Pathology > Study Images > Urinary Tract > Tumors of the Kidney > Benign Tumors
Objectives Anat & Hist Introduction Benign Tumors Malignant Tumors

III. Benign Tumors


After completing this tutorial the student will be able to:

  • list benign epithelial and messenchymal tumors of the kidney
  • describe clinical features of benign tumors of the kidney
  • describe gross and histologic features and identify lesions
  • state the distinguishing features between renal adenoma and renal cell carcinoma
  • describe an oncocyte and state where oncytomas occur

  1. Epithelial Tumors
    1. Cortical Adenoma

    Cortical adenomas are small well-defined tumors, which do not cause symptoms during life and are found commonly at autopsy in people over 40 years, the vast majority of whom are males. They form pale yellow to gray subcapsular, cortical masses usually less than 2 cm in diameter.

    Cortical adenoma in the upper pole of the kidney. Note its well circumscription.A smaller adenoma is also present in the lower pole.

    Histologically, most cortical adenomas show complex, branching papillary fronds that project into and a fill cystic space. Less often they form closely packed tubules or cords. The cells are uniform and small and usually lack cytologic features of malignancy. Occasionally they may be large with eosinophilic or clear cytoplasm.

    Cortical adenoma showing branching papillary fronds projecting into cystic spaces.

    Higher magnification showing the edge of the tumor (left) and papillary fronds lined by small, bland epithelial cells.

    Adenomas less than 3 cm in diameter rarely metastasize and the smaller the tumor, the more unlikely it is to metastasize. On the other hand larger tumors metastasize at a frequency that appears to be a linear function of their diameter. The dilemma is that all cancers begin as small lesions and no histologic criteria have been found, as yet, to distinguish a small carcinoma from an adenoma. Currently, the separation is based predominantly on the size of the lesion. Tumors less than 3 cm are called adenoma and larger ones are called carcinoma.

    1. Renal Oncocytoma

    These are almost always found incidentally and, unlike cortical adenomas, have typical gross appearances that differ markedly from the typical gross appearances of renal cell carcinoma and so can usually be clearly separated. The tumors are well circumscribed and the cut surface has a uniform mahogany-brown color without foci of hemorrhage or necrosis. Large oncocytomas commonly show a prominent stellate scar, which gives them a distinct gross appearance.

    Renal oncocytoma. Well-circumscribed lesion replacing one pole of the kidney.

    Microscopically, they are composed of large epithelial cells called oncocytes, which have abundant eosinophilic homogeneous or granular cytoplasm and uniform small nuclei. Electron microscopy shows these cells to be packed with abnormal mitochondria. Biochemical studies suggest a defect in mitochondrial ATP production with compensatory mitochondrial hyperplasia.

    Renal oncocytoma. Low power view showing edge of tumor with normal cortical tissue on the right.

    Higher magnification showing the abundant eosinophilic cytoplasm and regular, small nuclei.

    Electron photomicrograph of renal oncocytoma packed with numerous mitochondria.

    Question: In which other organs may oncocytomas be found?

  1. Messenchymal Tumors
    1. Renomedullary Interstitial Cell Tumor (Medullary Fibroma)

    These tumors virtually cause no symptoms and are noted as incidental findings at autopsy or nephrectomy for other reasons. They are small gray-white, well-circumscribed nodules, usually less than 1 cm in diameter, and located in the renal medulla.

    Histologically, they are composed of small stellate or polygonal cells and bundles of delicate fibers in a loose myxoid matrix. Although also called fibroma, the tumor cells are not of fibroblast origin. At the periphery, renal medullary tubules are often entrapped in the matrix.

    1. Angiomyolipoma

    Angiolipomas are benign lesions characterized by the presence of mature adipose tissue, smooth muscle and thick-walled blood vessels. The true nature of these lesions is uncertain, but they are usually regarded as hamartomas. About 33% of patients with angiomyolipoma have tuberous sclerosis and more than 80% of tuberous sclerosis patients have angiomyolipoma.

    Typically, the lesions are asymptomatic but may present with flank pain, mass, hematuria or a combination of these especially in patients with tuberous sclerosis. Grossly, the tumors are typically multifocal, bilateral and small in patients with tuberous sclerosis, and single, unilateral and large in those without. They are well circumscribed but not encapsulated and have varied appearances depending on the proportions of the constituent elements. In 25% of cases the tumors may be confused with malignancy because of extension outside the renal capsule.

    Angiomyolipoma. Tumor mass replaces the upper pole of this kidney. The irregular margins and capsular penetration wrongly suggest malignancy.

    The tumors show a wide variation in microscopic appearances due to differences in the amount of each component present in a particular tumor. There is no definite organization and the three components are haphazardly arranged. The bizarre mixture of elements may lead to confusion with a malignant.

    Histology of angiomyolipoma.

    Occasionally, lymph node involvement occurs but deaths from these "metastases" have never been reported and no systemic dissemination following nodal involvement has been documented.

    1. Juxtaglomerular Cell Tumor

    This is a rare, usually benign neoplasm of the juxtaglomerular cells of the kidney. Clinically, they occur mainly in young people, all of whom present with hypertension. Grossly, the tumors are small, usually less than 3 cm in diameter, well circumscribed and located in the cortex.

    Histologic appearances are variable.

    Question: What is the cause of the hypertension in patients with this tumor?

    Question: Which other renal tumors may present with hypertension?

  1. Embryonic Tumors

    Mesoblastic Nephroma

    This is a tumor that is usually found in the first three months of life and must be differentiated from nephroblastoma (see below). The tumor varies in size from a small to very large mass that bulges from the kidney. It is hard in consistency and cut surface is white and whorled.

    The tumor is composed of spindle cells and the margins are irregular with bands of tumor cells interdigitating with adjacent parenchyma. Mitoses are common but are normal as in other tissues of the rapidly growing infant.

    The tumor is entirely benign. However, if tongues of tumor cells at the margins are left behind after surgical resection local recurrence is possible.

Objectives Anat & Hist Introduction Benign Tumors Malignant Tumors