The disease can affect patients at any age but has a predilection for males in their teens and young adulthood. At the cellular level, the tumor is identical to osteoid osteoma; both tumor types show rich vascularization, irregular osteoid with osteoblasts and often osteoclast-type multinucleated giant cells. Differentiation CP 55,940 between the two tumor types is based on size. Osteoid osteoma has a limited growth potential and seldom exceeds 1 cm in largest diameter. In contrast, lesions larger than 2 cm are not considered to have a restricted growth potential and are referred to as osteoblastomas. Osteoblastoma typically shows a non-infiltrative growth pattern and when resected with free margins recurrences are uncommon. The treatment is therefore based on surgery alone and the prognosis is excellent. However, there is a group of intra-osseous osteoblastic tumors that can be diagnostically challenging at the histopathological level. These tumors have been referred to as aggressive, epithelioid or malignant osteoblastoma. Currently, they are considered within the morphological spectrum of osteoblastoma, and have the same clinical behavior. A very rare subtype of osteosarcoma exists, so-called osteoblastoma-like osteosarcoma, which shares some morphological features with osteoblastoma, but clinically behaves like CK 666 conventional highgrade osteosarcoma. Osteosarcomas, including these rare multiple aberrations. Low-grade central osteosarcomas do not pose a histological differential diagnosis and are characterized at the genetic level by frequent gains of MDM2. In osteoblastoma the genetic findings are heterogeneous ranging from single balanced structural rearrangements to multiple and complex changes. No recurrent, tumor-associated aberration has been described. In the present study, we have applied cytogenetic and single nucleotide polymorphism array analyses on osteoblastomas in order to identify recurrent genomic aberrations of importance for tumor development. We have also re-analyzed previously published global gene expression data on osteoblastoma, in order to evaluate the possible impact of genomic alterations. The genetic mechanisms underlying osteoblastoma development are largely unknown and no obvious genetic difference between conventional and so-called aggressive tumors has been identified. Cytogenetic information is available from six published osteoblastomas and complex rearrangements are detected in some of them whereas others display simple karyotypes with few changes, irrespective of whether the tumors are termed conventional or aggressive. In the present study, we found few or no genomic changes in nine conventional osteoblastomas while the two aggressive tumors displayed heavily rearranged genomes. The most common finding was deletion of whole or parts of the long arm of chromosome 22, which was detected by SNP array and/or cytogenetic analyses in three of the cases. Loss of chromosome 22 and rearrangement of 22q12, respectively, were reported in one case each of the previously published osteoblastomas.