Benign Kidney Tumors: #1 is Angiomyolipoma

Andrew Siegel MD   1/30/2021

A “tumor” is defined as a swelling caused by an abnormal growth of tissue. Fortunately, not all tumors are malignant and with respect to kidney tumors, more than 20% deemed to be suspicious for malignancy are actually benign, including complex cysts, angiomyolipomas, oncocytomas, etc.

Even though angiomyolipomas (AML) are present in only a small percentage of the population, they are the most common benign tumors of the kidney. Most AMLs are found incidentally on imaging studies, meaning they were unanticipated findings discovered on imaging studies done for other reasons.

“Angiomyolipoma” is a medical mouthful, but if you break it down into its components it is easily understandable. Angio- refers to dilated blood vessels , Myo- refers to smooth muscle, and Lipo- refers to mature fat cells.  Thus, an AML is a benign kidney growth composed of blood vessels, smooth muscle and fat, all normal components of the kidney but put together in variable proportions, like a recipe with the right ingredients, but the wrong proportions. Their highest prevalence is in middle-age females. AMLs are usually small, solitary and rarely progress to a clinical problem.

Because of the presence of fatty tissue, AMLs have a characteristic appearance on imaging studies and are usually therefore readily and easily diagnosed. See CT (computerized tomography) image below and note the kidney lesions (arrows) that have a high percentage of fat within. Notice how similar the fat density of the AMLs is to that of the fatty tissue surrounding the kidneys and the fatty tissue of the abdomen.

Hellerhoff, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0&gt;, via Wikimedia Commons

See microscopic image of AML below. Note angio- component (red blood vessels), myo- component (violet sheets of smooth muscle), and lipo- component (white fatty globules).

Sarahkayb, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0&gt;, via Wikimedia Commons

Most AMLs occur sporadically, although some are hereditary and under this circumstance usually associated with tuberous sclerosis (a genetic disorder characterized by the growth of numerous benign tumors in many parts of the body).  Hereditary AMLs tend to occur at a younger age, affect both genders equally, and are likely to be multiple, large, occur on both left and right sides, and prone to manifest earlier and act more aggressively.

AMLs rarely cause symptoms, but when they do so they typically present with bleeding due to the angio- component with its dilated blood vessels. This tends to happen with larger AMLs because the more sizable they are, the greater the potential for rupture and hemorrhage.

Because of their abundant fat, AMLs are usually easily recognized on ultrasonography as “hyper-echoic” (lots of internal echoes) lesions with acoustic shadowing (shadowing of the sound waves due to the presence of fat). However, about 5% of AMLs have insufficient fat to allow them to be easily distinguished from other renal masses, including cancer.

Small, asymptomatic AMLs do not require treatment, requiring only periodic monitoring. Treatment is indicated for symptomatic AMLs, when they occur in women of childbearing age, when there is a suspicion of malignancy, and when they are large. Traditionally, large is defined as greater than4 cm. in size, although the exact size at which intervention is necessary is subject to debate.  

Management may include: active surveillance (watchful waiting with periodic ultrasonography); selective renal artery embolization (interventional radiology procedure to choke off the arterial blood supply to the AML); partial nephrectomy (removal of the AML and a small margin of normal kidney tissue, leaving the remainder of the kidney intact); nephrectomy (removal of the entire kidney); cryoablation (freezing the AML); radio-frequency ablation (heating the AML); and treatment with medications including mTOR inhibitors, sirolimus or everolimus (a class of drugs that inhibits the development of blood vessels).  

Wishing you the best of health,

2014-04-23 20:16:29
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Dr. Andrew Siegel is a physician and urological surgeon who is board-certified in urology as well as in female pelvic medicine and reconstructive surgery.  He is an Assistant Clinical Professor of Surgery at the Rutgers-New Jersey Medical School and is a Castle Connolly Top Doctor New York Metro Area, Inside Jersey Top Doctor and Inside Jersey Top Doctor for Women’s Health. His mission is to “bridge the gap” between the public and the medical community. He is a urologist at New Jersey Urology, the largest urology practice in the United States.  His latest book is Prostate Cancer 20/20: A Practical Guide to Understanding Management Options for Patients and Their Families. 

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Video trailer for Prostate Cancer 20/20

Preview of Prostate Cancer 20/20

Andrew Siegel MD Amazon author page

PROSTATE CANCER 20/20 is now available at Audible, iTunes and Amazon as an audiobook read by the author (just over 6 hours). 

Dr. Siegel’s other books:

FINDING YOUR OWN FOUNTAIN OF YOUTH: The Essential Guide to Maximizing Health, Wellness, Fitness and Longevity

PROMISCUOUS EATING— Understanding and Ending Our Self-Destructive Relationship with Food

MALE PELVIC FITNESS: Optimizing Sexual and Urinary Health

THE KEGEL FIX: Recharging Female Pelvic, Sexual, and Urinary Health

Video on THE KEGEL FIX

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