Acute Lymphoblastic Leukemia Presenting as Soft Tissue Mass

For 3 months, a 9-year-old boy had swelling of the left upper arm. An MRI scan obtained at another facility 1 week after onset showed extensive edema of the soft tissue at the midhumeral level. Laboratory results, including complete blood cell (CBC) count and Lyme titer, were normal.

Two weeks before presentation, the swelling had worsened and associated pallor, fatigue, and lower extremity pains had developed. A repeated CBC count was consistent with leukemia: white blood cell (WBC) count, 336,000/μL (with 96% blasts); hemoglobin level, 5.2 g/dL; platelet count, 36,000/μL. The patient was referred for further evaluation.

On examination, the child was pale, with splenomegaly but no hepatomegaly or lymphadenopathy. The left arm mass was firm and nontender, without inflammation. Bone marrow aspiration revealed hypercellular marrow, with more than 95% blasts. Flow cytometry of the bone marrow aspirate was consistent with precursor B-cell acute lymphoblastic leukemia (ALL). Fluorescence in situ hybridization analysis revealed clonal rearrangement of the MLL gene at 11q23 (present in 87% of cells). Spinal fluid revealed no cells.

Before treatment, a repeated MRI scan showed a large (15 X 4.5- cm) mass medial to the humerus infiltrating the biceps brachii, brachioradialis, and triceps muscles with extension into the subcutaneous fat. The ulna neurovascular bundle, brachial artery, and vein were all encircled by the mass. On T-1 pulse sequence, the mass was isointense to the muscle with a normal bone cortex. The bone marrow showed low signal intensity, which signified an infiltrative process. A T-1 fat-saturated sequence revealed high signal intensity of the bone marrow, related to an absence of normal fatty bone marrow. T-1 fat-saturated pulse sequence postgadolinium injection showed marked enhancement of the soft tissue mass and bone marrow.

Virtually the entire humerus was involved, with no cortical erosion (A). ALL is one of the most common malignancies in children, accounting for 30% of all childhood cancers.^1,2 This primary malignancy arises from bone marrow. It can occur in any age-group and is slightly more common in girls in the first decade of life, with a peak incidence between 2 and 5 years. Leukemic skin infiltrations can involve the epidermis, dermis, and subcutaneous fat. These lesions are seen in 1% of patients.³

Children with ALL usually present with cytopenia, fever, and bone pains.⁴ Although asymptomatic skeletal involvement may be seen in 40% to 60% of patients at presentation, extramedullary leukemic masses outside the CNS and testes are exceedingly rare.^5,6 The few that are seen typically involve the eyes, ovaries, kidneys, tonsils, mediastinum, masseter muscle, scrotum, bone, or paraspinal areas.⁶ Such extramedullary leukemic masses are almost always a manifestation of relapsed ALL and not the primary presenting feature. However, it is important to consider ALL in the differential diagnosis when evaluating patients with a soft tissue mass.

This patient received standard chemotherapy consisting of prednisolone, vincristine, daunorubicin, PEG asparaginase, and intrathecal methotrexate. Within a week, his WBC count decreased to less than 1000/μL, with no blasts on the blood smear, and flow cytometry showed no abnormal immature B-cell populations. These findings were consistent with remission.

An MRI scan obtained 6 days after therapy showed marked decreased enhancement of the soft tissue (B). On all sequences (T-1, T-1 fat-saturated, and fat-saturated with contrast), the size of the mass had decreased and normal muscle tissue was visible. However, the bone marrow signal was still abnormal, signifying persistence of the infiltrative process replacing the normal bone marrow. This patient will continue to receive close monitoring by the hematology/oncology department.

References:

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2. Franck P, Duffner U, Schulze-Seemann W, et al.Testicular relapse after 13 years of complete remissionof acute lymphoblastic leukemia. Urol Int. 1998;60:239-241.
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4. Jonsson OG, Sartain P, Ducore JM, BuchananGR. Bone pain as an initial symptom of childhoodacute lymphoblastic leukemia: association with nearlynormal hematologic indexes. J Pediatr. 1990;117(2, pt 1):233-237.
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