Osteoblastoma is a rare benign bone tumor, accounting for less than 1% of all primary bone tumors.1 The most common anatomic sites of occurrence are the spine, femur, tibia, and mandible, with the pelvis being a rare location. Radiographically, it presents as a lesion greater than 1.5 to 2.0 cm with reactive sclerosis, new bone formation, and a mineralized internal matrix.2 Pathologically, the tumor presents with features similar to those of an osteoid osteoma but with the possibility of continued growth over time.3
Osteosarcoma is a primary malignancy of bone characterized by atypical-appearing stromal cells and osteoid matrix production that is most commonly located in the distal femur, proximal tibia, and proximal humerus.3,4 The similarities between osteoblastoma and osteosarcoma, both clinically and radiographically, can make the correct diagnosis challenging, and there have been reports of inaccurate pathological diagnoses.5 The transformation of an osteoblastoma to an osteosarcoma has been documented in small case series and case reports; however, it is rare and, to the authors' knowledge, has only been reported once in the ischial ramus but not in the tuberosity.6–13
A 12-year-old black boy initially presented to the authors' institution in October 2014 with reports of right hip pain and a limp that began after sustaining an injury in a sporting event 1 year earlier. He reported having pain at night but denied constitutional symptoms such as fevers, weight loss, or fatigue. All past medical histories and prior developmental history were unremarkable.
Initial radiographs showed an expansile, bone-forming lesion of the ischial tuberosity (Figure 1A). Subsequent magnetic resonance imaging of the pelvis with and without contrast showed an enhancing lesion within the right ischial tuberosity with expansion of the bone and significant surrounding edema, but no extraosseous extension or other aggressive features. The patient underwent an incisional biopsy of the lesion, with outside expert pathological opinion confirming an osteoblastoma. He subsequently underwent extended curettage of the lesion along with allogeneic bone grafting. The patient was followed routinely up to 7 months postoperatively without radiographic recurrence of osteoblastoma.
Preoperative anteroposterior pelvic radiographs showing osteoblastoma (A) and osteosarcoma (B) of the ischial tuberosity.
The patient was subsequently lost to follow-up. He returned to the authors' clinic 2 years after the index surgery with persistent right ischial pain and a firm, palpable mass. Radiographs (Figure 1B) and cross-sectional imaging showed exuberant bone formation in the ischium measuring 5×4×5.6 cm. An incisional biopsy was again performed given the increased growth of the lesion, with outside expert pathological opinion confirming a high-grade osteoblastoma-like osteosarcoma. Given the diagnosis, the pathology from the initial biopsy and extended curettage in 2014 was re-reviewed and did not show any evidence of osteosarcoma (Figure 2).
Specimens showing osteoblastoma (hematoxylin-eosin, original magnification ×4) (A) and osteoblastoma-like osteosarcoma (hematoxylin-eosin, original magnification ×10) (B).
The patient initially underwent standard staging studies, the results of which were negative for distant metastatic disease, and received high-dose methotrexate plus doxorubicin and cisplatin chemotherapy as per Children's Oncology Group Protocol AOST 0331. Local control was then performed with wide resection of the ischium, inferior pubic ramus, and posterior acetabulum with periacetabular reconstruction and constrained total hip arthroplasty (Triflange custom acetabular component, Freedom constrained acetabular liner, and Echo-Bimetric femoral stem; Biomet, Warsaw, Indiana) (Figure 3). Tumor necrosis was greater than 90% on the resection specimen, with negative surgical margins. The patient then completed standard adjuvant chemotherapy according to the same Children's Oncology Group protocol listed above. Seven months postoperatively, he was ambulating without assistive devices. Surveillance imaging continues to show no evidence of distant metastatic disease or local recurrence.
Postoperative anteroposterior pelvic (A) and lateral right hip (B) radiographs showing periace-tabular reconstruction and constrained total hip arthroplasty.
Osteosarcoma is a malignant bone-forming tumor that most commonly presents in the metaphyseal region of long tubular bones such as the distal femur, proximal tibia, and proximal humerus.4 Pelvic osteosarcomas tend to occur less commonly than osteosarcomas of the appendicular skeleton and are often thought to have a worse overall prognosis.14 There is a preponderance of occurrence in adolescent and young adult patients in the second decade of life, with a second peak incidence in older patients or in patients with preexisting Paget's disease or radiation exposure.1,15
Multiple histological variants of osteosarcoma have been previously described, with conventional osteoblastic and chondroblastic variants being the most common.15 Osteoblastoma-like osteosarcoma is a rare variant of osteosarcoma that has only been previously reported in small case series and case reports, making up only 1% of all osteosarcomas and histologically resembling osteoblastoma.8,16 Both tumors can present with varying amounts of lace-like osteoid production with spindled stromal cells with or without significant nuclear atypia or increased mitotic rate. The diagnosis is highly dependent on the bone–tumor interface, with osteosarcoma showing bone marrow permeation and osteoblastoma showing a distinct margin between the tumor and normal host bone.16–18
It is critical to establish an accurate diagnosis when dealing with bone-producing tumors, as the treatments can vary by histology. However, these tumors, whether benign or malignant, can often prove to be a diagnostic dilemma from a pathologic standpoint. Bertoni et al8 reviewed a series of 11 patients with osteoblastoma-like osteosarcoma, with 1 instance of an ischial location. In 9 of these cases, the initial diagnosis was believed to be an osteoblastoma, with an osteosarcoma only being recognized on recurrence. In those 9 cases, it was not clarified whether an osteoblastoma-like osteosarcoma was believed to exist on initial presentation and was only diagnosed as such on retrospective review, or whether a true de novo malignant transformation occurred from a benign osteoblastoma to an osteosarcoma.
In the current case, the pathology from the initial presentation was re-reviewed by an outside pathologist with expertise in bone sarcomas. No histological evidence was found for an osteosarcoma being present initially, supporting a true de novo transformation from a benign to a malignant entity. This transformation has been supported by previous reports.6–12 Lucas et al5 reviewed 306 cases of histologically confirmed osteoblastoma. Of these, 2 cases were found to have undergone malignant transformation to an osteosarcoma.
Long-term follow-up for patients who have been previously treated for osteoblastoma at any anatomic location remains important. Recurrence of an osteoblastoma at the initial site without malignant transformation remains more common1; however, being able to diagnose and treat in a timely fashion a tumor that has undergone malignant degeneration remains paramount. Multiple case reports, however, show a wide range of intervals (4 months to 5 years) between the treatment of an osteoblastoma and the diagnosis of an osteosarcoma.6–12 Transformation to an osteosarcoma can occur at variable times after an index presentation, with as long as 13 years being reported.19 A recommended duration of follow-up for osteoblastoma does not currently exist in the literature, leaving this to the experience and expertise of the treating physician.
This case supports the notion of malignant transformation of osteoblastoma to osteosarcoma and describes an anatomic location that, to the best of the authors' knowledge, has not been documented. This report highlights the importance of establishing an accurate diagnosis with bone-forming lesions, as the pathology can often be misleading if benign and malignant entities exist within the same specimen, and the need for long-term follow-up of patients with osteoblastoma to ensure malignant transformation does not occur.
- Zhang Y, Rosenberg AE. Bone-forming tumors. Surg Pathol Clin. 2017;10(3):513–535. doi:10.1016/j.path.2017.04.006 [CrossRef]
- McLeod RA, Dahlin DC, Beabout JW. The spectrum of osteoblastoma. AJR Am J Roentgenol. 1976;126(2):321–325. doi:10.2214/ajr.126.2.321 [CrossRef]
- Atesok KI, Alman BA, Schemitsch EH, Peyser A, Mankin H. Osteoid osteoma and osteoblastoma. J Am Acad Orthop Surg. 2011;19(11):678–689. doi:10.5435/00124635-201111000-00004 [CrossRef]
- Green JT, Mills AM. Osteogenic tumors of bone. Semin Diagn Pathol. 2014;31(1):21–29. doi:10.1053/j.semdp.2014.01.001 [CrossRef]
- Lucas DR, Unni KK, McLeod RA, O'Connor MI, Sim FH. Osteoblastoma: clinicopathologic study of 306 cases. Hum Pathol. 1994;25(2):117–134. doi:10.1016/0046-8177(94)90267-4 [CrossRef]
- Seki T, Fukuda H, Ishii Y, Hanaoka H, Yatabe S. Malignant transformation of benign osteoblastoma: a case report. J Bone Joint Surg Am. 1975;57(3):424–426. doi:10.2106/00004623-197557030-00028 [CrossRef]
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- Bertoni F, Bacchini P, Donati D, Martini A, Picci P, Campanacci M. Osteoblastoma-like osteosarcoma: the Rizzoli Institute experience. Mod Pathol. 1993;6(6):707–716.
- Görgün Ö, Salduz A, Kebudi R, Özger H, Bilgiç B. Malignant transformation of aggressive osteoblastoma to osteosarcoma. Eklem Hastalik Cerrahisi. 2016;27(2):108–112. doi:10.5606/ehc.2016.23 [CrossRef]
- Wozniak AW, Nowaczyk MT, Osmola K, Golusinski W. Malignant transformation of an osteoblastoma of the mandible: case report and review of the literature. Eur Arch Otorhinolaryngol. 2010;267(6):845–849. doi:10.1007/s00405-009-1172-8 [CrossRef]
- Grace J, McCarthy S, Stankovic R, Marsden W. Malignant transformation of osteoblastoma: study using image analysis microdensitometry. J Clin Pathol. 1993;46(11):1024–1029. doi:10.1136/jcp.46.11.1024 [CrossRef]
- Kunze E, Enderle A, Radig K, Schneider-Stock R. Aggressive osteoblastoma with focal malignant transformation and development of pulmonary metastases: a case report with a review of literature. Gen Diagn Pathol. 1996;141(5–6):377–392.
- Mayer L. Malignant degeneration of so-called benign osteoblastoma. Bull Hosp Joint Dis. 1967;28(1):4–13.
- Parry MC, Laitinen M, Albergo J, et al. Osteosarcoma of the pelvis. Bone Joint J. 2016;98-B(4):555–563. doi:10.1302/0301-620X.98B4.36583 [CrossRef]
- Bertoni F, Bacchini P. Classification of bone tumors. Eur J Radiol. 1998;27(suppl 1):74S–76S. doi:10.1016/S0720-048X(98)00046-1 [CrossRef]
- Bertoni F, Unni KK, McLeod RA, Dahlin DC. Osteosarcoma resembling osteoblastoma. Cancer. 1985;55(2):416–426. doi:10.1002/1097-0142(19850115)55:2<416::AID-CNCR2820550221>3.0.CO;2-5 [CrossRef]
- Bonar SF, McCarthy S, Stalley P, et al. Epiphyseal osteoblastoma-like osteosarcoma. Skeletal Radiol. 2004;33(1):46–50. doi:10.1007/s00256-003-0716-z [CrossRef]
- Oliveira CR, Mendonça BB, Camargo OP, et al. Classical osteoblastoma, atypical osteoblastoma, and osteosarcoma: a comparative study based on clinical, histological, and biological parameters. Clinics. 2007;62(2):167–174. doi:10.1590/S1807-59322007000200012 [CrossRef]
- Dorfman HD, Weiss SW. Borderline osteoblastic tumors: problems in the differential diagnosis of aggressive osteoblastoma and low-grade osteosarcoma. Semin Diagn Pathol. 1984;1(3):215–234.