Diagnosis
Diagnosis
Diagnosis: Alveolar Rhabdomyosarcoma (ARMS)
Discussion
ARMS is a high-grade malignant soft tissue tumor most frequently seen in children and adolescents and less common in adults > 45 years. It is the second most common type of rhabdomyosarcoma (RMS) after embryonal RMS. It often arises in the deep soft tissues of the extremities, but can occur in multiple sites including the head and neck, trunk, paraspinal region, perineum, and retroperitoneum. Early clinical presentation may mimic benign entities such as vascular malformations, cysts, or inflammatory masses. Usually presents as a rapidly enlarging mass.
Histologically, it is a highly cellular neoplasm with sheets and nests of primitive round cells. Fibrous septa create an alveolar architecture with central discohesion. Rhabdomyoblasts, multinucleated tumor giant and neoplastic cells with clear cell change might be seen occasionally. Mitotic activity is usually increased and variable tumor necrosis can be present. The solid subtype is composed of sheets of neoplastic cells, without septa or discohesion (as seen in our case).Usually ARMS are positive for desmin, with strong and diffuse nuclear myogenin expression and variable MyoD1. A PAX3-FOXO1 gene fusion is found in the majority of ARMS with a smaller subset characterized by PAX7-FOXO1 gene fusions. Fusion-positive ARMS carries a worse prognosis. PAX3–FOXO1 gene fusion typically confers a poorer outcome than PAX7–FOXO1. Fusion-negative ARMS behaves more like embryonal RMS
Differential diagnosis includes embryonal RMS, which is the most common type of RMS. It is more common in the younger age group in the head and neck and genitourinary system. Morphologically it is heterogeneous; may show spindle areas or myxoid stroma with more overt rhabdomyoblastic differentiation. Myogenin expression is less diffuse and it lacks FOXO1 translocations.
Spindle cell/sclerosing RMS affects infants, children and adults. Head and neck region is the most common site of involvement followed by the extremities. In the pediatric population, spindle cell/sclerosing RMS arises most often in the pretesticular region. Histologically it has cellular fascicles of spindle cells that have pale eosinophilic cytoplasm and blunted centrally located nuclei. It shows prominent hyalinization/sclerosis with tumor cells arranged in cords, nests and trabecular. Desmin is positive with focal expression of myogenin. It also lacks FOXO1 rearrangement.
Other small round blue cell tumors are also in the differential like Ewing sarcoma, lymphoma, neuroblastoma and melanoma.
As a follow up our patient completed adjuvant chemotherapy and radiation therapy and remains disease-free at 10-year follow-up.
This case highlights the importance of considering RMS in the differential diagnosis of even small, benign-appearing pediatric soft tissue masses. Accurate histopathologic assessment, supported by immunohistochemistry and molecular testing, is essential for establishing the diagnosis and guiding appropriate therapy.
Reference
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