The findings of CT and MRI in patients with metanephric adenoma

MA is the occurrence of an uncommon renal tumor and was first described as a bilateral and diffuse tumor by Bove et al. in 1979 [7]. The histologic origin of MA remains controversial, but most scholars consider it to be derived from metanephric blastema. In recent years, emerging MA cases have been reported. MA generally occurs in adults, with a peak age of occurrence in the fifth or sixth decade of life, but it is also diagnosed in children [8, 9]. In the present study, patients’ average age and the age range at diagnosis was 43.4 years and 17–67 years. It is worth noting that three cases were under 30 years of age. Unilateral renal onset is more common, and all our patients were taken bad for unilateral kidney. However, there are also bilateral renal cases in a previous report [10]. In addition, MA seems to be more common in women than in men [2]. Nevertheless, no gender difference was observed in this report due to the small sample size.

As was the case with our patients, 50 % were clinically asymptomatic and diagnosed during physical examination [2]. The symptomatic cases of MA manifested as gross hematuria and flank pain in this study. Other symptoms, including polycythemia, a palpable mass, backache, abdominal pain, and fever, have been frequently presented in previous reports [11]. It has been also demonstrated that some patients have symptoms of urinary tract infections (UTIs). MA has been reported to have the highest level of polycythemia among all kidney tumors [2], which is probably related to the production of erythropoietin and multiple cytokines by MA [12]. Asymptomatic cases go against the discovery of MA. Therefore, physical examination is important for detection in early stages.

The clinical and imaging features of MA are complex and varied. It is difficult to give a final conclusion through preoperative diagnosis, due to the fact that it is often misdiagnosed as renal cell carcinomas, renal cysts, and other kidney diseases [3, 13]. In this report, six patients were preoperatively misdiagnosed as renal cell carcinoma and two were preoperatively misdiagnosed as renal hamartoma. Currently, the final diagnosis has to rely on pathology [3]. CT is the main imaging method for the diagnosis of MA. However, no typical radiological features of MA have been identified [14]. In this study, plain CT showed that all MA tumors were solid and 87.5 % of the lesions were identified to be isodense and equal to the normal renal parenchyma. Dynamic contrast-enhanced CT is the principal characteristic of MA, and it revealed that enhanced degrees of the tumors in the renal cortex, renal parenchymal, and pelvic phase were mainly progressive and the levels in all three phases were lower than that of the normal renal parenchyma. MA lacks a blood supply, and the nutrient artery and neoangiogenesis are rarely observed in lesions. However, we observed the nutrient artery in one patient. The features described above help to identify MA and renal clear cell carcinoma. Renal clear cell carcinoma is the most common tumor in the kidneys with an abundant blood supply and the enhancement pattern of rapid rise-rapid fall, and the enhanced degree of this tumor is generally higher than that of the normal renal parenchyma. Calcification is considered a critical indicator for the diagnosis of MA [15]. Only one of the eight MA tumors showed calcification in this report. In addition, cystic changes and necrosis were confirmed in 37.5 % of MA tumors. We found that MA lesions were located both in the renal cortex and medulla in 87.5 % of patients. Zhu et al. reported that seven of eight MA tumors were centered in the renal medulla [16]. However, a recent study shows that only 16.7 % of tumors were located entirely within the renal parenchyma and 83.3 % were located at the periphery of the renal cortex without involvement of the renal collecting system [17]. In a previous report, the renal cortex has been indicated as the predilection site of MA [2]. The sample size greatly contributes to the conflicting CT findings for MA. In addition, we found that the lesions projected outside of the renal contour exhibited exophytic growth, which is one of the principal characters of MA.

The MRI findings of MA are relatively limited. The typical MRI finding of MA is hypointense (or isointense) on T1WI and T2WI [4, 11]. In our report, one case of MA was isointense on T1WI and isointense on T2WI with some slightly hyperintense areas in the center. Due to the diversification of MRI findings, more cases need to be collected and analyzed.

Rare metastatic MA has been reported. In addition, metastasis of the retroperitoneal enlarged lymph nodes, renal vessels, and the inferior vena cava was not observed in any case. However, lung metastasis and lymph node metastasis have been found in a few MA patients [18, 19]. A follow-up study is important for verifying metastatic MA.

The histological features of MA are distinctive and are characterized by epithelial cells with different amounts of cytoplasm. The adenomatous components are composed of uniform small cells arranged in tubular or papillary architectural patterns [11]. In the present patients, immunohistochemical staining showed that neoplastic cells were positive for vimentin, CK, and EMA. Previous findings demonstrate that CD57, AE1/AE3, and CAM5.2 are also positive in MA, while NSE, CEA, CgA, Syn, actin, desmin, and AMACR are negative [3, 20]. Pathological detection remains the most effective method for MA diagnosis.