Retroperitoneal Alveolar Soft Part Sarcoma Mimicking Paraganglioma: A Case Report

Alveolar soft part sarcoma (ASPS) is an uncommon soft tissue malignancy that rarely arises in the retroperitoneum. Its marked hypervascularity on imaging creates diagnostic confusion with paraganglioma, potentially leading to misdiagnosis. We present a case of a 33-year-old male patient with a seven-month history of a progressively enlarging left iliac fossa mass. Cross-sectional imaging demonstrated a 17×10×8 cm hypervascular retroperitoneal mass arising from the left iliopsoas region and displacing adjacent structures, with intense arterial enhancement, suggesting paraganglioma. Plasma metanephrines were normal. Surgical debulking was performed. Histopathology revealed characteristic organoid-alveolar architecture with nests separated by fibrovascular septa and pseudoalveolar spaces. Tumour cells exhibited abundant eosinophilic granular cytoplasm with PAS-D positive intracytoplasmic crystals. Immunohistochemistry showed strong nuclear TFE3 positivity whilst neuroendocrine, epithelial, renal, myogenic, and melanocytic markers were negative, confirming ASPS. This case highlights that retroperitoneal ASPS can convincingly mimic paraganglioma radiologically. Recognition of characteristic histomorphology and TFE3 immunoexpression is essential for accurate diagnosis. Surgical management and long-term surveillance are crucial given the propensity for late metastases.