cyst, proximal humerus, AP radiograph

The simple or unicameral bone cyst is a common lesion with unknown etiology and typical radiographic findings.

Location

Frequently found in the metaphysis of the proximal humerus and femur.

Margin

Well-defined margins and, in this example, a pathologic fracture that is bicortical. This is very common for a UBC of the proximal humerus.

Density

Cystic/lytic with thinned cortices, expansile, homogeneous in appearance with irregular septa. In this case the pathologic fracture has caused a fragment of cortical bone to fall into the cyst (arrow). This is called the "fallen leaf sign."

Summary

The differential for a bone tumor can be narrowed considerably if the age of the child, symptoms, location, margins and density are all considered. If the provider has concerns regarding a definitive diagnosis, the patient should be referred to an orthopedic specialist.

Bibliography

  1. Betsy M, Kupersmith LM, Springfield DS. Metaphyseal fibrous defects. J Am Acad Orthop Surg. Mar-Apr 2004;12(2):89-95
  2. Stahelli, Lynn (2006). Practice of Pediatric Orthopedics 2nd ed. Lippincott Williams & Wilkins.
  3. Stahelli L, Song K. Pediatric Othopaedic Secrets 3rd ed. Chapter 73: Initial Evaluation of Musculoskeletal Tumors by Ernest Conrad III (p423-426).

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