Low-grade central osteosarcoma of the metatarsal bone: a clinicopathological, immunohistochemical, cytogenetic and molecular cytogenetic analysis. Low-grade central osteosarcoma (LGCOS) is a very rare low-grade malignant neoplasm that is often confused with a variety of benign fibro-osseous lesions. It rarely involves the small tubular bones of the feet. We present an unusual case of LGCOS arising in the third metatarsal bone of a 16-year-old boy. The radiographic appearance was suggestive of a benign lesion. An open biopsy was performed and the initial diagnosis was fibrous dysplasia. The patient underwent curettage of the lesion and packing of the bony defect with a synthetic bone substitute. Histologically, the curetted specimens consisted of spindle cells admixed with irregular bony trabeculae and osteoid. The spindle cells were fairly uniform with mild atypia, and cellularity varied from low to high. Immunohistochemistry showed that the tumor cells were focally-positive for cyclin-dependent kinase 4 and p53, but negative for murine double minute-2. The MIB-1 labeling index was 36.7% in the highest focus. Cytogenetic analysis exhibited the following clonal karyotypic abnormalities: 48,XY,del(6)(p11),add(8)(q24),add(12)(p11.2),+mar1,+mar-2. Spectral karyotyping demonstrated that marker chromosomes were composed mainly of chromosome 6. Metaphase-based comparative genomic hybridization analysis showed a high-level amplification of 6p12-p21 and gains of 8q21-q24, 10p15, 12q13-q15, and 16q23-q24. Based on these findings, the final diagnosis was revised to LGCOS and the patient was treated with an additional wide excision, followed by reconstruction with a free-vascularized osteocutaneous scapular flap. At 18 months of follow-up, the patient is well with no evidence of local recurrence or distant metastasis. Our case highlights the diagnostic difficulty of this tumor with limited tissue samples and the importance of immunohistochemical and molecular cytogenetic analyses in ambiguous cases.