CTA defines the size, location, internal architecture, enhancement, and vascularity of cystic renal masses.
is an Associate Professor of Radiology and the Chief of Abdominal
Imaging, University of North Carolina, Chapel Hill, NC.
At the University of North Carolina (UNC), most cystic renal
masses are removed by laparoscopy. Computed tomography (CT) plays a
critical role in the success of the procedure by defining renal
pathology and creating detailed vascular roadmaps for the surgeon
to use in treatment planning.
Table 1 outlines the UNC CT protocol for the staging of renal
masses. We do not use oral contrast. We use a 16-slice
multidetector scanner, a detector collimation of 0.75 mm, a table
speed of 24 mm/sec, a gantry rotation of 0.5 seconds, and a total
scan time of 24 seconds. We obtain 3 sets of axial images, each of
which is reviewed at 5-mm slice thicknesses at the picture
archiving and communications system (PACS) workstation.
We first obtain unenhanced images. Next, we obtain images during
the nephrographic phase, 100 seconds after intravenous
administration of 100 mL of contrast medium, 350 mgI/mL, at 3
mL/sec through a 20-gauge antecubital line. We then obtain images
during the excretory phase, 3 minutes after completion of
We have found that the first set of enhanced images is adequate
for review of vascular structures. Not all institutions obtain
excretory-phase images, but they can be useful for evaluation of
the ureters. If desired, an arterial-phase set of axial images can
be obtained with a delay of 25 to 30 seconds and substituted for
the excretory-phase images. We then create coronal and sagittal
reformatted images, using 5-mm-thick slices reconstructed at 3-mm
To reduce the risk of contrast-induced nephropathy, we require
that the serum creatinine level be checked in all hospitalized and
emergency room patients within 48 hours prior to the CT scan.
Outpatients complete a 6-question form. If the patient has known
renal disease, is currently undergoing chemotherapy, or has a long
history of treatment with nonsteroidal anti-inflammatory drugs
(NSAIDs), we require that the creatinine level be checked. In all
other patients, we do the CT scan without determining the
If the creatinine level is >1.8 mg/dL, a radiologist
evaluates the patient and considers whether to use isosmolar
contrast medium and pre-hydration, 2 methods that have been shown
to reduce the risk of contrast-induced nephropathy.
In some cases, we may also turn to an alternative imaging method,
such as magnetic resonance imaging. At this time, we do not use
sodium bicarbonate or acetylcysteine for the prevention of
The Bosniak classification system is useful for describing renal
Bosniak I lesions are simple cysts with no enhancement, no visible
wall, no internal architecture, and clear fluid densities. They are
benign lesions, and no follow-up is necessary. Figure 1 shows a
simple cyst in a patient with hematuria.
Bosniak II lesions demonstrate no enhancement, a few fine septa,
thin rim calcification, and fluid densities (Figure 2).
Proteinaceous cysts usually belong in this category. Bosniak II
cystic renal masses are usually benign lesions, with a <5%
likelihood of malignancy. They usually require no follow-up.
Bosniak IIF lesions do require follow-up. These are small
(<1.5 cm), high-density lesions in which densitometry is
difficult and the wall cannot be identified. These are usually
cortical lesions, usually occur in young patients, and are
difficult to diagnose with confidence. Current recommendations call
for a follow-up every 6 months for a total of 2 years. Any increase
in the size of the lesion indicates a vascular neoplasm in need of
ablation or surgical removal, depending on its location.
Bosniak III lesions have thick septations, a visible enhancing
wall, coarse calcifications, and areas of enhancement >15 HU.
Enhancement is the single strongest predictor of outcome,
identifying a 25% to 45% likelihood of malignancy.
Figure 3 shows a patient with a cystic renal tumor. The mass has
>3 thick septa. Intrusion into the renal hilum necessitates
Figure 4 shows a rare case of a multilocular cystic nephroma in
a middle-aged woman. There are multiple septa within the lesion, as
well as a few calcifications. Delayed imaging illustrates classic
herniation into the renal pelvis. Capsular feeder vessels can be
seen below the tumor. It is critical to identify these vessels
prior to laparoscopy, so that the surgeon can plan trocar insertion
and lesion removal accordingly.
Bosniak IV lesions are cystic masses with a solid wall nodule
and enhancement >15 HU. These malignant lesions are almost
always renal cell carcinoma (Figure 5).
With advanced CT scanners, we have found it necessary to modify
our criteria for identifying malignant lesions. In the past, with a
4- or 8-slice scanner, we used an enhancement cut-off of 10 HU.
Be-cause of the pseudoenhancement that occurs with beam hardening,
we found it necessary to raise the cutoff to 15 HU with the
16-slice scanner. Some university medical centers use a cutoff of
It is also necessary to be more circumspect when evaluating
heterogeneity in the internal architecture of a lesion. This
criterion has long been an important aid in the identification of
tumors. With 16- and 64-slice scanners, however, there is much less
homogeneity in normal tissue, particularly when automatic dose
reduction beam modulation programs are used.
Figure 6 shows a minimally enhancing renal cell carcinoma.
Attenuation on the noncontrast image was 44 HU at the upper pole
(Figure 6A), only a little higher than the 35- to 38-HU
characteristic of normal kidney parenchyma. On the
contrast-enhanced, nephrographic-phase scan, attenuation measures
60 HU (Figure 6B). There is some mottling inside the lesion (Figure
6C). The combination of enhancement >15 HU and architectural
heterogeneity defines this lesion as minimally enhancing renal cell
Reading the scan
The following information should be included in the
radiologist's formal re-port to the surgeon:
• The size, location, and number of renal masses. Partial or
heminephrectomy is usually contraindicated for removal of a large
polar mass deforming an adjacent calyx, one that abuts the center
of the renal hilum.
- The presence or absence of significant enhancement (>15
HU) on nephrographic-phase images obtained using a 100-second
- The relationship of the tumor to the renal vasculature and
- The presence or absence of enlarged adjacent lymph nodes
(>1 cm), and loco- regional or distant metastases.
- The location of capsular feeder vessels above and below the
Hemorrhage is the most common complication of laparoscopic
partial nephrectomy, occurring in approximately 10% of cases.
Approximately 25% to 30% of patients have multiple renal arteries.
CT can depict accessory renal arteries as small as 2 mm in width.
Three-dimensional reformatted images easily show polar arteries,
capsular branches, and early hilar branching.
It is critical that the surgeon also have information on renal
CT can depict circumaortic, retroaortic, and multiple and early
dividing renal veins. It can also reveal the presence or absence of
splenorenal shunts, renal vein thrombosis, and intravascular tumor
extension. CT aids in the detection of collecting system and ureter
anomalies as well. Using excretory-phase imaging, we see bifid
renal pelvis in approximately 10% of patients and ureteral
duplication in approximately 1%.
Radiation dose is a concern in CT evaluation of kidney masses.
Our 3-scan protocol can expose the patient to a total radiation
dose of 10 to 12 cGy. Automatic dose modulation is one possible
solution, but it can result in a poor signal-to-noise ratio and
questionable HU measurements, thereby undermining the critical role
of densitometry in differentiating benign from malignant cystic
renal masses. Our solution has been to use an automatic dose
modulation protocol with thin patients only. For patients weighing
>200 pounds, we select an mA equal to the body weight.
Beam hardening can cause an artificial increase in densitometry
measurements, particularly in lesions surrounded by bright cortex.
Fortunately, studies have consistently shown that the increase in
density is <10 HU.
Therefore, most centers now use 15 HU to define vascular
enhancement. If enhancement is indeterminate, the best approach is
to consider morphology, using the most worrisome feature to guide
Traditionally, biopsy of renal masses has been uncommon. In the
case of solid renal masses, for example, most radiologists believe
the probability of renal cell carcinoma is so high that a biopsy
seldom influences outcome.
However, two recent studies suggest that renal biopsy of complex
cystic renal masses may be useful in identifying patients with
benign masses who can safely avoid surgery.
Some centers are now performing biopsies of renal lesions, but in
my opinion, this approach has not yet been adequately studied. Of
primary concern is the possibility that biopsy may miss malignant
tissue within a cystic lesion.
Renal biopsy is indicated in certain cases. In a patient with
known lymphoma or lung cancer, a renal mass represents a primary
tumor in approximately half of cases and metastases in the other
half. To make this distinction and determine the best course of
treatment, it is necessary to do a renal biopsy.
Renal biopsy is also indicated in patients with superimposed
renal disease. For example, polycystic kidney disease with a
tortuous residual cortex can mimic a renal mass. Similarly, in a
patient with a superimposed chronic infection, a biopsy is
sometimes necessary to confirm or rule out the presence of a renal
Finally, in a patient with a solitary kidney, it is appropriate
to biopsy any mass. Nephrectomy will necessitate dialysis, and a
biopsy can assure both the surgeon and the patient that surgery is
the right course to take.
With the routine use of laparoscopic approaches for both partial
and total nephrectomies, radiologists must provide a complete data
set for surgical planning. This includes more than the routine size
and location of the mass. A radiologic report now delineates the
arterial and venous drainage patterns of the tumor, including the
presence of capsular vessels. With respect to cystic renal masses,
careful application of the Bosniak criteria, with special attention
to lesion morphology and enhancement on multidetector scanners,
should lead to excellent results.
ELLIOT K. FISHMAN, MD:
Thanks very much, Julia. Let me start off the questions. If someone
presents with hematuria, what's your basic protocol?
JULIA R. FIELDING, MD:
Painful hematuria is a renal stone protocol. Painless hematuria
goes to a CT urogram. A CT urogram is done instead of an IVP in the
majority of patients now, and is basically a two-run series where
we do a 75-second delay, and then we do a 5-minute run. I happen to
use a compression device on my patients, rather than use a diuretic
or something like that. But it is important to get the urine out
into the collecting system and ureters. I have very good results
What kind of contrast protocols are you using for these
I use 100 mL of 350 mgI/mL contrast injected at about 3 mL/sec most
of the time. We have a 16-slice scanner. I use a 0.75-mm
collimation for the CT urogram. So it's a hefty dose again. I'm
trying to keep it down to a dull roar, especially for the
In terms of looking beyond the axial plane, is that just for
problem solving, or is that something you do routinely?
We do it for every single case. Every single case of painless
hematuria, characterized renal mass, and surgical planning for
renal mass is done with 3D imaging. So it is pretty routine for us
KAREN M. HORTON, MD:
So you don't do noncontrast CT? Or do you think you can pick up the
stones on the 75-second delay scan?
No, because what I'm doing is I'm trying to separate the patients
into 2 groups, basically. It's true that you can have hematuria
from stones that are just rattling around, I guess, inside your
inter-renal collecting systems. But I have elected to avoid a third
run right now. So it may be that I miss small stones, but I'm
hoping that I get all the cancers.
Do you have any difficulty with Bosniak II lesions or high-density
renal cysts simulating a tumor?
Sure. Everybody runs into difficulty with that.
But, if you have a noncontrast CT, the main reason to me is that
you can always see the stones. Are the small stones important?
Okay, but remember, though: What bleeds? Not renal cells. Renal
cell bleeds rarely. Transitional cell bleeds all the time. So I've
actually tailored the two protocols separately, a renal cell
protocol for a renal mass. A cortical mass is a different protocol
that does include a pre-contrast study. But, to me, an urothelial
issue doesn't require one.
I don't disagree. Just statistically, the amount of TCCs we see
compared with renal cell carcinoma is very small. But when a
patient's referred to you from the urologist and comes down with
hematuria, where it's basically "find something." That's kind of
what we're typically looking for.
Right. I had to give them a check-box form that I sent up to them.
The check-box form had six boxes on it. It says "painful hematuria.
I think this person has a stone-check." Or, "painless hematuria.
Assess for urothelial lesion." Or, "preoperative planning for renal
cell carcinoma." So they ended up getting a sort of drop-down menu
that they use to send things to us.
There certainly are cases where I have someone come in with
plain old hematuria, but so far, I guess because I have good help
working in a resident-located place, we figure out what's going on
from our computer records and decide which test we think is best.
So I'm not for one-protocol-fits-all renal cases. Partly that's
because I'm a GU radiologist, so I guess I'm more of a splitter
than a lumper for that kind of stuff.
GEOFFREY D. RUBIN, MD:
I think that was a nice comprehensive overview of renal mass
assessment. How frequently are your surgeons doing the
nephron-sparing surgery laparoscopically and requiring you to work
out some of the smaller venous anatomy? You mentioned the capsular
vessels and such?
As long as I can give them the capsular vessels and whether there's
an early or a large draining vein, that seems to be okay. The
obvious big renal venous anomalies, I think, are okay. So I'm not
required to find smaller items than that.
Do you do those studies biphasically to look for venous and
arterial anatomy? Or you try to get them in one phase?
I get them at one time. That's what I'm doing right now, which I
know is a little unusual.
So what's your delay and how long of an injection are you
I give a bolus of 100 mL at about 3 mL/sec, and I use a 100-second
And you still have arterial opacification?
I do. I know it's a weird protocol, but to me, it's so critically
important that I get the correct enhancement on those things, which
is why I've chosen that. But you could certainly do an
arterial-phase image first.
I guess if it was important for my surgeons to know whether there
was a tiny accessory or capsular artery, I would be concerned that
waiting 100 seconds with effectively a 30-second bolus would leave
me with insufficiently opacified arteries. What do you do?
We do a lot of partial nephrectomies. That's one of the most common
indications we have for doing CTs. We will do it with
arterial-phase imaging, because they demand really detailed
arterial maps from us. We do the arterial, venous, and we might get
a delayed image. To evaluate a big renal mass, just to stage the
tumor, I don't worry about noncontrast scans.
But the other thing you can do just for noncontrast scans
through the kidneys is 5-mm scans; I always can see stones on
arterial-phase imaging. If the stones are there, you see them
anyway. So the only value to me of the noncontrast CT, quite
frankly (besides knowing the kidneys are where they're supposed to
be) is determining lesion density. So if I had a high-density renal
cyst, I could look sequentially from non-contrast through delayed
Particularly in this era of 64-slice scanners, we can get
fantastic angiographic maps that I think really make the difference
in whether or not they can do partial nephrectomies in borderline
cases. The partial nephrectomies are also done routinely
laparoscopically, which makes a significant difference. If you go
by strict criteria, previously it was 4 cm, exophytic, and
peripheral--those are the simple cases. If you find those, it's not
a major to-do. But the surgeons are getting more and more
aggressive, particularly as we're picking up patients in their 50s
with renal masses. So our experience has been that they want that
level of detail. Obviously, you want to make sure that the patients
you're selecting for partial nephrectomies will end up with good
five-year results. So we will do that arterial-phase image.
I can see what Geoff was alluding to, that with the 100-second
delay, I can see the renal arteries, but I just don't have the
detail. Also, in terms of venous mapping, if I'm doing really good
venous mapping, a 70-second delay tends to be good. We're typically
using 100 mL of contrast at 4 mL/sec on a 16- or 64-slice scanner.
That gives a really good venous opacification. I've not made any
mistakes in terms of flow-related change in either the IVC or the
renal vein. When the tumor does extend beyond renal vein into IVC,
we've been very good at being able to determine exactly how far the
IVC goes, whether it's above the diaphragm or not.
As you said, the postprocessing is critical. We do a
combination, a little like what you showed, 3D mapping using volume
rendering and MIP, and then some coronal displays, which tend to
work fairly well. We do use sagittal images occasionally,
particularly if we're uncertain if a lesion is renal, adrenal, or
liver. But I tend to be one who likes that early-phase imaging.
Particularly recently, with the 16-row and faster scanners, if you
trigger right on the arterial enhancement, you can routinely see
interlobar arteries of the kidney. Then it becomes a question as to
whether the surgeons are going to use that information in planning
their surgery, and it becomes a bit of an
"if-you-build-it-they-will-come" phenomenon. If they've never known
that you can show it to them, then they might say, "Oh, what you're
giving is just fine." But if you can show them intrarenal branches
and their relationship to the tumor, they might say, "Gosh, I'd
like that in every case." Are you alluding that the demonstration
of intrarenal branches is impacting on the surgeon's decision?
Yes. I think the big thing now is that surgeons are becoming more
aggressive as we give them more detailed mapping. As I said, they
used to go after the peripheral exophytic lesions. Now they going
for more of the central ones, where they do much finer dissection
than they did before. I think the other issue is that you might
have a problem with the venous invasion. But if you're just looking
in terms of defining venous anatomy, typically at 30 or 40 seconds,
you have all the venous anatomy anyway. So the only time the venous
phase per se helps is if you have IVC extension, and maybe at 40
seconds you have some flow-related changes. But in terms of
defining venous anatomy, Satomi Kawamoto at Hopkins actually wrote
an article in
that said that basically from the 30 seconds you get all the
arterial and venous images; and that's based on looking at renal
Are you doing renal donors uniphasic now?
We are starting to, yes. Now it's interesting, as Geoff said, in
some ways, what you provide the referring docs is what they expect
you to provide. So on all of our hematuria cases and in all our
renal cases, we routinely do CT urography. Since you need
delayed-phase imaging anyway and delayed phase will give you that
differential enhancement. We always reconstruct with CT urography
routinely, in every patient. So, if you start looking at numbers,
for your arterial-phase image, 85% of renal cells enhance. So right
away, you can have some really high numbers. So, when looking at a
large number of cases, the reality is that if you have
arterial-phase and excretory-phase images, you always see
Now the times it may not happen are small. If you have a
relatively hypovascular renal cell, it may not be a substantial
difference between the early-phase and late-phase imaging; but
otherwise, it does work pretty well. I think the issue is also that
if it weren't a radiation issue, you'd get four phases in
everybody; that would be very simple.
So the question is, how do you work around minimizing the number
of acquisitions while maximizing the information?
ELLA KAZEROONI, MD:
How much of the postprocessing is physician-driven? Is it an
entirely physician-driven process for renal mass?
It's done by technologists for the pancreas, for the liver, for the
kidneys--basically, the visceral organs. Those have standard
technological processing that's done by the technologist. Then we
have a workstation in our room that we can do extra stuff on. But I
don't do it as often as you do. I would say that maybe 20% of the
time I want to look at it some other way and I cut it some other
I think that's true in our practice for solid organ work. Our renal
postprocessing protocol is very specific from the 3D lab and it's
usually sufficient. We get thin-slab MIPs along the axis through
the axis of the kidneys, and then we get the whole kidney views. We
also measure cortical and whole renal volumes on every patient.
That's something that our urologists and nephrologists are
demanding. They really like that, and it's a very quick
segmentation of cortical volumes.
For most of the visceral stuff, it is actually pretty easy to just
set and go.
Well, I'll disagree 100%! We do them all ourselves.
But is that not a waste of physicians' time?
No. I think we have the best technologists at Hopkins, or as good
as anybody. They know how to do 3D. But the bottom line is the way
you look at a kidney is in 3D. You can look at the combination of
axials, coronals, and sagittals. The only reasons you do it the way
you do it now are tradition or the lack of available computer
It's also use of physician time.
Not to be argumentative, but I guarantee you that I could do a 3D
faster than you could look at axial images.
But you are an isolated case.
No, all you've got to do is put the time in.
Do you save the 3D images for the physicians on the PACS? Or do you
only process them for your own use?
We're doing about 150 CTAs a week, or more. When I look at them, I
film the key images as I'm looking--these key images are between 20
and 30 images, depending on the case. They're printed out like
high-res film and they're delivered to the physicians at two
o'clock every day.
What about when you go filmless?
The thing is, this has nothing to do with being filmless. You can
have filmless radiology; you can have everything on the Web. But
physicians like to have images in their hands. Now we don't give
them 14″×17″film; the film is 8″×11″. I can tell you that
pancreatic surgeons and renal surgeons take those images, they look
at them, and they show them to their colleagues. They put them in
the patient's report so they can bring them up and take them to
surgery. So, although you're filmless, the cost for cameras is not
much, less than $500. You plug them on your workstation.
Do you take the pictures, too?
Yes, because you do it when you're looking and no one knows better
than you. If you tell me that technologists know as much as you do,
then we have a problem with reimbursement. Then you're being
But you're being underpaid if you're wasting your time doing all
No, I'm not being underpaid. That's why you have 2 hands and 10
fingers. You do everything at one time--multitasking. So as far as
I'm concerned, you look, you film, you talk, it's over. Boom.
That's it. I don't have to go get the films to look at what someone
else did. I do it at one time. It's over.
When I bring up a CT, I'd like to see the images at least in the
coronal, axial, and sagittal planes, and be able to scroll through,
thicken or thin them up as I need to, and adjust window settings,
interactively. For specialized protocols, like coronary CT
angiography, I spend much more time using the advanced processing
workstations. The place-holding function or "file room" function of
having some standard proto-col-driven images is done by our 3D lab
techs. I can then do additional processing on the cases I feel it's
needed on. Then, as far as film, I can't imagine giving film to one
of my surgeons anymore. It's all filmless. Or, I can send images to
our server and put them quickly into PowerPoint so I can e-mail
them, and their response to the latter is always very positive and
I'm met with a lot of gratitude.
We do that, too. We save interesting images. But it's an
interesting philosophy. What volume of CTAs are you doing in a week
We're divided up into organ specialties. Just in thoracic radiology
alone we're probably roughly doing 50 to 60 a week. But we have
thoracic, abdominal, and neuroradiology divisions all doing
One of the simple things that we find is that with our pancreatic
surgeons and most of our GI surgeons, not only do they want to see
the images, they want to see them in real-time. So even sending
them images, that's cool. They want to physically come down. John
Cameron, who is professor of surgery and ex-chair of surgery, who
has done more Whipple procedures than anybody in the world, will
routinely not do a Whipple without seeing the study in 3D. You
know, he wants static images, but he wants you to be roaming
through the data sets.
So one of the things you're seeing changing is what the
referring physicians expect to get from us. For years, referring
physicians got films of a few images, or they got a few things on
the PACS. That's not good enough. Our guys don't want to see axial
images, and they don't even want to see a few 3D images. They want
the interactivity. That's where I really see things going, because
as workstations get faster, as you get the load times, the
interactivity increases. That's why 64-slice CT is a disruptive
technology. If you're giving axial images or just a few simple
things, it really minimizes what you get out of the data set.
Technology is wonderful, right? Whether it's sending a few
specialized coronal MIPS or whatever it is you want to do, you
could send a coronal stack of 30 images in a PowerPoint file and
they could open it up whether they're at home or in their office or
in the surgical suite. They can have the semblance of interactivity
by scrolling through it in a PowerPoint file, similar to what they
could appreciate on a PACS workstation.
But what I think is that you tend to underestimate your expertise.
The thing that you get paid for as a radiologist is your expertise.
As Geoff was showing those images, it's choosing the right image
that tells the answer. You could have 100 images that don't tell
the story as well as the 1 right image. That's what I get paid for,
that 1 image, not the other 97 images. So I think it's an evolving
process. We've been evolving for 20 years and I said the same thing
in 1986. So it's just getting closer to the truth now!
It also will impact on reimbursement. The reimbursements for 3D
will be different. If a technologist is generating these coronal
and sagittals, you get zero reimbursement now. That's gone. So
unless you're doing postprocessing yourself, you will not get
reimbursed. That's a very practical aspect of reality--if you do
it, you will get paid physician time.
I do think that it's important to dispel the notion that all
technologists are capable of doing is pumping out coronals and
sagittals. They certainly can be trained to do more.
Techniques for lower-extremity CT angiography