Abstract
BACKGROUND: Solitary extramedullary plasmacytoma (SEP) is a rare proliferative clonal plasma cell disease. However, its occurrence in the sternoclavicular joint region has not yet been reported. Owing to the complex anatomical structure of this area and the lack of specific clinical manifestations, early diagnosis is challenging, and it is often misdiagnosed as other common bone and joint diseases. CASE PRESENTATION: We present the case of a 62-year-old male patient with a 2-month history of progressive swelling and pain in the left sternoclavicular joint. Upon admission, elevated levels of inflammatory markers were observed. Blood Next-Generation Sequencing (NGS) indicated the presence of Pseudomonas, and the soft tissue culture of the sternoclavicular joint revealed the presence of Pseudomonas aeruginosa and Proteus mirabilis. Due to poor response to anti-infectious and anti-inflammatory treatments, malignancy could not be excluded. Biopsy was performed, the lesion was resected by thoracoscopy, and SEP was confirmed by pathological examination. The patient underwent local post-operative radiotherapy. After nine-months of follow-up, the patient's joint pain resolved. CONCLUSION: Sternoclavicular joint swelling and pain are observed in spondyloarthritis, SAPHO syndrome, infections, and tumors. Involvement of the sternoclavicular joint by SEP has rarely been reported. Timely pathological examinations are crucial for patients with an unclear diagnosis. Given that SEP may progress to multiple myeloma (MM) within five years, early diagnosis and timely intervention are critically important.