Metanephric adenoma: association between the imaging features of contrast-enhanced ultrasound and clinicopathological characteristics.

Background: This study aimed to improve the understanding of metanephric adenoma (MA) by retrospective analysis of contrast-enhanced ultrasound (CEUS) findings and clinicopathological characteristics of MAs.

Methods: Gray-scale ultrasound (US) and CEUS findings of 7 adult MA patients, confirmed by postoperative pathology, were summarized via collection of clinicopathological and ultrasonographic imaging data, including tumor location, size, echo intensity, color flow, presence or absence of calcification, and liquefactive necrosis, contrast-enhanced pattern, enhancement characteristics, and contrast wash-out compared with adjacent parenchyma, and the presence or absence of a pseudocapsule. Histopathological analyses, including hematoxylin and eosin (HE) and immunohistochemical (IHC) staining, were conducted with the EnVision method.

Results: All 7 participants were female, aged 29-73 years (mean age, 54 years), with flank pain (3/7). All tumors were solid (7/7) with sizes of 2.0-5.0 cm (mean diameter, 3.07 cm), including 4 in the left kidney, 3 in the right kidney, 2 in the renal pelvis, and 5 in the renal parenchyma. On the gray-scale US, MA was shown as hypoechoic (4/7), slightly hyperechoic (2/7), isoechoic (1/7), and with a defined border. The morphology was regular and rounded (7/7), internal echogenicity was homogeneous (5/7), and no calcification was seen (7/7). The CEUS showed clear boundaries (7/7), homogeneous isodensity (5/7), with calcification (0/7), necrosis (2/11), heterogeneous hyperattenuation (2/7), pseudocapsule (2/7), and medullary phase fast wash-out (7/7). The surgical methods were radical nephrectomy (4/7) and partial nephrectomy (3/7). The duration of follow-up period for all participants was 3-74 months, and no local or distant recurrences were found. The IHC staining showed that most tumor cells were positive for WT1, cytokeratins AE1/AE3, vimentin, and CD57, and exhibited focal positivity for CK7, while negative for CD10, AMACR, and CK720. The proliferative index (Ki-67) was 2-3%.

Conclusions: On gray-scale US, MA appears as a solid nodule with a well-defined boundary, regular morphology, and homogeneous echogenicity; CEUS shows slow progression and slightly lower homogeneous enhancement and fast wash-out in the medullary phase. These findings may provide insight into the progression of MA and aid in the development of diagnostic and therapeutic strategies.