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Pathology for Urologists

Renal Angiomyolipoma (AML)


Image A
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Image B
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  • Benign tumor composed of fat, smooth muscle cells and thick-walled vessels; believe to originate from perivascular epithelioid cells (PEC) (aka, PEComa).
  • Related to other PEComas such as lymphangioleiomyomatosis, "sugar" tumor of lung, cardiac rhabdomyomas, etc.
  • May arise in association with tuberous sclerosis:
    • >50% of patients with TS develop AMLs; <50% of AML has TS
    • Tend to be asymptomatic, bilateral, small, and multiple.
    • Associated with genetic alteration in TSC1 (chr 9q34) and TSC2 (chr 16p13.3)
  • Abundant fat containing tumor recognizable radiologically.
  • Gross:
    • Range from 1-20 cm (average 9 cm).
    • Golden-yellow appearance is common, but depends on the ratio (>fat looks like lipoma; > smooth muscle looks like leiomyoma) (image A).
    • Usually well-demarcated but not encapsulated.
    • May show local "invasion" or extension into perinephric or sinus fat.

Image C
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Image D
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  • Histology:
    • Consists of varying amounts of smooth muscle cells, mature fat and thick-walled vessels
      (image B) and (image C).
    • Vessels abnormal, often hyalinized and thick with eccentric lumen.
    • Smooth muscle cells appear to originate and "radiate" off the vessels (image D).
    • AML with benign epithelial cysts (AMLEC) is a rare variant.
  • IHC: spindle cells reactive with melanoma marker (HMB-45+, Mart1+) and actin+ thereby distinguishing it from other spindle cell lesions of the kidney.
  • DDX: lipoma (for fat predominant AML), leiomyoma (for smooth muscle predominant AML).
  • Benign course.
  • Surgical intervention recommended for tumors >4 cm (retroperitoneal hemorrhage is common complication).

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