PRNETs are most commonly found in individuals aged between 50 and 60 years, predominantly in the right kidney, with no significant gender difference observed (2, 14). Typically, these tumors present with non-specific clinical symptoms and are often discovered incidentally during routine physical exams. When symptoms do appear, patients commonly experience back pain, abdominal distension, abdominal mass, and hematuria (2, 8). Well-differentiated NET cells possess neuroendocrine functionality and can secrete vasoactive substances such as dopamine, histamine, and prostaglandins, which may result in carcinoid syndrome. Studies have shown that approximately 15% of patients with well-differentiated NETs develop carcinoid syndrome, presenting symptoms such as facial flushing, systemic edema, diarrhea, and asthma (10). This type of tumor usually spreads locally and metastasis is rare, but the frequency of metastasis increases with the increase of tumor size (15).

Renal NETs usually present as solid masses with clear boundaries, mostly located near the renal hilum of the renal parenchyma, while those with horseshoe kidney showed cystic-solid masses, and the lesions were mostly located near the isthmus (2). These tumors are generally hypovascular or even avascular, leading to either no enhancement or mild enhancement, with larger tumors occasionally showing heterogeneous enhancement. Approximately 25% of renal NETs exhibit internal calcification, and about 50% of patients have retroperitoneal lymphadenopathy (8, 16, 17). Some studies suggest that calcification is associated with the indolent nature of the tumor, while hemorrhage and necrosis are linked to more aggressive growth and a poorer prognosis (2). Radionuclide-labeled octreotide can specifically bind to somatostatin receptors in tumor tissue, aiding in the detection of primary carcinoid and metastatic lesions. This method has been reported to have a sensitivity greater than 85% for detecting carcinoid tumors, including small and asymptomatic lesions (18). FDG-PET is commonly used to assess tumor metabolism in cancer patients in various stages of the disease (19). In our case, the tumor was located in the left kidney, displaying mild heterogeneous enhancement on contrast-enhanced imaging with calcification, but without retroperitoneal lymphadenopathy. Currently, there is limited imaging data on PRNETs. Based on this case and a review of the literature, the following imaging features can be summarized: heterogeneous density, mild to moderate enhancement, and calcification, which are helpful for diagnosing renal NETs. Most lesions exhibit heterogeneous signal intensity across different sequences, with varying signal characteristics that are not specific and may be related to the presence of cystic degeneration, hemorrhage, or necrosis.

Conventional imaging techniques often struggle to accurately differentiate renal NETs from other renal malignancies, such as renal cell carcinoma, due to overlapping imaging features. This overlap is a key factor contributing to the misdiagnosis in this patient. Clinically, renal NETs must be distinguished from the following conditions: (1) CCRCC: On CT, these tumors typically present as low-density masses with a common pseudocapsule. Microcystic degeneration, hemorrhage, and calcification are often seen. On MRI, clear cell carcinoma usually shows high signal intensity on T2WI, and some lesions may not demonstrate low signal intensity on the ADC map. Post-contrast imaging typically reveals significant enhancement, with a “fast-in, fast-out” enhancement pattern. (2) Chromophobe renal cell carcinoma: These tumors are characterized by a high degree of homogeneity. On CT, they typically show equal density, while MRI often reveals equal signal intensity on T1WI. Enhanced CT and MRI show uniform enhancement, and DWI displays high signal intensity. Delayed imaging may reveal a central strip-like low signal, known as the “spoke-wheel sign” (20, 21). (3) Fat-Poor Renal Angiomyolipoma: These lesions exhibit a typical homogeneous high density on CT and a characteristic homogeneous low signal on T2WI and ADC maps. They lack a capsule, calcification, or hemorrhage. The common enhancement pattern is “fast-in, fast-out,” although approximately one-third of lesions may show delayed enhancement (22, 23).

Although the calcification, mild to moderate enhancement and heterogeneous density in imaging examination are helpful for the diagnosis of well differentiated renal NETs, the final diagnosis and differential diagnosis mainly depend on the results of pathological and immunohistochemical examination. The commonly used neuroendocrine markers are chromogranin A (CgA), neuron-specific enolase (NSE), synaptophysin (Syn) and CD56. Recently, insulinoma-associated 1 (INSM-1), a zinc finger transcription factor, was also found to be expressed in NETs (24). The specificity of CgA in the diagnosis of carcinoid was 97%, while that of Syn was as high as 100% (2). Ki-67 is a reliable marker of pathological grading, and its index reflects cell proliferation activity to a certain extent. The immunohistochemical results of this patient showed that the tumor cells were positive for CK, Syn, CgA, NSE, and INSM-1, and the Ki-67 proliferation index was 2-3%. According to the European Neuroendocrine Tumor Society (ENETS) guidelines and the 2010 World Health Organization (WHO) classification, the NETs of this patient was graded as G1 (25). The final diagnosis was: (left) renal neuroendocrine tumor (NET, G1, carcinoid). Erythropoietin immunohistochemistry could not be performed owing to insufficient residual tissue, a limitation that, although mitigated by the diagnostic support provided by characteristic morphology and standard immunohistochemical markers, should be acknowledged.

Although primary renal NETs are low-invasive tumors in terms of tumor biology, they still have invasion and metastasis, so they should be treated as soon as possible. Surgery is the best treatment for the disease, and the current gold standard for the treatment of this type of tumor is partial nephrectomy or radical nephrectomy. The extent of surgery mainly depends on the size and location of the tumor, as well as the status of invasion and lymph node metastasis. Radical nephrectomy can be used for large tumors or tumors that cannot be completely resected by partial resection. Lymph node dissection is recommended during surgery, especially if there is any evidence of lymph node enlargement. So far, no systemic therapy has been established for adjuvant or metastatic setting of PRNETs (6, 25, 26). Octreotide is a long-acting somatostatin analogue used for both detection and as a first line antineoplastic systemic therapy. Although radiation therapy has not been extensively studied, radiolabeled octreotide is effective in reducing tumor volume and improving clinical symptoms in patients with metastases (5, 17).

Renal NETs have a lower degree of malignancy and a good prognosis, but long-term follow-up is still needed to monitor the patient’s condition. The following major prognostic factors have been identified. Patients older than 40 years tend to have faster disease progression and more severe initial symptoms. Tumors with a maximum diameter of less than 4 cm or confined to the renal parenchyma usually have fewer metastases and have a better prognosis. Other important prognostic factors include high mitotic rate greater than 2/10 HPF, lymphovascular invasion, atypical cytology and necrosis (17, 26). Given the nonspecific nature of CT and MRI, postoperative imaging should include somatostatin receptor scintigraphy to detect metastatic disease. Long-term follow-up is recommended because metastatic disease can occur even within five years after diagnosis. Follow-up should include physical examination, biochemical laboratory examination and chromogranin A (CgA) levels, along with imaging studies every 3-6 months (6).