case_images: PMC10324607_01
case_images: Coronal view of PET scan, showing the 6cm hepatic mass and no clear primary tumor to explain metastasis, December 2022.
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case_images: In outpatient, the patient underwent PET scan, which did not identify a primary site of malignancy ( Figure 1 ). Imaging of the patient's retroperitoneal adenocarcinoma and liver metastasis are summarized in  Figures 2 ,  3  and  1 , respectively.
case_images: Axial and sagittal view of MRI abdomen, showing a 5.4 x 5.1 x 4.1cm T1 and T2 hyperintense lesion with areas of nodularity and thick septation along the anterior edge, November 2021.
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case_images: f2
case_images: Imaging of the patient's retroperitoneal adenocarcinoma and liver metastasis are summarized in  Figures 2 ,  3  and  1 , respectively.
case_images: Axial view of CT Abdomen with contrast, showing a 6cm hypodense hepatic lesion, suspected to be metastatic in origin. Satellite lesion not pictured. December 2022.
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case_images: f3
case_images: Imaging of the patient's retroperitoneal adenocarcinoma and liver metastasis are summarized in  Figures 2 ,  3  and  1 , respectively.
case_images: Pathology findings. (A, B) Fine needle aspiration of abdominal cyst. Atypical cells with clear cytoplasm arranged in nests(*). Not shown are subsequent immunohistochemical stains on the cell block marking these cells as positive for CK7 and negative for GATA3, PAX8, and OCT3/4. (C) Macroscopic gross appearance of retroperitoneal cystic tumor. The specimen measures 8.5 x 6.2 x 4.5 cm, weighs 90 g and upon sectioning, reveals a multilocular cystic mass (*; 6.5 cm largest cyst size) with variably thickened walls. The cysts contain necrotic material and serous-mucoid fluid, with patchy areas of papillary excrescences. (D, E) Microscopic appearance of retroperitoneal cystic tumor. D. shows a low-power view of invasive adenocarcinoma ( ) with cystic changes (*). E. shows malignant epithelial cells with cytoplasmic vacuoles and foamy appearance. Not shown are extensive perineural invasion and subsequent immunohistochemical stains marking the tumor cells as positive for CK7, CK19 and negative for CK20, CDX2, GATA3, PAX8, OCT3/4, AFP, Glypican-3, and CD30. (F) Liver biopsy. Liver parenchyma ( ) positive for moderately differentiated adenocarcinoma (*). Not shown are subsequent immunohistochemical stains marking the tumor cells as positive for CK19 and negative for CDX2, PAX8, AFP, and CD30.
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case_images: f4
case_images: Additionally, the patient's pathology findings are summarized in  Figure 4 .