Low-grade osteogenic tumor with secondary aneurysmal bone cyst: Distinguishing osteoblastoma from low-grade osteosarcoma in the setting of noncanonical molecular findings.

Low-grade osteogenic tumors in the appendicular skeleton can show overlapping radiologic and histologic features, especially when secondary aneurysmal bone cyst (ABC) change is present. Osteoblastoma (OB) with secondary ABC and low-grade osteosarcoma (LG-OS) with secondary ABC may be difficult to distinguish, and noncanonical molecular alterations such as KMT2D or ZRSR2 variants may further complicate interpretation. We present an 18-year-old male presented with 6 months of atraumatic hip/knee pain and progressive difficulty weight-bearing. Imaging demonstrated a proximal femoral intramedullary lesion with cortical thickening, impending breakthrough, and surrounding marrow/soft-tissue edema. The differential diagnosis included OB vs LG-OS. Two image-guided biopsies favored OB. The patient underwent wide resection with allograft prosthetic composite reconstruction; margins were negative, and overall pathology supported OB with secondary ABC. Recovery was uncomplicated aside from transient urinary retention. By postoperative week 8, the patient had progressed to full weight-bearing with minimal pain. CT chest demonstrated stable benign nodules under surveillance. The integrated radiologic, histologic, immunophenotypic, and molecular profile supports a low-grade osteogenic tumor with secondary ABC.