The latest 2016 World Health Organization classification of tumors of the urinary system and male genital organs now includes RH as a mesenchymal tumor, which is similar to the CNS hemangioblastoma. However, a correlation between VHL syndrome and VHL gene mutation has not been reported, and the biological behavior of the tumor is benign. RH is a rare, slow-growing, and benign renal tumor. It usually affects the middle-aged and elderly but can also occur in younger patients[18, 20]. Compared with women, men are 1.5 to 2 times more likely to develop RH. In sporadic cases like the two described here, there is no VHL-related disease and no associated family history of VHL disease in the patient (so-called “isolated” RH).
The gross features of RH include a predominantly solid tumor rich in capillaries with a well-demarcated border from the surrounding renal parenchyma, which is consistent with what we saw in the surgical specimens. Microscopically, RH is a morphologically distinctive vascular neoplasm with rich capillary networks and lipid-rich stromal cells[18, 20, 29]. Based on the stromal cell components, they could be divided into three types: capillary-dominated, interstitial cell-dominated, and classic type (intermediate between the other two types). Unfortunately, although the stromal cells of RH appear normal, they may show obvious nuclear pleomorphism, similar to malignant tumors. Therefore, RH can be easily misdiagnosed as RCC and other malignant tumors.
Because it is difficult to distinguish RH from renal clear cell carcinoma by routine hematoxylin and eosin (HE) staining alone, immunohistochemical examination is helpful for differential diagnosis. Studies have shown that labeling with inhibin alpha, S-100, and CD10 help to distinguish RH from renal clear cell carcinoma. Inhibin alpha and S-100 are usually positive in RH and negative in renal clear cell carcinoma and contrast; CD10 is usually positive in renal clear cell carcinoma and negative in RH[30-33]. Our cases are also consistent with the results of the above studies.
CT and MRI are very useful examination methods that have been widely used in the diagnosis and differential diagnosis of diseases in various systems. Both have gained increased acceptance for accurate diagnosis and differential diagnosis of renal tumors[34-37].
Most hemangioblastomas are in the cerebellum and can be divided into three types: solid, solid-cystic, or predominantly cystic with small mural nodules[38, 39]. The most common and typical radiologic findings correspond to the last type and show a markedly enhanced small mural nodule attached to a large unenhanced cyst wall[40, 41]. There is limited literature describing the imaging findings of RH, but they report RH as a solid, heterogeneously enhanced mass, which is consistent with our cases[17, 22, 23]. However, they do not correspond with the most common and typical radiologic findings of hemangioblastoma in the cerebellum. This discrepancy may be related to the different tumor growth environments.
In addition to being solid tumors, the CT and MRI findings of our two patients were similar to that of cavernous hemangiomas of the liver, including peripheral nodular enhancement in the corticomedullary phase, progressive centripetal enhancement in the nephrographic and delayed phases, and sometimes complete “filling in” in the delayed phase[42, 43]. Other renal tumors do not exhibit these enhancement patterns, which could be unique to RH. At present, these findings have not been mentioned in the literature. It may be that others have not noticed this feature, or they may not have performed contrast enhancement multiphase scanning with CT and MRI. T2WI also helps during differential diagnosis. The high signal intensity on T2WI indicated slow blood flow in the neoplastic vascular channel, which is of great significance to angiogenic tumors. The first case was less typical due to bleeding, but the second case was similar to cavernous hemangioma of the liver, showing a significantly high signal intensity known as the “light bulb sign”.
Because RHs are indolent neoplasms, asymptomatic tumors may be managed with observation. Gross total resection is the most suitable treatment if intervention is required. If the correct preoperative diagnosis was made, these two patients may have received different treatments. Unfortunately, RH can easily be misdiagnosed as renal clear cell carcinoma, the most common malignant tumor of the kidney, which can lead to overtreatment. Characteristic CT and MRI manifestations may help guide preoperative diagnosis. At the same time, once RH is surgically confirmed, a comprehensive examination should be performed to determine if the tumor is associated with VHL disease.