Primary osseous leiomyosarcoma of humerus misinterpreted as aneurysmal bone cyst: A case report and literature review.

Rationale: Primary leiomyosarcoma of the bone (LMSB) is a rare aggressive sarcoma with limited treatment options. Histopathologic and immunohistochemical features are similar to their more common uterine and soft tissue counterparts. However, its broader spectrum of histopathologic features and rarity make diagnostic challenges.

Patient concerns: We present a case of LMSB in a 20-year-old female who presented with left shoulder aching pain for 3 months. An osteolytic intramedullary lesion was found in the left proximal humeral epi-metaphysis.

Diagnoses: Initial open biopsy showed a giant cell tumor of bone with aneurysmal bone cyst (ABC)-like changes. However, an open biopsy followed by extended curettage showed LMSB with ABC-like changes.

Interventions: Wide excision of the lesion and bipolar hemiarthroplasty followed by concomitant chemoradiation therapy was conducted. The mass was completely removed without significant problems.

Outcomes: Complete mass excision and symptomatic improvements were achieved, and no subsequent relapses were observed.

Lessons: The authors encountered a rare case of LMSB. Most occurrences are in the lower extremity and trunk, respectively. ABC-like changes in bone tumors can lead to misdiagnosis. In this case, the ABC-like changes developed from the underlying LMSB as a secondary alteration. A careful examination of the underlying bone tumor is crucial to avoid misdiagnosing it as ABC or exhibiting ABC-like changes. Moreover, there has been no case report of LMSB with secondary ABC-like changes in bone.