RESUMO
INTRODUCTION: Myeloid sarcoma (MS) is a rare extramedullary tumor composed of malignant myeloid cells that most commonly arise in patients previously diagnosed with myeloproliferative disease. However, they can still occur in isolation and without bone marrow disease. CASE PRESENTATION: An 8-year-old girl who had a history of acute myeloid leukemia and was off treatment for four years presented to the clinic with a history of on and off left knee swelling and pain without any direct trauma to the knee over the last two years. Knee Magnetic resonance imaging (MRI) showed diffused joint effusion with proximal tibia focal edema. A diagnosis of juvenile rheumatoid arthritis was suspected, and the patient was started on treatment, but the problem did not resolve. Eventually, the patient underwent a repeat MRI and showed increased joint effusion with an increase in the focal edema. An open bone biopsy of the lesion was taken, and the histopathology showed sheets of primitive mononuclear cells positive for CD33 and CD117 and negative for CD34, myeloperoxidase, CD10, CD20, and CD68, indicating myeloid sarcoma. CLINICAL DISCUSSION: Histological examination and immunohistochemistry are the most important in diagnosing myeloid sarcoma. Previously, before the introduction of chemotherapy and stem cell transplant, such cases of proximal tibia MS were treated with surgical resection of the bone. However, chemotherapy with the possibility of an allogeneic hematopoietic stem cell transplant (alloHSCT) has changed the view of survival in such cases. CONCLUSION: Isolated proximal tibia myeloid sarcoma is a rare occurrence that can be misdiagnosed and lead to delayed treatment. Bone biopsy, Immunohistochemistry, and cytogenetic studies play a critical role in differentiating MS from other types of tumors.
