Malignant Fibrous Histiocytoma of Bone and Osteosarcoma

Introduction

Osteosarcoma is the most common primary malignant tumor of bone, excluding plasma cell myeloma. Approximately 75% of all osteosarcomas are of the classic or conventional type, and the remaining 25% comprise the osteosarcoma variants, which are the subject of this article.1 The variants are a heterogeneous group of osteosarcomas with a range of different imaging and behavioral features.

Pathophysiology

Osteosarcoma is a malignant mesenchymal sarcoma characterized by the direct formation of bone or osteoid by tumor cells. For further information on the individual pathologic characteristics of the osteosarcoma variants, the reader is directed to the References section.

Frequency

United States

The peak incidence of osteosarcoma occurs in the second decade of life, which corresponds to the maximal period of skeletal growth. The incidence of osteosarcoma in persons younger than 20 years is 4.8 cases per million population. Approximately 75% of these cases are conventional osteosarcomas. Frequency data for the individual osteosarcoma variants is difficult to calculate because many are rare tumors. The table below shows the relative percentages of the osteosarcoma variants.

Frequency of osteosarcoma variants in the United States Open table in new window

Tumor	Frequency, %
Telangiectatic	3.5-11
Parosteal	3-4
Periosteal	1-2
Gnathic	6-9
Small cell	1
Intraosseous, low grade	<1
Surface, high grade	<1
Secondary	5-7

Mortality/Morbidity

The overall prognosis for patients with osteosarcoma depends on the stage of the tumor at presentation. Without metastases, long-term survival is in the order of 60-85%.

• Telangiectatic osteosarcoma has been considered more aggressive than classic osteosarcoma, but studies of long-term survival after optimum treatment now indicate that the aggressiveness of telangiectatic osteosarcoma is similar to that of the classic type.
• Intraosseous low-grade osteosarcoma generally has a good prognosis.
• Gnathic osteosarcoma is less frequently associated with metastatic spread than is conventional osteosarcoma, but local disease recurrence is often problematic.
• The prognosis for intracortical osteosarcoma is unclear because of its rarity.
• Both small-cell and secondary osteosarcoma are generally associated with a poor prognosis.
• High-grade surface osteosarcoma has a prognosis similar to that for a conventional osteosarcoma.
• The prognosis for periosteal osteosarcoma is better than that for conventional osteosarcoma.
• The prognosis for a parosteal osteosarcoma is generally excellent.
• The prognosis for multicentric osteosarcoma is dire.

Race

Osteosarcoma occurs in all racial and ethnic groups, but its frequency is slightly greater in African Americans than in Caucasians.

Sex

Most variants have a sex distribution similar to that of conventional osteosarcoma. That is, they are slightly more common in males than in females. Gnathic osteosarcoma and intraosseous low-grade osteosarcoma are believed to show a more equal sex distribution, and some studies of parosteal osteosarcoma have suggested that the tumor is more prevalent in females.

Age

Conventional osteosarcoma has a wide age range, with a peak in the second decade. This peak is thought to be associated with the period of maximal intensity of skeletal growth.

• Most of the osteosarcoma variants show a similar age distribution, with the exception of intraosseous low-grade, gnathic, and parosteal osteosarcomas. These have a peak slightly later, in the third decade of life.
• A second, smaller peak in the distribution of osteosarcoma as a whole is seen in late adulthood. This second peak is principally due to the occurrence of secondary osteosarcoma at this stage of life.

Anatomy

Conventional osteosarcoma is most frequent in areas of high skeletal growth, especially the metaphyseal regions of the distal femur, proximal tibia, and proximal humerus. Most osteosarcoma variants follow a similar distribution, with the exception of gnathic (mandible and maxilla) lesions, intracortical lesions (rare but more typically diaphyseal), periosteal lesions (more typically diaphyseal), and secondary osteosarcomas. The last osteosarcomas frequently occur in the pelvis and proximal femur, often in association with Paget disease.

Presentation

Most osteosarcoma variants have presentations similar to that of a conventional osteosarcoma, with pain, swelling, and a palpable mass that may have been present for weeks or months. Parosteal osteosarcoma is notable for the presence of a mass that in some cases may have been present for years. Intraosseous low-grade tumors may have a presentation that is similarly prolonged.

Multicentric osteosarcoma may result from 1 of 2 processes. Multiple primary tumors may occur either synchronously or asynchronously. Alternatively, multicentric disease may occur from a single primary lesion with metastases to other skeletal sites at presentation. Regardless of controversies over which mechanism is more likely, when multicentric disease is initially present, the prognosis is poor.

Preferred Examination

Preferred modalities for evaluating primary disease are radiography, magnetic resonance imaging (MRI), and sometimes computed tomography (CT) scanning. Staging is always performed by using chest CT scanning to detect pulmonary metastases. Isotopic bone scanning is generally used to detect skeletal metastases or synchronous tumors, but whole-body MRI may replace this study.

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