An 18-year-old man was referred to the Mayo Clinic for further evaluation of possible malignant tumor of the left pelvis. In 1994, the patient underwent upper and lower endoscopy because of weight loss and diarrhea, but the findings were not specific. During this evaluation, left hip pain developed. Abdominal computed tomography (CT) was performed, and enteropathic arthritis was diagnosed. The patient was examined by a rheumatologist and told that he had inflammation of the left hip.
Subsequently, he underwent physical therapy for 4 months and took sulfasalazine, without any sign of improvement. At that time (1 year after the initial presentation), magnetic resonance imaging (MRI) showed a mass in the left pelvis. A needle biopsy was performed, which was nondiagnostic, and tibe patient was referred to us for further evaluation.
Physical examination showed a significant limp and wasting of the left thigh muscles. Motion of the left hip was limited, with range of motion from 20° to 70° of flexion and essentially no abduction, adduction, or internal or external rotation. Pain was especially increased by hip motion. No other abnormalities were noted on physical examination.
Plain radiographs of the pelvis showed a mixed lytic and sclerotic lesion involving the left acetabular area, including the supraacetabular portion of the ileum and the lateral aspect of the superior pubic rami (Fig 1). The left femoral head was partially obscured by the lesion.
Bone scintigraphy showed increased uptake of radioisotope in the left acetabular region (Fig 2). Computed tomography and MRI of the pelvis showed an expansile mass involving the anterior aspect of the left acetabulum, with some associated cortical destruction and soft-tissue extension throughout the cortex (Figs 3-4). In addition, there was thickening of the adjacent obturator internis muscle. Staging studies were performed, including a bone s.can (negative except for increased uptake in the left acetabular area), chest radiography (negative), and CT of the chest, which showed two indeterminate nodules (3 mm and 5 mm) in the right lower lung.
Open biopsy was performed after a CT-guided biopsy failed to establish the diagnosis. Histologic review of the frozen specimen showed the tumor was an osteoblastoma (Fig 5). The patient underwent intralesional excision of the tumor and cauterization of the tumor bed with phenol; a bone graft from the anterior iliac crest was used for reconstruction of a portion of the anterior column of the acetabulum (Fig 6). Touch weight bearing using two crutches was recommended for 3 months and partial weight bearing for another 6 weeks.
At latest follow-up, 2 years after the initial surgical procedure, the patient was able to walk freely with a minimal limp. His pain had resolved completely, and he had normal hip range of motion. Radiographs showed incorporation of the bone graft (Fig 7). Computed tomography of the pelvis showed satisfactory articular congruity of the left hip joint (Fig. 8). Repeat CT of the chest showed disappearance of the lung nodules.
Osteoblastoma is a rare benign tumor of the bone that consists of abnormal bony trabeculae and osteoid, and is usually larger than that of osteoid osteoma.1,2 Osteoblastoma occurs approximately four times less frequently than osteoid osteoma, accounting for <1% of all primary bone tumors.2 It is also about 20 times less common than osteosarcoma, with which it may be confused.3
Benign osteoblastoma has been reported only sporadically in the literature, with the posterior elements of the spine and long bones of the appendicular skeleton affected in most cases.2"4 Benign osteoblastic tumors rarely occur in pelvic bones.5"9 Jackson et al7 found 8 (4.3%) of 184 osteoblastomas in the pelvic bones. The anterior pelvis is an uncommon location for osteoblastoma.6 Huvos8 found 364 reported cases, with only 1 case of the tumor in the ischiopubic region.
Fig 1 : AP radiograph of the left hip showing mixed lytic and sclerotic changes involving the left acetabular area.
Fig 2: Bone scintigram of the pelvis showing increased uptake of the radioisotope in the left acetabular region.
Fig 3: Coronal (A) and transverse (B) MRIs of the pelvis showing expansile mass of the anterior column of the left acetabulum.
Most osteoblastomas occur in patients between the ages of 10 and 25 years. In die largest series (306 cases) reported,4 the mean age at presentation was 20.4 years (range: 6 months to 75 years). The area most commonly affected is the vertebral column, including the sacrum.4 Sites less commonly reported include the pelvis, scapula, patella, ribs, and clavicle.2
Early diagnosis of osteoblastoma of the pelvis is a challenge. In many reported cases, the time from the onset of symptoms to the correct diagnosis is long.5,6,9 The low incidence of osteoblastoma in the pelvis may be one of the reasons for delayed diagnosis. Standard radiographs are used most frequently for diagnostic purposes. Most lesions are intracortical and show cortical expansion, making the tumor radiographically similar to an aneurysmal bone cyst.10,11 Unlike osteoid osteomas, which rarely have a maximal dimension >115 cm, osteoblastomas can be several centimeters in diameter.11
Technetium bone scan and CT are die best diagnostic aids.9 The characteristic hot spot on the bone scan and CT scan may lead to the correct diagnosis. The role of CT is to study the tumor morphologically, localize it, and assess its extent. Computed tomography is also essential in planning the most favorable surgical approach in difficult areas such as the acetabular zone.
In the case presented, the diagnosis of osteoblastoma was considered because of the site and radiographic features of the lesion, the patient's age, and clinical presentation. However, the clinical and radiographic features of the patient could suggest the diagnosis of other lesions in this location such as eosinophilic granuloma,2 chondroblastoma,12 aneurysmal bone cyst,10,13 giant cell tumor,2 aggressive osteoblastoma,14 and osteosarcoma.15
The diagnosis in this case was established by biopsy of the lesion. Whenever feasible, trocar biopsy is preferable. This usually is performed with CT and is especially indicated for iliac, supra-acetabular, and acetabular lesions. In any pelvic lesion, the biopsy tract should always be placed along the incision of a possible exposure of the pelvis so mat it can be excised completely at definitive surgery.
Gross pathologic study usually reveals a highly vascular lesion, typically red or reddish brown with a granular texture. The sclerotic reaction around the tumor usually is larger than that observed in osteoid sarcomas. The classic histologic appearance of osteoblastoma is a network of interlacing trabecular or woven bone lined with a variable number of osteoblasts in a fibrovascular stroma. The intertrabecular stroma contains a proliferation of capillaries and loosely arranged spindle cells without atypia.2,3 The amount of osteoid may vary within the same tumor, and there may be foci of chondroid matrix.16 Usually, the lesion is demarcated sharply from normal host tissue. The variable histologic features that overlap with those of osteosarcoma15 demonstrate the necessity of a good biopsy specimen and an experienced pathologist to interpret the histologic specimen.
The treatment of pelvic osteoblastoma may be complex because of the large size of the tumor and its tendency to recur after curettage.5 Moreover, the tumor may bleed profusely during curettage, which is an obstacle to complete removal of neoplastic tissue. Wide resection can be effective for osteoblastoma of the iliac wing, pubic rami, or ischium because the consequent functional impairment is not severe.5,6 However, the acetabular area, which is the most common location of pelvic osteoblastomas, presents major difficulties because of the need to completely excise the tumor but spare hip function.6
Fig 4: CT scan of the pelvis showing an expansile mass involving the anterior aspect of the left acetabulum with some cortical destruction and soft-tissue extension.
If the weight-supporting area of the acetabular wall is surrounded by the tumor, reconstruction with a bone graft is required after careful removal of neoplastic tissue. This was performed successfully in the case reported here. The two risks associated with mis operation are recurrence, if remnants of the tumor are left, and hip disability, if the joint is not reconstructed perfectly.
A conventional osteoblastoma has a reported recurrence rate of approximately 10% to 20%.4,7,17 Recurrence can develop up to 18 months postoperatively.4 The use of adjuvant treatment such as phenol or cryosurgery is recommended to improve the chance of success. As an adjunct to surgical treatment, arterial embolization may be used preoperatively to reduce hemorrhaging associated with osteoblastomas, especially for lesions that show secondary aneurysmal bone cyst degeneration.5
In the past decade, several reports have considered the aggressiveness and potential for malignant transformation of benign osteoblastoma. Mayer18 described a benign osteoblastoma of the acetabulum that recurred several times and transformed into an anaplastic osteosarcoma 9 years after the initial diagnosis. Schajowicz and Lemos19 described eight cases of aggressive osteoblastoma called "malignant Osteoblastoma." These tumors appeared to be locally aggressive and were separated from conventional osteosarcomas not only because of their peculiar histologic pattern but also because they had different clinical and radiologic features and a better prognosis.
Fig 5: Histologic features typical of osteoblastoma. Note the presence of irregular fragments of osteoid trabeculae in a loose fibrovascular connective tissue stroma (A) (hematoxylin eosin; original magnification x 100.) Numerous osteoblasts rim the osteoid and prominent stromal vascularity is noted (B). (hematoxylin eosin; original magnification ×200.)
Fig 6: Postoperative AP radiograph of the left hip joint after resection of tumor and reconstruction of the anterior acetabular column.
Fig 7: Postoperative AP radiograph of the left acetabulum showing incorporation of the bone graft.
The aggressive tumors in the cases reported by Mirra et al,20 Schajowicz and Lemos,19 Dorfman and Weiss,15 and Kenan et al14 did not metastasize despite a lapse of years before definite treatment was achieved. These tumors should be termed "aggressive osteoblastoma" rather than "malignant osteoblastoma," because of the prolonged clinical course and the lack of evidence of distal metastasis.14,21 Therefore, the treatment of choice appears to be further en bloc resection, which would avoid the morbidity associated with chemotherapy and ablative surgery.
Fig 8: Postoperative CT scan of the pelvis showing satisfactory articular congruity of the left hip joint.
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