Abstract Background/Aims Autoimmune myositis typically presents with symmetrical proximal muscle weakness, elevated creatine kinase (CK), and a good response to corticosteroids. In elderly patients, atypical features such as asymmetrical involvement, normal inflammatory markers, and poor steroid response should prompt consideration of alternative diagnoses, including malignancy. Diffuse large B-cell lymphoma (DLBCL) involving skeletal muscle is very rare but can closely mimic autoimmune myositis. Methods We present the case of a 76-year-old woman with unilateral thigh swelling and elevated CK, ultimately diagnosed with DLBCL. Results A 76-year-old woman presented with a two-week history of painful swelling and weakness in her left thigh, along with a one-month history of general malaise and fatigue. Her medical history included aortic valve replacement, hypercholesterolemia, and a previously treated right thigh melanoma 20 years prior. There were no associated fevers, night sweats, or weight loss. There were no symptoms suggestive of connective tissue disease. No recent travel, medication triggers, or substance use were identified. Examination revealed a swollen and tender left thigh with reduced proximal power in the left lower limb (MRC 2/5). Other muscle groups were unaffected. There were no clinical signs of inflammatory arthritis or stigmata of dermatomyositis. Investigations revealed a raised creatine kinase (CK) of 1,039 U/L, with normal C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and white cell count. Infective myopathy screen was negative. LDH was elevated at 748 U/L. Doppler ultrasound excluded deep vein thrombosis. MRI showed soft tissue oedema in the left iliopsoas and adductors, suggestive of an inflammatory process but with no features of malignancy or lymphadenopathy. Given the atypical presentation, statins were discontinued, and an urgent muscle biopsy was arranged. She was started on oral prednisolone 20 mg daily, later increased to 40 mg due to lack of clinical improvement. Repeat CK remained elevated at 1,090 U/L. While investigations continued, her platelet count dropped from normal to 66. Autoimmune serology showed anti-centromere antibody positivity, but no clinical signs of systemic sclerosis. Myositis-specific antibodies were sent and are still pending. CT chest and MRI abdomen/pelvis were unremarkable. Muscle biopsy confirmed diffuse large B-cell lymphoma (DLBCL) with necrosis and infarction. PET-CT showed intense FDG uptake in the left thigh and iliopsoas, with subtle pleural thickening and retroperitoneal oedema. She was urgently referred to haematology and commenced on Pola-R-CHP chemotherapy. Conclusion Primary skeletal muscle involvement in diffuse large B-cell lymphoma (DLBCL) is rare and can mimic autoimmune myositis. In this case, asymmetrical thigh swelling, normal inflammatory markers, and poor steroid response raised suspicion, leading to a diagnostic muscle biopsy that confirmed DLBCL. Anti-centromere antibody positivity was likely incidental. This case underscores the importance of considering malignancy in atypical presentations and interpreting serology within clinical context to avoid diagnostic bias. Disclosure S. Baloch: None. B. Vasta: None.
Baloch et al. (Wed,) studied this question.