Differential Diagnosis for AE1/AE3 and OSCAR Positive Retroperitoneal Tumor

Single Most Likely Diagnosis

• Metastatic Carcinoma: Given the positivity for AE1/AE3 (a broad-spectrum cytokeratin marker indicating epithelial origin) and OSCAR (a marker that can be positive in various carcinomas), the most likely diagnosis is a metastatic carcinoma to the retroperitoneum. This could originate from a variety of primary sites, including the lung, breast, gastrointestinal tract, or genitourinary system. The retroperitoneum is a common site for metastasis due to its rich lymphatic and vascular supply.

Other Likely Diagnoses

• Sarcomas with Epithelial Differentiation: Some sarcomas, such as epithelioid sarcomas or carcinosarcomas, can express epithelial markers like AE1/AE3. While less common than carcinomas, these tumors can present in the retroperitoneum and may stain positive for OSCAR.
• Primitive Neuroectodermal Tumor (PNET)/Ewing Sarcoma: Although primarily known for their neural markers, some PNETs/Ewing sarcomas can express cytokeratins and might be considered in the differential, especially if they exhibit epithelial differentiation.

Do Not Miss Diagnoses

• Lymphoma: While lymphomas are typically diagnosed with lymphoid markers, some cases, especially those with significant epithelial infiltration or specific subtypes like anaplastic large cell lymphoma, might express cytokeratins or other epithelial markers. Missing a diagnosis of lymphoma could lead to inappropriate treatment and poor outcomes.
• Melanoma: Metastatic melanoma can present almost anywhere, including the retroperitoneum, and can express a wide range of markers. Although AE1/AE3 and OSCAR positivity is not typical for melanoma, the clinical context and additional immunohistochemical stains (e.g., S-100, HMB-45) are crucial for diagnosis.

Rare Diagnoses

• Desmoplastic Small Round Cell Tumor (DSRCT): This rare, aggressive tumor often presents in the abdominal or pelvic cavity of young males. It can express epithelial, mesenchymal, and neural markers, including cytokeratins, making it a consideration in the differential diagnosis of retroperitoneal tumors with AE1/AE3 and OSCAR positivity.
• Biphasic Synovial Sarcoma: Although typically occurring near the joints of the extremities, synovial sarcomas can rarely present in the retroperitoneum. They are known for their biphasic pattern (epithelial and spindle cell components) and can express cytokeratins, among other markers.