case_images: PMC6056779_01
case_images: Results of initial imaging evaluation. (a) Longitudinal ultrasound image
  demonstrating an oval hypoechoic mass superficial to the left kidney with
  mild mass effect on the left kidney. (b) Axial T2 fat saturation MRI shows
  the lesion as a fluid signal oval mass exerting mass effect upon the left
  kidney. Extension of intermediate signal along the left kidney in the
  perirenal space is also seen. (c) Axial T1 fat saturation MRI demonstrating
  heterogeneous, predominantly peripheral, and progressive enhancement of the
  left perirenal mass. The mass extends laterally beyond the posterior renal
  fascia and invades the intercostal musculature (white arrow). (d) Axial
  image from a subsequent CT of the abdomen with IV contrast shows loss of the
  fat place between the intercostal musculature and the low-density left
  perirenal mass.
case_images: 10.1177_2036361318789724-fig1.jpg
case_images: PMC6056779_01_10.1177_2036361318789724-fig1.jpg
case_images: fig1-2036361318789724
case_images: Sonographic images of the abdomen demonstrated a 5.5 cm hypoechoic mass with internal vascularity in the left perirenal space which was potentially suspicious for malignancy (Figure 1(a)). Further evaluation with a gadolinium-enhanced magnetic resonance imaging (MRI) revealed a 4.9 x2.9 x 5.1 cm enhancing retroperitoneal mass with T2 hyperintensity which exerted mass effect on the left kidney (Figure 1(b)). In addition, MRI demonstrated heterogeneous enhancement, restricted diffusion, and invasion into Gerota's fascia, all of which were concerning for malignancy (Figure 1(c)). Computed tomography (CT) of the chest, abdomen, and pelvis, obtained for formal staging evaluation and surgical planning, revealed no evidence of metastases and confirmed the MRI findings of an extra-renal mass abutting the kidney with no clear evidence of invasion of the renal parenchyma (Figure 1(d)).
case_images: Results of repeat imaging studies. Axial T1 fat saturation post contrast
  image from a follow-up MRI performed 3 months after initial presentation
  showing progression of disease in the index left perirenal mass with greater
  enhancement and increased mass effect upon the left kidney (white
  arrows).
case_images: 10.1177_2036361318789724-fig2.jpg
case_images: PMC6056779_01_10.1177_2036361318789724-fig2.jpg
case_images: fig2-2036361318789724
case_images: Resection gross specimen and histopathology. (a) Gross resection specimen
  showing relationship between mass (double arrow) and renal parenchyma
  (single arrow). (b) Abundant myxoid material with scattered foci of
  inflammatory cells (H&E, 40x). (c) Relationship between the mass (left)
  and renal cortex (right) (H&E, 40x). (d) Focus of inflammatory cells
  with erythroid precursors (arrowhead) (H&E, 200x). (e) Larger atypical
  cell consistent with a megakaryocyte (arrow) (H&E, 400x).
case_images: 10.1177_2036361318789724-fig3.jpg
case_images: PMC6056779_01_10.1177_2036361318789724-fig3.jpg
case_images: fig3-2036361318789724
case_images: As shown in Figure 3(a)-(c), the mass was immediately adjacent to the renal capsule and had a similar histologic appearance to the prior core biopsy. Although the diagnosis of soft tissue myxoma was previously considered, immunohistochemical staining for CD61 was positive, highlighting rare large atypical cells which were consistent with megakaryocytes (Figure 3(d) and (e)).