Teaching Case

A 46-year-old male presented with a 4-month history of left loin pain, with no history of fever, weight loss, or dysuria. No palpable mass was found on examination. Serum creatinine was 128 micro mol/L, CRP 11.4 mg/dL, ESR 23, and LDH 232. HIV and TB tests were negative.

Ultrasound was partly limited due to bowel gases however showed bulky hypoechoic left psoas muscle and mild fullness of the left kidney. Computed tomography was therefore performed that showed heterogeneously enhancing mass-like abnormality within the left psoas muscle causing its moderate bulkiness, extending to retroperitoneal and left paraortic regions causing lateral displacement of the left kidney. No calcification was seen. The adjacent vertebra and disc spaces were normal. No other abdominal lymphadenopathy, focal solid visceral lesions or splenomegaly was seen. A whole-body gallium-67 scan and SPECT/CT were performed that showed abnormal uptake only within bulky left psoas muscle. A CT-guided Tru-Cut®  biopsy was then performed.

Biopsy of left psoas abnormality revealed Primary B-cell lymphoma. Development of Non-Hodgkin Lymphoma (NHL) within a skeletal muscle is very rare, although extra-nodal sites containing lymphoid tissue can develop NHL [1]. Diffuse large B-cell lymphoma accounts for 50% of soft tissue NHL, and the most common type of skeletal muscle lymphoma as well [2]. We did not perform magnetic resonance imaging in our patient as there was no vertebral or disc abnormality detected on computed tomography. However, a few reports in the literature have described its high T1 and T2 signal intensity on MR imaging with diffusion restriction due to high cellularity [3]. Differential diagnoses include spontaneous psoas hematoma and psoas abscess. Spontaneous psoas hematoma is rare and is usually associated with anticoagulation therapy with a high rate of mortality. The MRI appearance of blood is typically changing over time [4]. Psoas abscess can be primary (due to hematogenous spread of infection from an occult source or direct extension from TB spondylitis) or secondary (Crohn’s disease is the commonest cause), is common in young and Asian population [5]. History of contact with TB person raised ESR, bony or chest findings, and laboratory tests are usually sufficient. Clinicians and radiologists should be aware of this entity and must include in their differential possibility of any muscle mass.

Written informed patient consent for publication has been obtained.