Periosteal Fasciitis With Unusual Radiologic Features Harboring a Novel STAG1::USP6 Fusion Gene

Area of Science:

• Oncology
• Molecular Pathology
• Radiology

Background:

• Periosteal fasciitis is a rare subtype of nodular fasciitis originating from the periosteum.
• Diagnosis can be challenging, requiring correlation between imaging and pathology.
• USP6 gene rearrangements are characteristic of nodular fasciitis and related lesions.

Observation:

• A case of periosteal fasciitis with metaplastic bone formation was incidentally detected during breast carcinoma screening.
• Radiologic examination showed a heterogeneous, enhancing lesion in the right femoral distal metaphysis, initially concerning for chondrosarcoma.
• Histological analysis revealed bland spindle cells with myofibroblastic and osteoblastic features, alongside immature bone and cartilage.

Findings:

• Molecular analysis identified a novel STAG1::USP6 gene fusion.
• This finding was crucial for accurate diagnosis of periosteal fasciitis.
• The discovery expands the known molecular profile of USP6-associated neoplasms.

Implications:

• Accurate diagnosis of periosteal fasciitis can be improved with molecular analysis.
• The identification of the STAG1::USP6 fusion broadens the spectrum of USP6-related tumors.
• This case highlights the importance of integrating radiologic, pathologic, and molecular data for diagnosing bone lesions.