Pathology case study 3 ~ Education

The following review of two separate cases illustrates a common kidney tumour. Both were small incidental findings discovered on CT. Both lesions originate around the capsule.

Case 1
Low power histology of angiomyolipoma

Low power - Clearly circumscribed, eosinophilic nodule pushing onto renal parenchyma. No obvious fat in the nodule.

Histology of angiomyolipoma

Medium power of AML showing vessels (angio-), smooth muscle (myo-) and fat (lip-) components.and 'lip-'

High power histology of angiomyolipoma

High power of AML showing vessels, smooth muscle and fat

A51y.o. female underwent a partial nephrectomy. The specimen contained a circumscribed, pale grey nodule, 14mm in diameter that was clear of all margins.



Case 2
Macroscopic appearance of AML

Macroscopic appearance of cortical based lesion with vague central yellow colouring suggestive of adipose tissue.

Low power histology of angiomyolipoma

Low power of tumour with visible intralesional adipose tissue (clear vacuoles) and adjacent renal parenchyma

High power histology of angiomyolipoma

High power of AML with vessles, smooth muscle and adipose tissue.

A 64y.o. male underwent a partial nephrectomy. The specimen contained a circumscribed, fatty yellow nodule, 11mm in diameter that was clear of the renal bed margin.



Diagnosis: Angiomyolipoma

Discussion: Both these cases are angiomyolipomas (AML). These are mesenchymal tumours derived from perivascular epitheliod cells (PEC).  The tumour is composed of a variable amount of three components; blood vessels (angio-), plump spindle cells (myo-)  and blood vessels (lipo-). Almost all classic angiomyolipomas are benign but they do have the risk of rupture with bleeding or secondary damage/destruction of surrounding structures as they grow. There is a special variant called an epithelioid angiomyolipoma; this is composed of more plump, epithelial looking cells, often with nuclear atypia, and these have an risk of malignant behaviour including metastasis.

Angiomyolipoma are most commonly seen in the kidney (approximately 3% of adult renal masses) but are described in other organs. They can be seen as part of the genetic disease tuberous sclerosis, where they are commonly multiple and/or bilateral.

Advancements in radiology and changes in requesting of radiological testing has changed the presentation of renal masses, with angiomyolipoma now most commonly identified incidentally on CT; they are often diagnosed relatively confidently by the mixed composition and circumscribed nature, but this ‘textbook appearance’ is not always present and these lesions can pose a diagnostic dilemma. As pathologists we are seeing more core biopsies taken to try and identify the tumour type. On histology the typical three components (vessels, smooth muscle and fat) are seen in variable proportions, but ‘monophasic’ (only one component) and ‘biphasic’ (two components) can be seen, especially if the tumour is only minimally sampled (eg core biopsy). Tumour cells are usually positive on immunohistochemistry for HMB45, CD117, CD63, and negative for cytokeratin and other epithelial markers.

Laparoscopic partial nephrectomy is also becoming the treatment of choice in order to preserve nephrons. In partial nephrectomy cases, we determine the tumour type and assess pathological margins.  There is some debate as to whether a clear margin is necessary.  In angiomyolipomas, because they are benign, the margin may not be important.

This case is de-identified and intended for education purposes only.

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