DEFINITION AND PATHOGENESIS
- Variants of osteosarcoma that display low-grade histologic anaplasia coupled with a low biologic grade (metastatic rate in the range of 10%) are defined as a low-grade osteosarcoma
- The "sclerosing" variant of osteosarcoma has extremely innocent, "normalized" nuclei that may show a cytologic grade of 0, but are typically of biologic grade 2-3 (metastatic rate in the range of 40-50%)(cannot be defined as low grade)
IMPORTANCE
- When difficult to diagnose, lesions are often mistaken for a benign bone tumor
- Misdiagnosis is associated with an ?risk of transformation to a conventional, high-grade osteosarcoma with full metastatic potential
- <2% of osteosarcomas are low-grade lesions
- The overall incidence of biopsy-analyzed primary bone tumors is therefore 1% (about equivalent to the frequency of chondroblastoma)
- On average, patients are about one decade older than those with conventional osteosarcoma (70% 18-40 yrs of age)(wide range: 15-83 yrs of age)
CLINICAL FEATURES
- Pain and/or swelling (average duration of sxs 44 mos, ranging from 1-180 mos)
RADIOLOGIC FEATURES
- Almost any bone can be involved, but sites are predominantly in long bones (85%), esp LEs
- Most affect the metaepiphyseal region of the bone, in contrast to the more usual metaphyseal to metadiaphyseal location of conventional osteosarcoma
- The typical case is characterized by:
- Bone expansion caused by slow growth of the tumor
- Bone production in most instances, imparting a hazy to "ground glass" appearance similar to fibrous dysplasia
- Irregular cortical erosion, imparting a coarsely trabeculated pattern
- A generalized lack of symmetry or uniformity
- Unusual findings
- Pure intralesional lucency
- Densely blastic
- Surrounding host bone sclerosis
- Ominous periosteal reactions
- Osteochondroma-like nubbins
GROSS PATHOLOGY
- Most lesions are white and gritty, related to the production of fibrous and bony tissues
HISTOLOGIC FEATURES
- At least four histologic variants are noted:
- Fibrous dysplasia-like
- Nonossifying fibroma-like
- Osteoblastoma-like
- Chondromyxoid fibroma-like
- Diagnosis is made be finding one or more of the following:
- Subtle cytologic anaplasia and increased mitoses beyond the range of the benign lesion it mimics
- Abnormal radiologic patterns that are consistent with a low-grade malignancy but not with the benign histologic lesion it mimics
- Unexplained stubborn recurrences for as many as 12 years after the diagnosis of one of the four benign lesions mentioned previously, coupled with the eventual development of clear-cut anaplasia and/or metastases
- Most such cases with obvious malignant change are misdiagnosed as a benign lesion that has undergone malignant transformation rather than as a low-grade osteosarcoma with eventual change into a conventional osteosarcoma
DIFFERENTIAL CLINICOPATHOLOGIC DIAGNOSIS
- Fibrous dysplasia
- NOF
- Enchondroma
- UBC
- Chondrosarcoma
- Desmoplastic fibroma
- Chondromyxoid fibroma
DISEASE COURSE AND TREATMENT
- As long as the tumor is of low grade, the metastatic rate is low (about 10% or less)
- "The intramedullary counterpart of parosteal osteosarcoma"
- Wide resection (intralesional excision results in 100% LR)
- LR can be higher grade or even dedifferentiated
- 5- and 10-yr survival 90% and 85%
- Reconstruction of a 20cm tibial defect after wide resection using an Ilizarov technique has been described
SPECIAL CONSIDERATION
- MULTICENTRIC LOW GRADE OSTEOSARCOMA
- Has been reported
FIGURES
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REFERENCES
Barbara C, Tornetta P, III, Vigorita VJ, Zilles M, Etienne G: Leg pain in an 11-year-old boy. Clin Orthop 363:264-7,70-2,1999.
McCarthy EF, Tolo VT, Dorfman HD: Multicentric metachronous, low-grade sclerosing osteogenic sarcoma. Skeletal Radiol 16:592-6,1987.