Issues in Imaging the Kidney

Introduction

People with VHL are at increased risk for kidney cancer.  With our worldwide advisors, we have worked out a system of “watchful waiting,” monitoring the growth of kidney tumors, and ablating or removing them before they reach the 3 cm level.  While size is not a precise measure of the metastatic potential of a tumor, it is the best indicator we currently have, and it has proved to be very reliable.

In order to do that “watchful waiting,” it is important to obtain images of the kidney on a regular basis (with many recommending that this imaging be performed once a year) to check on the size and rate of growth of the kidney tumor(s) being watched.  Until a few years ago, we were using both CT and MRI relatively freely. 

VHL tumors of the kidney are highly vascular, and are best seen when a contrast medium has been administered.  The contrast is a substance injected into your bloodstream that colors the blood vessels and gives the radiologist a better outline and view of the tumor, and a more accurate measurement of its size. 

Contrast medium for CT and MRI

Although the contrast medium used for CT (iodinated contrast medium) is well tolerated in people whose kidneys are functioning normally, it has been known for many years that this contrast can damage the kidneys in people who already have compromised renal function.  Due to this problem, many physicians turned to the use of contrast-enhanced MRI, as an alternative imaging study to evaluate the kidneys in people with kidney disease. (MRI contrast medium, which is completely unrelated in structure to iodinated contrast material, does not have this same potentially damaging effect on the kidneys as does iodinated CT contrast, when the MRI contrast medium is used at standard doses).

Nephrogenic Systemic Fibrosis

Beginning in about 2003, reports began to emerge of a nasty complication that occurred among people with poor creatinine clearance, a complication called nephrogenic systemic fibrosis (NSF).  In April 2006, it was first noticed that most patients who developed NSF had received an injection of contrast medium for MRI (which are gadolinium-based contrast material) within weeks to several months of their developing symptoms.  The association between bad kidney function, MRI contrast-medium, and NSF has since been confirmed in many other studies. 

NSF produces stiffening of the skin and can become so severe that affected individuals lose their ability to move their joints and may even become immobile.  NSF can sometimes affect internal organs as well.
VHL and NSF

The question for people with VHL is: how can we get the best possible images of the kidneys without putting the kidney at risk, and without causing NSF?  Since the kidneys can only be evaluated well if some type of contrast material is administered, the option of doing an unenhanced CT or an unenhanced MRI, as an alternative, is not a very good one.

It is important to emphasize that people with VHL who have normal kidney function are not at any greater risk of NSF than anyone in the general population.  VHL is not a “renal disease” in that it does not erode kidney function.  You can have multiple kidney surgeries and still have perfectly good renal function.  But there are some people with VHL who will be at increased risk for developing NSF, and these people should confer with their own urologist and radiologist about how best to proceed.  These are people who have lost so much kidney tissue, due to prior surgeries, that their remaining kidney function is severely limited.

How is kidney function measured?

There are two ways in which kidney function is now measured.  The older method involves obtaining a serum creatinine level.  A normal creatinine level is usually about 1.4 mg/dl or lower.  A higher creatinine level indicates that your kidney is not functioning normally.  Many physicians believe that a measurement of creatinine clearance or glomerular filtration is more accurate.  This is not easily obtained directly.  Instead, formulae have been developed that estimate glomerular filtration rate (eGFR).   These formulae use several variables, including the serum creatinine level, but also age, gender, and race.  Normally, the calculated eGFR should be over 90 ml/min. A lower eGFR indicates that your kidney is not functioning normally. 

As far as we know at this time, all MRI contrast media can be used safely, without concern for NSF, in patients who have normal or even mildly elevated serum creatinine levels or eGFR levels of 30 ml/min or higher.  It is only those patients with severe renal insufficiency/failure who are at increased risk (GFR<30 ml/min).

Does the type of MRI contrast medium matter?

The incidence of NSF in high risk patients also may vary depending on which gadolinium-based contrast medium was used.  The chemical composition of each brand of MRI contrast media is different.  To our knowledge, there are several brands that do not yet have a single case of NSF definitely associated only with them, so use of one of these agents may lower one’s risk.

So what to do?

At the present time, many experts have made a number of specific recommendations.  First of all, if you have any concerns about your kidney function, you can ask your physician to get a blood test so that your serum creatinine level can be determined and your eGFR calculated.

1. If your kidneys are working normally:
If you have normal kidney function, you can be imaged with either contrast-enhanced CT or MRI without fear of damaging your kidneys or of risking NSF. 

2. If you are already on dialysis and further kidney damage is not a concern:
If your kidney function is damaged so badly that you are already on permanent dialysis AND there is no concern about making your kidneys function more poorly, your kidneys can (and probably should) be imaged by CT rather than MRI, with iodinated contrast medium administered. 

3. If you have mild or moderate kidney damage:
If you have mild kidney damage (with an eGFR of 30 ml/min or more) a contrast-enhanced MRI is probably preferable to a contrast-enhanced CT.  The risk of NSF here is minimal.

 

4. If you have severe kidney damage and are not yet on dialysis:
If, on the other hand, you have severe kidney damage (with an eGFR < 30 ml/min) and are not yet on permanent dialysis, then the decision is more difficult and should be made after carefully consulting with your physicians.  If a decision is made to perform a contrast-enhanced MRI, it is suggested that one of the contrast agents that has not been observed to produce NSF be used and that the standard recommended doses of the MRI contrast-agents not be exceeded.  This means not having magnetic resonance angiography (MRA), which may use higher doses of MRI contrast medium. 

What is the actual risk in high-risk populations?

It should be remembered, however, that even in high-risk patients, and even when using high doses of the brand of contrast medium with the highest reported association to NSF, the risk is no more than 5-10%.  More than 90% of patients injected with this contrast medium did not develop NSF.  We now believe that when alternative MRI contrast-agents are utilized at lower doses, the risk of developing NSF is probably much lower. 

For example, the radiology department at Johns Hopkins University (JHU) School of Medicine reviewed all their patient cases (not just VHL) from 2003 through 2006 and worked to reduce the risk of NSF.  They published the recommendations shown in Figure 1.  Using these guidelines, they reduced the rate of NSF at JHU from 36.5 cases per 100,000 gadolinium-enhanced procedures to four cases per 100,000, “suggesting that the new policy was associated with a significantly lower incidence of NSF.”  Using the precautions mentioned above, at the University of Michigan, no cases of NSF have been reported since late 2006, even though contrast-enhanced MRI is still performed in some patients with severe kidney disease.

Again, there is no pat formula that works for everyone, and we will continue to learn as new information becomes available.  We encourage you to have a conversation about this with your urologist and with your radiologist.  You and your team should make the decision together as to how to proceed.

Questions to ask your doctor:

• What is my estimated glomerular filtration rate?
• Do you recommend a CT or an MRI?
• Am I at increased risk of developing NSF?
• What precautions do you recommend I take to make sure that my risk of further damaging my kidneys or of developing NSF is minimized while getting the best possible images for my urologist?
• Would it be helpful for me to drink more water before or after the imaging procedure? (this is not a good idea for everyone, so be sure to ask your doctor)

We should also add that this article is not intended as an absolute recommendation, but rather as mechanism for providing you with information to assist you in having an informed discussion with your own doctors, so that you can participate in determining the best course of action for you.

References:
1. Wayne Forrest, “JHU Study: Screening patients prior to MRI lowers NSF risk”,
http://www.auntminnie.com/index.asp?Sec=sup&Sub=mri&Pag=dis&ItemId=84478
Perez-Rodriguez et al, ” Nephrogenic systemic fibrosis: incidence, associations, and effect of risk factor assessment--report of 33 cases.” http://www.ncbi.nlm.nih.gov/pubmed/19188312

American College of Radiology, Manual on Contrast Media, see esp. the chapters on “Contrast Nephrotoxicity” and “Nephrogenic Systemic Fibrosis”.  http://www.acr.org/SecondaryMainMenuCategories/quality_safety/contrast_manual.aspx

As printed in the VHL Family Forum 17:2, May 2009. For permission to reprint, please contact VHL Family Alliance, editor@vhl.org. Further information is available from the VHL Family Alliance, info@vhl.org.