Benigntumours of the kidney are frequent incidental findings: renal adenomas, renal oncocytomas, angiomyolipomas are tumours composed of smooth muscle, fat, and large blood vessels. Renal fibromas are very common small benign tumours of spindle cells.

Adenoma. Traditionally, this tumor has been defined as a renal epithelial neoplasm that is less than 2.5 cm in size. Most of these lesions are associated with a benign course; however, approximately 10% of the lesions that are between 1 and 3 cm behave in a

malignant fashion. These 10% are best considered small renal cell carcinomas of low malignant potential.

Angiomyolipoma is a benign tumor composed of a mixture of fat, blood vessels, and smooth muscle tissue. These tumors are of significance because clinically and radiologically they often are misdiagnosed as carcinomas. Macroscopically angio-myolipomas range in size from a few centimeters in diameter to quite large. They affect the cortex and medulla equally and have no predilection for the right or left kidney. On cut surface, they exhibit a yellow-to-gray color, depending on fat content. Microscopically there is an admixture of mature adipose tissue, thick-walled vessels, and varying amounts of smooth muscle tissue.

Mesoblastic nephroma (benign nephroblastoma) is a congenital hamartoma that commonly is confused with Wilms' tumor. Most mesoblastic nephromas are diagnosed in the early months of life. Macroscopically mesoblastic nephromas vary in size and are unilateral. On cut section, they have a characteristic firm, whitish surface that resembles smooth muscle. Necrosis usually is absent, but cystic changes may be present. The tumors lack clear encapsulation. Microscopically the predominant features are interlacing bundles of mature connective tissue, which entrap the glomeruli and other renal elements. Rarely, foci of dysplastic cartilage and embryonic mesenchyme may be present.

Renal adenocarcinoma is the most common malignanttumour of the kidney.

Renal cell carcinoma (hypernephroma) is an adenocarcinoma that arises from the proximal or distal convoluted tubule.

Etiology and pathogenesis. The cause of these tumors remains obscure, but they have been produced in laboratory animals using chemical, physical, and viral agents. The chemicals used include aromatic hydrocarbons, amines, amides, and such aliphatic compounds as aflatoxins (metabolic products of the fungus). Partial deletion of the short arm chromosome 3 is a frequent cytogenetic finding in renal cell carcinoma.

Pathology. Renal cell carcinoma may affect either kidney and has no predilection for a specific location within the organ. Macroscopically the tumor protrudes from the renal cortex as an irregular bosselated mass, which, on cut section, has a characteristic yellow-orange appearance. Hemorrhage and necrosis are commonly seen. At the periphery of the tumor, the normal parenchyma is compressed, forming a pseudocapsule. Foci of myxoid degeneration and calcification may be present. Microscopiccally several patterns can be seen, including papillary, tubular, granular, solid, or sarcomatoid. Most tumors are composed of clear cells with distinct cytoplasmic membranes, abundant cytoplasm, and eccentric nuclei. The lesions are markedly vascularized, with little

stroma between the cells; occasionally clusters of histiocytes and inflammatory cells are present. Renal cell tumors frequently show areas of pleomorphism and giant cells and, thus, resemble different types of sarcomas.

Renal-cell carcinoma metastasizes mainly through the bloodstream and lymphatic spread also is possible.

Nephroblastoma (Wilms, tumour) is anembryonal tumour derived from the primitive meta-nephros. Wilms' tumor (nephroblastoma) is a mixed neoplasm composed of metanephric blastema and its stromal and epithelial derivatives at variable stages of differentiation. Wilms' tumor is the most common malignancy of renal origin in children.

Etiology and pathogenesis. Very recent work has located a tumor suppressor gene on chromosome 1 Ipl3. Thought to repress transcription and designated WT-1, this gene is lost or deleted in sporadic and hereditary Wilms' tumor. Nephroblastomas may be combined with several other congenital malformations, such as sporadic aniridia, microcephaly, mental retardation, and spina bifida.

Pathology. Wilms' tumor can be unilateral, bilateral, or multifocal in its involvement of the same kidney. Macroscopically the tumors are large, well delineated, and well encapsulated. On cut section, they often appear grayish-white, with areas of hemorrhage and occasional cystic changes. The junction between the tumor and the kidney is sharp, often with a rim of

normal kidney parenchyma. Microscopically the lesions are characterized by formation of abortive or embryonic glomerular and tubular structures surrounded by an immature spindle cell stroma. The epithelial elements may be scanty or predominantly tubular. The stroma may show different elements, such as skeletal muscle, cartilage, and fat.

Frequent metastasis of the tumor are lymphogenic and hematogenic. The renal hilar and paraaortic lymph nodes are involved. The lungs, liver, adrenal glands, diaphragm, retroperitoneum, and bones also are commonly involved.

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