Pseudopapillary Osteoblastic Tumor With Psammomatous/Dot-Like Calcification of the Jawbone: A Report of a Hitherto Undescribed Morphologic Variant of Osteoblastoma Supported by Molecular Analysis.

Osteoblastoma (OB) is a bone-forming tumor typically characterized by FOS rearrangements. When arising in the jawbone, the histologic features overlap with those of cementoblastoma (CB). We report the case of a 20-year-old man with a histologically uncategorized osteo-cementoblastic tumor in the jawbone. The tumor showed sheet-like or pseudopapillary structures with abundant calcifications, resembling psammoma body, cementum, or dot-like calcifications. Osteoid formation was minimal and the calcified materials displayed atypical features for OB, complicating definitive diagnosis. To elucidate the molecular basis of these unique features, we performed whole-genome sequencing and nanopore sequencing-based methylation analysis. These analyses confirmed the characteristic FOS rearrangement commonly observed in OB and revealed a novel fusion gene, FOS::FN1::FOS , which has not been reported previously. In addition, DNA methylation profiling confirmed clustering with OB, and genomic analysis demonstrated an almost flat copy-number profile, consistent with the typical features of OB. We hypothesize that this novel fusion gene, in combination with the unique anatomic site of the jawbone, may have contributed to the distinct histologic features. We propose this tumor as a hitherto undescribed morphologic variant of OB.