# Expanding the differential diagnosis of bone-forming tumors: a new entity characterized by a NIPBL::BEND2 fusion

## TL;DR

A new type of bone-forming tumor is identified, driven by a rare NIPBL::BEND2 fusion, expanding the known range of tumors associated with this genetic event.

## Contribution

The paper reports two new cases of low-grade bone-forming tumors linked to a NIPBL::BEND2 fusion, expanding the clinical spectrum of this rare genetic entity.

## Key findings

- Two new cases of bone-forming tumors with a NIPBL::BEND2 fusion were identified in patients with distinct clinical presentations.
- These tumors lacked features of high-grade osteosarcoma or phosphaturic mesenchymal tumors, suggesting a novel clinical entity.
- The findings suggest that low-grade bone-forming tumors can now be added to the range of phenotypes associated with the NIPBL::BEND2 fusion.

## Abstract

We report two bone-forming tumors driven by a newly recognized fusion. The first is a lesion in the distal radius of a 59-year-old female subject, which had been present for 10 years. Imaging showed an intramedullary tumor with an exophytic dorsal component and marked sclerosis, histologically best classified as low-grade osteosarcoma. The second was a destructive lesion in the left ulna of a 13-year-old female with a short history of elbow pain. Imaging showed a lytic lesion with subtle tumor ossification, histologically best classified as an atypical osteoblastoma-like tumor. A NIPBL::BEND2 fusion was found in both cases on whole genome sequencing. This fusion has been previously reported in three bone lesions, two of which were reported as phosphaturic mesenchymal tumors and the other exhibited features of a high-grade osteosarcoma. These newly reported cases were bone-forming, but histologically indolent, with no features of high-grade osteosarcoma, and did not demonstrate the typical features of a phosphaturic mesenchymal tumor nor evidence of tumor-induced osteomalacia. The World Health Organization has not established a specific name for tumors associated with a NIPBL::BEND2 fusion, and their biological potential cannot currently be predicted. These cases add to the expanding clinical and histological knowledge of this entity and illustrate that, in addition to the other rarely reported phenotypes, low-grade bone-forming tumors may also be caused by this exceptionally rare fusion.