Anaplastic large cell lymphoma mimicking a psoas abscess
Anaplastic Large Cell Lymphoma (ALCL) is a type of Non-Hodgkin lymphoma (NHL). Of the childhood malignancies, the Non-Hodgkin lymphomas are more common, often presenting at an advanced stage with peripheral lymph node and extra-nodal involvement and accompanied by B symptoms. Common extranodal sites include skin, liver, lung, soft tissue and bone. Here present the case of Anaplastic Lymphoma Kinase 1 (ALK1) positive ALCL mimicking a psoas abscess in a 6-year-old girl and contributing to a significant diagnostic dilemma.

Here present case of a 6 year old girl who was seen in hospital with iron-deficiency anaemia (microcytic anaemia with low serum ferritin) and a two-month history of low-grade fevers, intermittent night sweats, marginal weight loss, decreased appetite, lethargy and intermittent abdominal and back pain.
She was diagnosed with iron-deficiency anaemia with viral overlay. An assessment by a paediatric haematologist led to an outpatient bone scan and PET scan being performed. The PET scan revealed focal increased vascularity in the right pelvis suspicious for an inflammatory mass. Following this, an abdominal ultrasound and MRI were performed, demonstrating extensive inflammatory change surrounding the right iliopsoas muscles, and right iliac chain and inguinal lymphadenopathy.

Under the supervision of the infectious diseases team, the girl was commenced on a four-week course of antibiotics for a suspected psoas abscess. She represented to hospital 8 days later with reduced fevers and night sweats, but with new urge incontinence, incomplete bladder emptying and antalgic gait with pain on right hip flexion, extension and internal rotation. A diagnostic laparoscopy was performed to biopsy the mass about 3 weeks after presentation, along with cystoscopy and right ureteric catheter placement. The cystoscopy showed extrinsic deformation of the bladder. Upon laparoscopy, a moderately large peritoneal mass was identified abutting the right iliopsoas muscle, which encased the iliac vessels and displaced the right adnexae .

The mass was biopsied with the sample sent for histologic and microbiology testing. Histology confirmed the mass was a ALK1+ anaplastic large cell lymphoma, with reactive lymphadenopathy of the inguinal and femoral lymph nodes (no malignancy was seen in these nodes). The girl was subsequently commenced on chemotherapy ALCL99 protocol. A repeat MRI was performed nine months following commencement of chemotherapy which demonstrated complete resolution of the primary lesion. Now, 2 years since diagnosis, she remains event free.

In conclusion, clinicians should be aware of the rare possibility of a NHL presenting as an iliopsoas lesion in a child. A significant lesion of the iliopsoas muscle in a child identified on imaging, with limited signs of psoas involvement, may warrant an early biopsy.

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