Renal oncocytoma overview

# Overview
Renal oncocytoma is a relatively rare and benign tumor. The incidence of  is approximately 3% to 7% of solid renal resected tumors and it is more common in male patients. The majority of patients with renal oncocytoma are asymptomatic. Symptoms of renal oncocytoma include hematuria, flank pain, abdominal or flank mass and weight loss. Physical examination of patients with renal oncocytoma is usually normal. An abdominal or flank mass may be palpayed during physical examination. Surgery is the mainstay of treatment for renal oncocytoma. Although the nature of renal oncocytoma is benign and the prognosis is excellent, since the definite diagnosis can not be obtained before operation, surgical resection is a choice of treatment.

# Historical perspective
Renal oncocytoma was first discovered by Zippel, in 1942 and Klein and Valensi were the first to demonstrate the pathologic characteristics of renal oncocytoma as proximal tubular adenoma with oncocytic features in 1976.

# Classification
There is no established system for the classification of renal oncocytoma.

# Pathophysiology
The exact pathogenesis of renal oncocytoma is not completely understood. Although some mechanisms are suggested in the pathogenesis of this disease that include, lossing of chromosome 1 and dysfunction of mitochondrial enzymes which is caused by alterations in the mitochondrial DNA. DNA diploidy is seen in 96% of patients with renal oncocytomas. The development of renal oncocytoma is the result of multiple genetic mutations such as deletion of chromosome 1, deletion of the sex chromosome, translocation of chromosome 11q13, sporadic or no chromosomal alteration. Renal oncocytoma can be associated with familial renal oncocytoma or Birt-Hogg-Dube syndrome. On gross pathology, tan to brown surface color , well-encapsulated with a thick, well-defined, fibrous capsule, central scar, and homogeneous appearance without any hemorrhage or necrosis inside it in the tumor cut are characteristic findings of renal oncocytoma. On microscopichistopathological analysis, renal oncocytoma characterized by "oncocytes". They are large, round to polygonal neoplastic cells accompanied by eosinophilicgranular cytoplasm and are organized in nested or organoid pattern. Although, renal oncocytoma is benign, atypia, prominent nucleoli, and pleomorphism may seen in microscopic examination.

# Causes
There are no established causes for renal oncocytoma.

# Differential Diagnosis
Renal oncocytoma must be differentiated from other diseases that cause abdominal mass, abdominal pain and hematuria such as wilms tumor, renal cell carcinoma, rhabdoid kidney disease, polycystic kidney disease, and other urogenital mass.

# Epidemiology and Demographics
The incidence of renal oncocytoma is approximately 3% to 7% of solid renal resected tumors. Patients of all age groups may develop renal oncocytoma. The age of patients can be differ from 10 to 94 years. The incidence of renal oncocytoma increases with age; the median age at the time of surgery is 62 to 68 years. Males are more commonly affected by renal oncocytoma than females. The male to female ratio is approximately 2 to 1.

# Risk Fctors
Patients with tuberous sclerosis complex and Birt-Hogg-Dube syndrome are at an elevated risk of developing renal oncocytomas (often bilateral).

# Screening
There is insufficient evidence to recommend routine screening for renal oncocytoma.

# Natural History, Complications and Prognosis
The median age at the time of surgery is 62 to 68 years. In 10% to 32% of patients with renal oncocytoma, coexcitente RCC are seen. Prognosis is generally excellent, There are only two cases of metastatic renal oncocytoma were reported. Since the definite diagnosis is maintain just after surgery, most of patients are undergone operation.

# Diagnosis

## History and Symptoms
The majority of patients with renal oncocytoma are asymptomatic. Symptoms are seen in almost only 17% to 21% of patients with renal oncocytoma. Symptoms of renal oncocytoma include hematuria, flank pain, abdominal or flank mass and weight loss.

## Physical Examination
Physical examination of patients with renal oncocytoma is usually normal. An abdominal or flank mass may be palpayed during physical examination.

## Laboratory Findings
Some patients with renal oncocytoma may have hematuria, in their urine analysis.

## X Ray
X-ray is rarely done for the diagnosis of renal oncocytoma.

## CT Scan
Abdominal CT scan may be helpful in the diagnosis of renal oncocytoma. Findings on CT scan suggestive ofrenal oncocytoma include solid renal lesion, centralscar (stellate scar), hypervascularity, hypodenseity ( most of the time), and homogenous enhancements.

## MRI
Renal MRI may be helpful in the diagnosis of renal oncocytoma. Findings on MRI suggestive of renal oncocytoma include central scar, satellite pattern, pseudo-capsule, hypointensity in T1 and hyperintensity in T2 weighted images. Although, none of these characteristics can differentiate between renal oncocytoma and RCC.

## Ultrasound
Renal ultrasound in renal oncocytoma patients may show: solid mass ( help to distinguish solid from cystic mass), central scarring, calcification, central necrosis which none of them characteristics for indicating a specific renal lesion.

## Other Imaging Findings
Renal angiography may be helpful in the diagnosis of renal oncocytoma. The spoke-wheeled appearance, lucent rim sign, homogeneous capillary flush in the nephrogram phase, lack of wild neoplastic vessels,and both hypovascular or hypervascular may seen on angiography of patients with renal oncocytoma. Although, none of them are specific for renal oncocytoma and they also may seen in RCCs.

## Other Diagnostic Studies
Renal mass biopsy may be helpful in the diagnosis of renal oncocytoma. However, distinguishing between oncocytoma and RCC by biopsy is difficult. Since this method only reserved for patients who are at high risk for an operation like very elderly or extremely sick patients.  Some complications may happen during renal mass biopsy which are perirenal hemorrhage, pneumothorax ( during biopsy of upper pole tumors) and tumor seeding.

# Treatment

## Medical Therapy
The mainstay of therapy for renal oncocytoma is surgery.

## Surgery
Surgery is the mainstay of treatment for renal oncocytoma. Although the nature of renal oncocytoma is benign and the prognosis is excellent, since the definite diagnosis can not be obtained before operation, surgical resection is a choice of treatment. Best option for surgery differs based on the mass characteristics, partial nephrectomy is done in polar lesions smaller than 4 cm in a normal contralateral kidney while, large solid renal masses which destroy most part of renal tissue or patients who have not candidate for nephron-sparing surgery are reserved for total nephrectomy.

## Primary Prevention
There are no established measures for the primary prevention of renal oncocytoma.

## Secondary Prevention
There are no established measures for the secondary prevention of renal oncocytoma.