Figure 1.

Case 1(A) Positron emission tomography/computed tomography (PET/CT) scan suggested a mass with a mildly inhomogeneous increase in glucose metabolism with an maximum standardized uptake value of 3.95. PET/CT images in axial, coronal, and sagittal planes showed a hypermetabolic lesion in the allograft kidney, suggesting high metabolic activity. (B) Corresponding noncontrast CT scan images in axial, coronal, and sagittal views demonstrated the anatomic structure of the renal mass with heterogeneous density. (C) Fused PET/CT scan images in axial, coronal, and sagittal planes clearly localized the hypermetabolic activity to the mass observed on CT scan, providing both metabolic and anatomic correlation. (D) Axial T1-weighted images in July 2024 suggested a 15.0 cm × 10.0 cm × 16.0 cm mass with a slightly hypointense signal. (E) Axial T2-weighted images with fat saturation showed that the mass was hyperintense. (F) Enhanced CT scan in December 2024 showed an increased size of 16.5 cm × 11.1 cm of the mass in the arterial phase. (G) Enhanced CT scan in December 2024 (venous phase). (H) The gross specimen of the tumor was irregularly shaped, with a maximum diameter of about 17 cm, and a grayish, partly mucoid cut surface. (I) The tumor showed expansive growth with an intact capsule and no clear infiltrative border (Δ) (hematoxylin and eosin, scale bar = 1.25 mm). (J) The tumor is mainly composed of adipocytes, myxoid stroma with spindled or stellate stromal cells (hematoxylin and eosin, scale bar = 200 μm). (K) Spindle-shaped tumor cells are arranged in bundles, with long oval nuclei, uniform chromatin, and with no nuclear pleomorphism or atypia (hematoxylin and eosin, scale bar = 100 μm). (L) Immunohistochemistry showed CD34 positive (scale = 200 μm). (M) Desmin is weakly positive. (N) Murine double minute 2 negative (scale bar = 200 μm). (O) S100 negative (scale bar = 200 μm).