Bone and Soft Tissue Pathology
Case 5 -
K. Krishnan Unni
Click on each slide thumbnail image for an enlarged view
A 22-year-old man presented with pain in the lower back and
constipation. Roentgenograms revealed a mineralizing mass involving the coccyx.
Case 5 - Figure 1 - CT scan shows expansile lesion of coccyx with mineralization.
Case 5 - Figure 2 - MRI shows mass lesion expanding into soft tissue but still well demarcated.
Case 5 - Figure 3 - Bone-forming neoplasm juxtaposed to soft tissue. However, there is no permeation.
Case 5 - Figure 4 - Bony trabeculae lined with a single layer of osteoblasts. There is capillary proliferation between bony trabeculae.
Osteoblastoma is a benign bone forming neoplasm with
histologic features similar to those of osteoid osteoma but with potential for progressive growth.
Osteoid osteoma and osteoblastoma have many similarities, especially histologically as emphasized by
the term giant osteoid osteoma, which Dr. Dahlin used to describe the entity in the early 1950s.
However, the term osteoblastoma, which was proposed by Jaffe and Lichtenstein a few years later, became
accepted. Although there are many similarities between osteoblastoma and osteoid osteoma, there are
important differences and hence it is reasonable to keep them as separate entities. The most important
reason for this separation is the difficulty in distinguishing osteoblastoma from osteosarcoma
occasionally. The distinction between osteoid osteoma and osteosarcoma are, however, straightforward.
Osteoblastoma is predominantly a disease of childhood and adolescents. In the Mayo Clinic files, the age
of the patients range from 4 to 75 years; 80% of the cases occur in the first three decades of life. In
a study involving a large number of patients, Lucas and co-authors found an age distribution ranging from
6 monthss to 75 years. Seventy-five percent of these patients were below 25 years. Osteoblastoma has a
distinct predilection to involve males. The predominant symptom is pain, usually mild and
long-standing. Patients with tumors involving the spine frequently present with neurologic symptoms.
There have been rare examples, as described by Mirra, of osteoblastomas presenting with systemic
toxicity. Mirra described a patient with a lesion involving the proximal femur who presented with fever,
loss of appetite, cachexia, clubbing of the toes and fingers and extensive periosteal new bone
formation. The patient, had relief of symptoms as soon as the lesion was removed surgically.
Osteoblastomas can involve any portion of the skeleton. However, there are distinct differences when
they involve the appendicular skeleton compared to the spine and to the jawbones. In the appendicular
skeleton, the tumor tends to involve the diaphysis more commonly than many other lesions. Osteoblastoma
has also a peculiar tendency to involve the cortex. In the series reported by McLeod and co-authors,
about 60% of the lesions were cortical, whereas 40% were medullary. However, some other series, such as
the report from the Netherlands Tumor Registry, reported only 42% involving the cortex. Occasionally,
osteoblastomas can involve the periosteum. Osteoblastoma of the spine tends to involve the posterior
elements. In the series reported by McLeod and co-authors 62% of the lesions were in the posterior
elements, 24% involved both the body and the posterior elements and only 14% involved the vertebral body
exclusively. In the jawbones, osteoblastomas tend to involve the root of a tooth producing a sclerotic
mass. This has led to the term cementoblastoma in the oral pathology literature. The roentgenographic
features of osteoblastomas vary according to the type of bone involved. In the long bones, the interior
of the lesion tends to be loosened with varying amounts of ossification. In the series reported by
McLeod and co-authors, about two-thirds of the lesions were purely lucent, whereas the remainder showed
varying degrees of ossification. The margins of the tumors are usually well defined and distinct. Only
about one-sixth of the tumors show indistinct margins. Approximately half of the lesions show
considerable reactive sclerosis, as seen with osteoid osteoma. A central lucent nidus as in an osteoid
osteoma is seen only in about one-fifth of all osteoblastomas. Frank cortical destruction and periosteal
new bone formation may be seen. These features may suggest a malignant neoplasm in about 25% of the
Grossly, most osteoblastomas are received in the pathology laboratory as fragments and not as an
intact lesion. The lesions tend to be red, granular and lack the fish-flesh appearance usually
associated with sarcomas.
Osteoblastomas have a classic histologic pattern of anastomosing bony trabeculae, irregularly arranged
in a loose fibrovascular stroma. The tumors tend to be sharply demarcated and may show connection to the
surrounding cortical bone. The amount of bone and osteoid in the tumor is quite variable. Occasionally,
the lesion is quite heavily mineralized suggesting bony trabeculae. The bony trabeculae are always lined
with a single layer of osteoblasts. Between the bony trabeculae there is a loose fibrovascular
connective tissue. The osteoblasts may have abundant pink cytoplasm and round nuclei with prominent
nucleoli suggesting an epithelioid appearance. Although chondroid differentiation is unusual, the
presence of clear-cut cartilage does not rule out the possibility of an osteoblastoma. Although
osteoblastomas typically show bone formation throughout the lesion, occasionally, one finds little
clusters of osteoblasts without matrix formation.
The term malignant osteoblastoma was introduced by Schajowicz and Lemos to describe a lesion with
features of osteoblastoma but with the propensity to recur locally. Dorfman and Weiss used the term
aggressive osteoblastoma to describe the same entity. They suggested that osteoblastomas with a
predominant proliferation of epithelioid osteoblasts have a more aggressive clinical behavior.
It may be possible to make a diagnosis of osteoblastoma on fine needle aspiration. It is unusual to
find diagnostic material from osteoid osteomas on fine needle aspiration. However, because
osteoblastomas are more loosely arranged, one may obtain osteoblasts with varying amounts of matrix in
The differential diagnosis involves many entities. As indicated previously, osteoid osteomas and
osteoblastomas share many similarities, including similar roentgenographic features. Osteoid osteomas
never have a limited growth potential and the nidus never reaches a size of over 2 cm.
The roentgenographic appearance of osteoid osteoma is also quite typical with a lucent nidus
surrounded by sclerotic bone. Osteoblastomas rarely have this appearance. Osteoid osteomas tend to
involve the cortex of long bones, whereas osteoblastomas have a distinct predilection to involve the
spine. Microscopically, osteoid osteomas and osteoblastomas are identical. The most important
differential diagnosis from osteoblastoma involves osteosarcoma. It is usually easy to make this
distinction. However, there is a subset of osteosarcomas which simulate the appearance of
osteoblastoma. The term osteoblastoma-like osteosarcoma has been used for this entity. The most
important distinction between the two involves the margins of the lesion.
Osteoblastomas are extremely well circumscribed and one should not find foci of tumor invading
surrounding bony trabeculae. Osteoblastomas have a loose arrangement of the tissue, whereas
osteosarcomas are more densely cellular. If one finds bony trabeculae with many layers of osteoblasts
rimming them, a diagnosis of osteoblastoma is expected. However, on limited biopsy material, the
distinction between osteoblastoma and osteosarcoma can be almost impossible. Giant cell tumors
occasionally will have new bone formation. Osteoblastomas can have areas with a lot of giant cells.
Hence, the distinction can be somewhat difficult. It is unusual for osteoblastomas of long bones to
involve the epiphysis, whereas giant cell tumors typically involve the ends of long bones. In the spine,
giant cell tumors typically involve the body of the vertebrae, whereas osteoblastomas tend to involve the
posterior elements. Aneurysmal bone cysts can simulate the appearance of osteoblastoma because of
reactive new bone formation. Both aneurysmal bone cysts and osteoblastomas involve the posterior
elements of the vertebrae. However, in osteoblastomas there is no proliferation of spindle cells,
whereas aneurysmal bone cysts are characterized by a loosely arranged spindle cell proliferation.
The treatment of osteoblastoma is surgical. Most osteoblastomas do not occur after spindle
curettage. However, the location in the spine may make surgical removal sometimes difficult. If an
osteoblastoma recurs after surgical removal, the possibility of under-diagnosing an osteosarcoma should
- Dahlin, D. C. And Johnson, E. W., Jr: Giant Osteoid Osteoma. JBJS, 36A:559-572, 1954.
- Dorfman, H. C. And Weiss, S. W. Borderline Osteoblastic Tumors: Problems In The Differential Diagnosis Of Aggressive Osteoblastoma And Low-Grade Osteosarcoma. Semin Diagn Pathol, 1:215-234, 1984.
- Lucas, D. R., Unni, K. K., Mcleod, R. A., O'Connor, M. I. And Sim, F. H.: Osteoblastoma: Clinicopathologic Study Of 306 Cases. Hum Pathol, 25:117-134, 1994.
- Mcleod RA, Dahlin DC, Beabout JW: The Spectrum Of Osteoblastoma. Am J Reontgenol. 126:321-335, 1976.
- Mirra JM, Theros E, Smasson J, Cove K, Rao P: A Case of Osteoblastoma Associated with Severe Systemic Toxicity. Am J Surg Pathol. 3:463-471, 1979.
- Schajowicz F and Lemos C: Malignant Osteoblastoma. J Bone Joint Surg. 58(B):202-211, 1976.