Lessons in clinical reasoning - pitfalls, myths, and pearls: shoulder pain as the first and only manifestation of lung cancer.

Objectives: Lung cancer is the leading cause of cancer-related death and poses significant challenges in diagnosis and management. Although muscle metastases are exceedingly rare and typically not the initial clinical manifestation of neoplastic processes, their recognition is crucial for optimal patient care.

Case Presentation: We present a case report in which we identify the unique scenario of a 60-year-old man with shoulder pain and a deltoid muscle mass, initially suggestive of an undifferentiated pleomorphic sarcoma.