Primary lymphoma of the acromion presenting with isolated shoulder pain.

Primary lymphoma of the bone is an uncommon malignancy that most frequently involves the long bones while the flat bone involvement is rare. We report the case of a man in his 20s who presented with progressive shoulder pain without systemic symptoms. Imaging demonstrated an intramedullary lytic lesion of the acromion with cortical thinning and minimal soft tissue involvement. Histopathological examination confirmed germinal centre type diffuse large B-cell lymphoma. Fluorine-18 fluorodeoxyglucose positron emission tomography/CT demonstrated a solitary hypermetabolic acromial lesion without nodal or visceral disease, consistent with stage I primary lymphoma of bone. The patient was treated with standard immunochemotherapy followed by consolidative involved-site radiotherapy. Interval imaging showed a marked reduction in metabolic activity and lesion size with sclerotic transformation, indicating a favourable treatment response. This case emphasises the rarity of acromial involvement and clarifies contemporary staging and tissue diagnostic considerations in suspected primary bone lymphoma.