Tumours of the Kidney

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Benign tumours of the kidney are commonly seen as an incidental finding and are of little clinical significance.
The main tumours of the kidney in adults are renal adenocarcinomas; metastatic tumours are seen, but they are uncommon.
Transitional cell carcinomas of the renal pelvis are considered in the section on disease of the lower urinary tract.
The only other common renal tumour is nephroblastoma (Wilms' tumour), which is seen almost exclusively in early childhood.

Benign tumours of the kidney are common incidental findings at post mortem examination
in about 20% of all patients, and may also be encountered as incidental findings on imaging.

Renal adenomas are benign epithelial tumours derived from renal tubular epithelium.
Their histological appearances overlap with those of renal cell carcinomas, making them
very difficult to distinguish. In order to separate adenomas from carcinomas, an arbitrary
cut-off of 3 cm in size has been adopted.

Some small lesions suspected of being adenomas may be carcinomas, going on to metastasize.
Renal oncocytomas are benign epithelial tumours composed of large cells with
eosinophilic cytoplasm filled with mitochondria.

They are best considered as a variant of adenoma.
Angiomyolipomas are tumours composed of smooth muscle, fat, and large blood vessels.
They are seen in association with tuberose sclerosis.
Renal fibromas are very common small benign tumours of spindle cells. Typically
3-10 mm in size, they are of no functional significance and are found in the medulla.
Aetiology and molecular pathology of renal carcinoma

Renal carcinoma: aetiology & prognosis

Renal adenocarcinoma is the most common malignant tumour of the kidney
Renal adenocarcinoma, derived from the renal tubular epithelium, accounts for 90% of
primary malignant renal tumours in adults.

Renal adenocarcinomas account for around 3% of all carcinomas in adults.
Macroscopically these tumours are usually rounded masses, with a yellowish cut face
marked with areas of haemorrhage and necrosis There are several
histological patterns of renal adenocarcinoma, the most common being the 'clear-cell
pattern', in which the tumour cells have clear cytoplasm due to the high content of
glycogen and lipid. Tubular and papillary carcinomas are characterized
by epithelial cells with a granular cytoplasm.

The tumour spreads by local expansion (breaking through the renal capsule into
perinephric fat) and by blood-borne metastasis (involving lungs, bone, brain and other
sites as a result of tumour invasion of the renal vein). A characteristic behavioural
feature is that large tumours may grow as a solid core along the main renal vein,
even entering the inferior vena cava.
A renal carcinoma appears as a yellow mass, with areas of necrosis and haemorrhage.
Renal adenocarcinoma.

Clear-cell carcinoma of the kidney is composed of uniform vacuolated cells.
Nephroblastoma is one of the common malignant tumours of childhood
Nephroblastoma (Wilms' tumour) is an embryonal tumour derived from the
primitive metanephros. Although it does occasionally occur in adults, it is
predominantly a tumour of young children, with a peak incidence between
the ages of 1 and 4 years.
Macroscopically, tumours are rounded masses that replace large amounts of the kidney,
appearing as solid, fleshy, white lesions with frequent areas of necrosis.
Histologically, there are various combinations of four elements, these being
primitive small-cell blastematous tissue resembling the developing metanephric blastema,
immature-looking glomerular structures, epithelial tubules, and stroma composed
of spindle cells and striated muscle.
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