Pitfalls: PET-CT, Part 1

Introduction to PET CT Lectures

I'm gonna do two lectures in PET CT and hopefully give you a fairly broad but comprehensive overview of some of the pitfalls that you'll encounter when reading PET CT.

In the first lecture, we're gonna cover neck and chest, and in the second lecture we will cover abdomen and pelvis as well as some artifacts that you may encounter.

Lecture Outline

The lecture outline is as follows. We will cover normal and false positive FDG activity, including looking at the glands and lymphoid tissue, muscle fat, iatrogenic and surgical causes of FDG activity, inflammatory infectious, as well as benign tumors that can cause increased FDG activity.

Then we'll talk about some size related pitfalls in PET CT as well as non FDG avid tumors. And then at the end we'll do some difficult case review.

Normal Uptake Patterns and Variants in the Neck and Chest

Starting with normal uptake patterns and variants in the neck and chest, what I'm gonna do is compare and contrast normal uptake from pathologic uptake. And I'm gonna use a purely practical approach given the 45 minute limitations. It'll be a case-based approach with some studies referring to the cases that I'm showing you, but otherwise not focusing on the literature.

Glands and Lymphoid Tissue

In terms of patterns of uptake, normal FDG uptake is usually symmetrical or diffuse. It can be asymmetrical though if the patient has had surgery, and sometimes dependent upon the physiological state of the patient, including patients that are lactating as well as other etiologies.

Thyroid Uptake

Just a look at thyroid uptake. This is a PET Zina patient who I believe was for a pulmonary nodule. We see a fairly diffuse intense FDG activity in an otherwise normal appearing thyroid gland.

The differential for bilateral areas of increased uptake in the thyroid includes physiologic uptake thyroiditis, or multinodular goiter. Generally if you see bilateral fairly symmetrical uptake, I generally recommend correlation with thyroid function tests and leave it at that.

Another example with a patient with multiple nodules, so multinodular goiter with diffuse FDG uptake, again, we're not too concerned with malignancy in this type of scenario.

However, if you see focal asymmetrical FDG activity, whether the thyroid gland looks normal or not, you should recommend a correlation with ultrasound and fine needle aspiration if there is a nodule as several studies. And this is just one looking at incidental increased FDG activity in certain organs. This study looking at thyroid uptake shows that about 24, 25% of patients with focal asymmetrical FDG activity can be incidental thyroid carcinoma.

However, just because it's asymmetrical doesn't mean that it's a malignancy. And the differential does include benign adenomas, which can also take up fairly intense FDG activity.

Salivary Glands

Moving on to the salivary glands, we've got parotid as well as the submandibular glands, and again, we're looking for symmetry. So on the coronal PET image here, we see fairly symmetrical diffuse intense FDG activity. And keep in mind that the range of normal in glands, lymphoid tissue, as well as muscle and fat can be anywhere from almost no uptake to very intense FDG activity. And it does not correlate generally with pathology. So just keep that in mind.

Here's an example of asymmetrical uptake in the right neck. And again, when you see something that's asymmetrical, you wanna look at the CT for correlation. And in this patient we see that the right parotid gland looks normal, whereas the left parotid gland is absent. So this is this should be almost expected when you're reading, and this could be intense uptake as I've showed you, and be normal. But it's asymmetrical only because the left parotid gland is absent, so that's asymmetrical.

So the differential diagnosis for asymmetrical gland activity, the most common is going to be a benign lesion such as pleomorphic adenoma or Warthin's tumor. About 60% of pleomorphic adenomas will take up FDG activity. So a parotid lesion is generally still going to be a benign lesion, even if it has asymmetrical intense FDG activity.

Upwards of a hundred percent of Warthin's tumor actually have intense FDG activity. However, it's pretty much impossible to exclude a neoplasm such as mucoepidermoid carcinoma, adenoid cystic, or even metastatic disease, as most of these patients do have a history of cancer.

And then keep in mind, again, if the patient has had contralateral surgery, that it's often normal to see asymmetrical FDG activity in the normal remaining gland.

So here's an example of a patient with intense focal FDG activity unrelated to the head and neck. We see that there is a enhancing nodule here in the right parotid. There is intense FDG activity correlating to that nodule. Again, the differential for this would be a benign lesion such as a pleomorphic adenoma Warthin's or less likely carcinoma. And you can say that in your dictation. However, we generally do recommend pathologic sampling because we can't exclude those types of malignancies.

In terms of the submandibular glands, same thing. Fairly symmetrical is what we're looking for. It can be anywhere from non visible almost or no FDG activity upwards of having very intense FDG activity as long as it's symmetrical and the glands look otherwise normal, it's normal.

Lymphoid Tissue

Moving on to lymphoid tissue, all of the structures in Waldeyer's ring can take up FDG activity. And again, the range is anywhere from almost no activity to very intense.

Here we see the adenoids with fairly intense symmetrical FDG activity. We also see some normal physiologic uptake in the oropharynx and soft palate.

Here we have bilateral symmetrical intense FDG uptake in the palatine tonsils. Again, this is normal, although you can see this in people who have had a recent upper respiratory infection. So other benign things can also cause bilateral symmetrical uptake, not just normal, it could be reactive tissue there.

Here we see physiologic intense symmetrical activity in the lingual tonsils, which are slightly more medial than the palatine tonsils on the base of the tongue. Again, this is normal. The tissue is very symmetric on CT. There's no mass, nothing to worry about here.

However, you can see asymmetrical FDG activity within lymphoid tissue. And the differential here would include a possible neoplasm, particularly if the patient has a strong smoking history. So you need to include that.

However, this is an example of hyperplasia. So if a patient had say a neck infection on one side, you can have reactive adenopathy or reactive hyperplasia on the ipsilateral side. And there have been several case reports showing asymmetrical uptake corresponding to benign follicular hyperplasia.

Here we see fairly symmetrical, but slightly asymmetrical on the left FDG activity. Again, I would probably read this out as unless the patient had a known history of lymphoma involving that tonsil, that there's fairly symmetrical uptake, although it's slightly more prominent on the left. On CT, there's no definite asymmetry or mass likely benign or physiologic.

Thymic Activity

Moving on into the chest. Thymic activity, this is a pretty good appearance for thymic activity. Basically, you see this upside down V-shaped appearance, and in younger patients you will see this a lot more commonly. You generally, in older patients, you shouldn't see uptake related to the thymus, with a few exceptions.

The differential for uptake in the thymus in older patients would be if patients have had chemotherapy. So you can see reactive tissue in the thymus. And in younger patients, this is completely normal. You do want to see that it is linear, generally on the coronal images. And then when you look on the axial images, we see a nice just normal soft tissue rind here corresponding to the thymus.

This is an 18-year-old with a questionable mediastinal mass. And again, on the CT we see just a normal rind of tissue in the anterior mediastinum, diffuse FDG activity.

If the patient has a history of Hodgkin's lymphoma, sometimes this can be difficult. And I'm gonna show you another case. Yeah, so this was actually a patient with Hodgkin's lymphoma who we treated, and then this was a follow-up PET CT and it looks fairly focal and asymmetrical on the right, however, her primary disease in the neck as well as she had axillary nodes actually resolved. So this was new activity, and unfortunately because it was asymmetrical and focal, the patient was a medical student and she wanted to know what this was. So they actually sampled this tissue and this was just reactive tissue.

So thymic uptake, this is a study looking at the degree or incidence of uptake in terms of age. And they found that 73% of untreated patients up to the age of 13 had thymic uptake as well as 8% of patients in the fourth decade of life. So you do see it occasionally in patients who were 40, even 50. But, and again, if you see it older than 40, 50 years of age, think of whether the patient's had chemotherapy. Generally, you see uptake in the thymus about six to eight months after initiation of chemotherapy.

Breast Tissue

Generally breast tissue does not take up FDG activity. It's not metabolically active, but in patients who are lactating, actively lactating, it can have fairly symmetrical but focal areas of intense FDG activity and this would be completely compatible with the patient's history of lactation. And then in the same patient, once they're done lactating, we see complete disappearance of uptake.

Now, keep in mind, just like the thyroid, that if you see focal intense FDG activity in the breast, those almost always are going to represent breast carcinoma. So unlike the thyroid where the incidence of carcinomas about 25 to 50%, when you have focal intense asymmetrical uptake in the breast, if you have focal intense uptake, it is pretty much going to be carcinoma. So recommend mammography, ultrasound and biopsy.

So in terms of glands and lymphoid tissue, the range of normal uptake from minimal to intense, as we said, symmetrical activity is generally benign. If asymmetrical and no history of surgery, it may be inflammatory or neoplastic. So you may want to recommend pathologic sampling even in a asymmetric palatine tonsil, for instance. And then asymmetrical focal uptake in the thyroid with nodule, recommend pathologic sampling.

Muscle

Okay, moving on to muscle, again, patterns usually symmetrical, although there are several muscles that can have asymmetrical uptake, particularly if they had, say a radical neck dissection and the sternocleidomastoid muscle's gone. The contralateral normal muscle will oftentimes have asymmetrical activity, and you always wanna correlate with the CT images.

And then if you have a patient who has a lot of muscular activity, you can actually decrease the amount of activity by giving pre-treatment or pre-scan benzodiazepines to reduce both muscular and fat activity.

Some muscles that are characteristically or commonly seen as asymmetric would be the diaphragm, neck muscles such as platysma, scalene muscles, the sternocleidomastoid, any of the swallowing muscles, as well as the mylohyoid. And I'll show you some examples of these asymmetrical physiologic uptake in these muscles. And then facial muscles, the obliques are notorious for being asymmetrically metabolically active, other muscles of mastication as well.

So here we see focal intense FDG activity in the paraspinal muscle. Now on the axial images, it would be difficult to exclude some sort of mass here, although I don't see anything on this non-contrast CT. So you always wanna look at all three orthogonal planes in PET CT imaging because it will give you generally the answer. And that is that there's linear uptake within the paraspinal muscles bilaterally. And again, the muscle belly looks unremarkable.

There's the trapezius muscle, we see that there's uptake unilaterally in it. On the axial images, these look concerning for a possible node. And on a non-contrast CT, it becomes very difficult to say for certain whether there's a lymph node here or is this just muscle. But when you look at the coronal plane, we see that there is this J shaped linear uptake, and then we see the other muscle here on the left is normal, so this is just normal uptake within the mylohyoid muscle.

So it is important to know a little bit of anatomy in the neck and be able to look at all three orthogonal planes and not just call something that looks focal asymmetrical on one orthogonal plane abnormal simply because you're not looking at all three orthogonal planes to see that it is actually linear and correlates to a muscle.

This was originally interpreted as abnormal, and we see focal, fairly focal, somewhat linear asymmetrical activity. And this is a patient who had a history of lymphoma, and we see that there is a structure here on the right that is asymmetrically larger than the left, and that's normal. And we see that there's intense FDG activity that corresponds to the right crus of the diaphragm. And this is a normal anatomical variant, physiologic variant as well.

So keep this in mind, asymmetrical uptake in the crus, and then sometimes the muscle can be asymmetrical based on things that we've done to the patient as well.

So this is, we see focal intense FDG activity in the tongue, in the left side of the tongue. Now the right side of the tongue looks different, and that's because they cut the 12th cranial nerve on the right. And so we see fatty replacement of the right side of the tongue. Now this is actually the opposite, so don't get confused by this. The artist that drew this for this case actually switched it. So I think he was actually looking at this activity and trying to make it correlate to this, and I didn't have 'em redraw it, but essentially the opposite.

So you get fatty atrophy if you cut the 12th cranial nerve during, say, a head and neck resection, and you can easily have physiologic focal asymmetrical activity. And that's normal in the contralateral normal tongue.

Cardiac Activity

So cardiac activity, we oftentimes will see the left ventricle. And if you look at, say, a 3D MIP image of a normal PET study, we will see intense uptake in the brain, always, because the brain is the only obligate glucose metabolizing organ in our bodies. The other thing that you will almost always see is some degree of left ventricular activity.

However, we see reduced cardiac activity with longer fasting periods. And that is because although the left ventricle likes to use glucose for metabolism, it switches over to fatty acid metabolism and shunts glucose to the brain in periods of fast. So all chambers can have uptake with pathology, however, so patients who have a dilated atrium from AFib or even cardiomyopathy with diffusely dilated or thickened chambers of the heart can have diffuse uptake. So we'll look at some examples of that.

So this is in the fasted state, we see no uptake in the left ventricle, whereas the non-fasted, same patient, just different scan, and we generally tell patients to fast four to six hours. Once they get to about six hours or so, it will reduce the cardiac activity.

Same thing here. Fasted state, we see generally some just mild uptake, whereas in the non-fasted state, fairly intense FDG activity in the left ventricle, whereas the rest of the heart, again, in a normal patient, does not have increased uptake.

Other patterns, sometimes you can see just portions of a chamber. So it's important to correlate to the CT part of the exam. And prior to PET CT, it was oftentimes difficult to reconcile some areas of uptake in the chest that may look like lymph nodes, but the even the aortic root can have areas of focal uptake. Same thing here we see a little bit along the aortic root.

And then this is a patient who had a longstanding history of AFib. We see that the atrium is dilated with linear rim uptake, and then a patient here with cardiomyopathy, diffusely dilated and thickened, both ventricles, we see uptake as well as in to a lesser degree, the atria.

So keep in mind that focal uptake in the chest, whether it's related to cardiac activity or not, is not always pathology, and you'll want to correlate with the CT part of the exam.

Fat (Brown Fat)

Let's move on now and talk about fat. And I'm not talking about normal fat, I'm talking about brown fat. So this is metabolically active fat, for those of you who've never heard of it, it's a thermogenic organ that's found in all mammals. So us being mammals, we have brown fat. I actually learned about it back in college and about hibernating bears. So they have a lot of brown fat and it's essentially just a thermogenic organ to generate heat while they hibernate, but we actually have it as well.

So they're unlike normal fat or adipose tissue, which has very low concentrations of mitochondria. Brown fat has extremely high concentrations of mitochondria, and the thing about it is that it actually has an on off switch. And that switch is the sympathetic nervous system. So even in the same patient on one scan, you could see a lot of brown fat activity, whereas the next scan, you might not see any. And it all depends on if the patient's stressed as well as if they're cold, it'll generally activate the sympathetic nervous system.

So ways to reduce brown fat would be, you can give them benzodiazepines as a pretreatment, you can keep them warm during the uptake after you've injected them with the FDG, and other things like that.

So this was the first description of brown fat from Johns Hopkins back in 2003. And we used to think that this was all related to muscle activity prior to PET CT. And then when we got PET CT, we saw that, wow, these areas really are not correlating to muscle. They correlate perfectly to areas of fat attenuation. And so the way to prove that something is brown fat is simply by looking at the CT and looking at the attenuation of the area. If it measures fat attenuation, by definition it's brown fat.

However, it's not always symmetrical in these cases, like I'm showing you. So these look fairly symmetrical. And how do I know this is not muscle? Because it correlates to brown fat not to muscle. Sometimes you can have both muscle and brown fat and it can be a little more difficult. It's more academic though, because it's all physiologic.

Now left paratracheal is a somewhat atypical area once it's very focal here, though, you don't have a muscle in this area. So left paratracheal is a good area for brown fat. Interestingly, when you look at the CT, the attenuation of brown fat tends to be a few Hounsfield units higher. So you can see that it almost looks like dirty fat. In this area it's very focal. It almost looks like it could be a node, but if you measure this area, it'll measure about negative 30 Hounsfield units. And by definition it's brown fat.

Other areas that can be asymmetrical and focal is anywhere in the neck. So posteriorly in the neck here, this looks somewhat concerning, particularly in a patient with say, head and neck carcinoma. And then when you look, now, you have to make sure that you have good coregistered CT and PET images. If the patient moves between the CT part of the exam and the PET part of the exam. For instance, if they're awake during the CT and their head is straight and they fall asleep and their head turns to the side during the PET part of the exam, the images will be misregistered. And so that's where this becomes more problematic.

But with accurately coregistered images, we can see very nicely that this is just a focal area of brown fat. It measures fat attenuation, by definition it's brown fat.

We actually published in 2009 a nice review of atypical locations and appearances of brown fat. This case was included, but this is a patient with breast cancer and it almost looks like she's got bilateral adrenal metastases, right? These are the kidneys here. So there's focal areas of intense FDG uptake in the suprarenal location, as well as in the axilla bilaterally. So you need to look at the CT, and when we look at it, here are the adrenal glands, right? That's the right there. It looks normal, and the left here, and that looks normal, and the activity is actually posterior. And if you looked on all three orthogonal planes, it's posterior and somewhat superior to the adrenal glands. And this is a fairly typical, I have no idea why, but this is an area where brown fat likes to live.

However, it can also be very focal and very rounded in any area of the body and can really be misconstrued as a focal node. But this is actually just brown fat. And we see a triangular area of brown fat here. This was the same patient scanned six months later. These are the same images though. Now, do you think that this patient was scanned in the winter in Pittsburgh? The patient was, so this was a patient that was scanned in the winter and then brought 'em back in the summer in July when it's nice and warm. And we see resolution of that activity with no treatment.

This is a patient with cervical carcinoma, and I'm gonna show you two different slides in different areas related to this patient. But this, these look to me originally as, okay, there's some focal uptake in those areas. And I thought, okay, this is gonna be posterior suprarenal, physiologic brown fat. But then I saw some asymmetrical areas in the left paraspinal region, and we see that something that almost looks like it has mass effect, but if you measure it, it measures fat attenuation. And these are just focal fatty deposits of brown fat. So keep that, keep it in mind that it can be very focal.

This patient almost had a cardiac biopsy. This luckily came to me, the person that read it said there's a mass in the atrium or in the interatrial septum that has intense uptake, recommend cardiac MR and biopsy. And if you look closely, the interatrial septum is a little bit fatty thickened here. And this uptake corresponds very nicely to an otherwise normal appearing, but slightly fatty hypertrophied interatrial septum. So lipomatous hypertrophy of the interatrial septum is oftentimes brown fat, or at least it's metabolically active. So keep that in mind. It can be very, very focal.

Now, I showed you other muscles that can be asymmetrical while the vocal cords can also be asymmetrical if the patient has had surgery. So this is a patient with thyroid carcinoma, and I'm gonna show you two cases with the exact same history and they're gonna look opposite.

Okay? So this is a patient who had known damage to the left recurrent laryngeal nerve during a thyroid surgery. And we see normal super physiologic activity in the contralateral remaining vocal cord. This was originally published back in before PET CT was even approved. This was when we had the prototype scanner and nice correlation visually with the atrophied left cord here.

Now keep that case in mind. I'm gonna show you a follow up to that.

So brown fat in summary, equally symmetrical and asymmetrical areas of uptake, it's more common in women, you see it more commonly in the winter months. And then keep in mind that locations besides the relatively common supraclavicular is left paratracheal, retrocrural pararenal, anywhere in the neck as well as in the interatrial septum.

Iatrogenic and Surgical Causes

Alright, what about other causes? Iatrogenic and surgical? There's a lot of things that we do to the patient, particularly patients who have cancer that will cause differing appearances of uptake that aren't related to malignancy.

So radiation pneumonitis, if you irradiate the lung, you can see uptake within the lung post-radiation that lasts generally around three to four months, but can be seen really as long as one year, even longer. Sometimes radiation fibrosis over time, that same area will decrease in FDG activity. And once there's no uptake in that area, it becomes essentially radiation fibrosis. You generally see contraction of tissue as well post-surgery.

Generally do not reevaluate patients after large surgeries before about six weeks, unless you're worried about areas outside of the surgical bed because you can have uptake in the surgical bed for about four to six weeks. And then marrow stimulating medications like GCSF and erythropoietin will also cause diffuse marrow activity from activation.

So there's a patient with a lung cancer, poor surgical candidate, and this is all they had. This was an old case, this was a staging PET study. And we see after radiation we see a very nice, and unfortunately the scans don't look like this anymore. This was pretty easy to see that you could almost draw with a fine pencil. The area where they irradiated, you know, an anterior to posterior port, now they have all these intersecting ports, it makes it much more difficult to read.

And this is again, pretty easy to see, okay, there's diffuse uptake in the where the port was, difficult to say what's happening to the underlying tumor, but I can certainly say is there any disease outside of that area if you restage the patient. But really impossible to say what's happening here. And same thing here, different patient, again, very, almost the same appearance. Very difficult to say what's going on with the underlying tumor at this point.

Now here is an example of radiation pneumonitis, and then watch here, as I show you the follow up, we see some contraction of the tissue, and this is radiation fibrosis. And again, this, the pneumonitis part can last for upwards of a year.

Okay, let's go back to this case now. And we're talking about iatrogenic and surgical causes. So I told you that the left was damaged and it's the same history now, and I'm gonna show you this same history and there's intense uptake now in the other side, but I know that the left recurrent laryngeal nerve was damaged. How could that be? Well, I didn't know this was an old case and we postulated whether there could be a second malignancy there, but in fact, they injected the patient's cord with Teflon. And so this was, I think one of the first cases that was published. Teflon thyroplasty can cause focal intense FDG activity and this will remain FDG avid essentially forever. We did, we've had follow ups for almost 10 years on this patient. And it has not changed at all.

This is an example when you give a marrow stimulating medication, and you see diffuse uptake throughout the marrow with erythropoietin, you generally don't see as much activity in the spleen. Generally spleen should have less activity than the liver. When you see diffuse uptake like this in conjunction with bone marrow activity, you should think about the patient being on GCSF.

So just some general considerations and recommendations, no minimum period, depending upon the information desired. What I mean in terms of follow up. So if a patient's had surgery and they say, well, can I get a scan two weeks later? Well, you can just realize that there's gonna be activity in the surgical bed, which will limit the evaluation of the bed. But otherwise, I can tell you other things, the longer you wait, the less false positives, that's the bottom line.

So if you wait a year, you're not gonna have uptake related to surgery or radiation or other things, but generally the clinicians want to know sooner than that. But if you just want general recommendations about four to six weeks after surgery, most patients will resolve activity in the surgical bed one month after discontinuing marrow medications. It takes about 28 days average for this to resolve two to three months post-radiation a little bit longer if it involves the lung. And again, this can last upwards of a year in some patients.

Infection and Inflammation

Talk about infection and inflammation now. So this patient had a left-sided head neck carcinoma, and this was a follow-up study. We see focal intense FDG activity that looks fairly worrisome based on the PET images alone. We look at the CT, there's really nothing abnormal, no soft tissue mass or anything. It looks like it correlates to the actual tooth or beside the tooth or maybe in the bed. They took the patient to surgery, took out two teeth, patient had a dental abscess.

This patient had a history of colorectal carcinoma and they knew about this hepatic metastasis. They were going to do a partial hepatectomy, so they did the right thing, which was to stage the patient before doing a laparotomy and whacking out half their liver. And they see that there's activity in the bilateral hilar and right paratracheal regions.

Now, when you're reading PET, it really is important to think about methods of spread. And this in general wouldn't make a whole lot of sense for say, a sigmoid colon carcinoma to go have one lesion in the liver and then have uptake in the mediastinum. It just, it's a little odd, but this is fairly common appearance for other things such as sarcoidosis. So they delayed the patient's surgery, they did bronchoscopy, sample the nodes, and it was non-caseating granulomas. The patient went for surgery, had their resection.

This is, unfortunately there's nothing that you can do for these patients. This is a patient who received talc pleurodesis from recurring pleural effusions. And it does a good job at reducing pleural effusions and by causing an inflammatory reaction of the pleura. Unfortunately, it also causes diffuse FDG activity, and it will last for a very long time. So it's difficult to say for sure whether there's any potential tumor along the pleural surfaces.

Sometimes if you see another example here, sometimes if you see these discontinuous areas of high attenuation, it can be very helpful in those situations like this patient here where we see it almost looks like a pleural plaque. But in general, asbestos related pleural plaques do not take up FDG. This is talc pleurodesis and we see intense FDG activity. So pleural plaques from asbestos, you generally don't see uptake, but with talc, high attenuation discontinuous areas, we oftentimes will see intense uptake.

Other Benign Processes

What about other benign processes? Well, obviously trauma will cause increased metabolism in the area that's traumatized. So we see multiple rib fractures here. We see the fracture on CT and there's uptake. This will resolve after the fractures heal.

What about now we're gonna switch over. So I've just told you a whole bunch about potential false positives and normal physiologic, atypical physiologic patterns in the neck and chest. Now let's talk a little bit about false negative PETs. And let's start with size because size is critical in PET imaging.

False Negatives: Size Related Pitfalls

Generally we don't see things less than about six millimeters. So keep in mind that the standardized uptake value, that's the value essentially that you get when you put a cursor on the computer over the area of interest, where the uptake is, it tells you how much activity is there. It's significantly underestimated with lesions near the scanner resolution. So the, and actually that starts to happen at about two centimeters. So at two centimeters, we're underestimating the SUV and we, when we get down to the scanner resolution at about six millimeters, we're underestimating the true SUV value by a factor of 10.

So if you see any activity in a tiny pulmonary nodule, you should get worried, although it is indeterminate. So what I tell people in terms of pulmonary nodule management, six millimeters or less, follow it with CT, or use the Fleischner criteria if the patient doesn't have risk factors and so forth. If it's six to 10, you can think about using PET CT, but it's probably gonna be confusing in terms of the PET part. If it's greater than one centimeter, the literature is overwhelmingly in favor of using PET CT. And you always need to look at the CT of course.

So this patient has a little bit of scarring in the left apex here, and then there's about an eight millimeter nodule here, slightly spiculated underlying emphysematous change. If I told you that the SUV in this lesion was in the range of 1.2, that generally would be construed as favors benign etiology, but in fact we're underestimating the amount of activity by probably a factor of 10 at the level of resolution. So the fact that you see anything above background lung in this is critical, and you really should look at this as suspicious.

So let me show you, this is poor form. So this is a six millimeter nodule here. They decided to do a PET and we see that there's some minimal activity there. And the person that interpreted the study said that it favors a benign etiology. This was cut and paste from the report. Now I disagree and I think it's actually not truthful. It's just inaccurate to say this, but unfortunately when you say this, then the clinicians get this false sense of security.

Well, they had a negative PET and so they did the right thing, which was to continue to follow the nodule, but watch what happened. So they did a three month follow up, and then they did a six month follow up, and then they did a nine month follow up, and then they did a 12 month follow up, and then they did about a 17 month follow up. Now, you don't do this regardless of what the PET says or regardless of who interpreted the PET says, and this is the problem with PET is that, you know, you can really get a false sense of security. And so at this point, so this is not only just a very nice example of the natural history of cancer, of lung cancer, but it's also great documentation from malpractice. You sat there and watched this nodule get three times the size.

Now watch what happens. So this is the nodule at six millimeters, but once it's 1.3 centimeters, that's the same nodule. But we're underestimating the true activity within that neoplasm because it's a small lesion and it's at the scanner resolution. So again, no matter what PET says, if you have an enlarging nodule, you need to biopsy it or take it out, period.

Non-FDG Avid Tumors

Okay? Other things that can be falsely negative prostate cancer. So prostate is the only indication that is not covered these days for at least one PET CT. It used to be that you only had about 5, 6, 7 indications for PET CT. And the good thing about those was that they were all reliably FDG avid. Now there are a whole host of tumors, these are all covered now for scanning with PET CT. And unfortunately, many of these are non FDG avid.

So if you don't have someone who's reading the scan that knows these appearances on CT, it can be, it can really have disastrous results. But renal cell carcinoma, certain lung cancers like bronchoalveolar carcinoma and other well differentiated adenocarcinomas, HCC cholangiocarcinoma, sarcomas and neuroendocrine tumors.

So this is an example of the pulmonary nodule variant of bronchoalveolar carcinoma, and we see that there's really no uptake beyond that of background lung. Now it's not that all BACs will have no uptake, actually about 50 to 60% of them will have some degree of uptake, but many of them will be non FDG avid. Now, if you do a PET, and this is the first scan, it doesn't mean that you just stop following the patient. These patients still have to be followed, or if you wanna know the answer, just put a needle in it.

But nonetheless, because you have a negative PET, it does not prove anything. And these patients all need to be followed for two years, just like we do with every other pulmonary nodule. And once it's at this size, you probably wanna biopsy it. Anyway, how about this?

I showed this to one of my PET fellows and I said, well, what do you think? What's going on here? What do you think? And he said, well, is there a pericardial effusion here? And I said, no. And so in fact, there's an endobronchial lesion here that's causing collapse of the left upper lobe, and there's not much uptake within that endobronchial lesion. And this, if you see endobronchial lesion with very little FDG activity, you should be thinking of carcinoid.

Keep in mind that some tumors, for whatever reason, sarcomas being notorious for them, can have a wide range of variability in the FDG activity associated with the metastatic lesions. So anytime I see sarcoma, I start to think, okay, when I start seeing these lesions without uptake, I know there's still lesions. They're just, for whatever reason, they're variable uptake. So the primary lesion can have intense uptake. These nodules in the lungs that are over a centimeter can have almost no uptake. You can see lesions that have mild to moderate uptake, and that's pretty much what you get with soft tissue sarcomas.

This is a patient who has some cardiac activity. We see some physiologic FDG in the ureters and in the bladder. Otherwise it looks reasonably good except for the fact that the liver's large and this patient has multiple lesions that are just not FDG avid. This was a widespread neuroendocrine tumor.

This was an interesting case, where the patient had known bronchoalveolar carcinoma and actually had their right lung taken out because they had multifocal disease on the right. Now, at a few institutions, including UPMC where I used to practice, they would consider the patient for transplant if you had disease that was localized to the other lung. So this was a patient that was being evaluated for transplant and we presented them at transplant conference and so forth. And I'll show you some images here now.

So multifocal pretty typical appearance for BAC, and I think you'll learn a little bit more about this in the chest sections here. So multifocal disease. Now on the CT of the soft tissues, we see maybe a small lymph node here. Pretty normal appearing pneumonectomy bed, but there's this, okay, so keep this in mind and then let's go down and look at the at these lesions here, lemme show you some fused images now.

And so I looked at this and I thought, geez, these are big lesions and there's mild uptake. And then these have really intense FDG activity and they're pretty small nodes. It wasn't really making sense to me. So I said, I wonder if there's something else going on here. And this would've made the patient non-surgical. So they did biopsies and this patient had sarcoidosis, so sarcoid and BAC, the patient went for transplant and there's the lung.

Difficult Case Review

Alright, so now we're gonna do just a few cases and I'm gonna ask you basically is it tumor or is it not tumor? One is you think it's cancer two, you think it's not cancer. Give you a little test here. No wishy-washy. There's no three. So you have to commit. Is it cancer, is it not? You can't just say, ah, it could be cancer, it could be benign, it could be anything.

So, okay, so 33% said malignant and the rest said benign. Well, there's actually a three centimeter renal cell carcinoma here. Solid enhancing lesion. There is contrast enhancement and that is actually this right there. So keep in mind that at about 60% of renal cell carcinomas may not have any metabolic activity, some will have minimal.

Okay, how about this? Hopefully Dr. Webb will get this right. It's not easy. Okay, so 91% got the correct answer. Hopefully you looked at the answer actually here is on the CT we see this crescentic shaped area of slightly higher attenuation and that's what this corresponds to. And this is an acute intramural hematoma. So keep in mind that acute injuries, acute types of processes even, I have a renal hematoma, perirenal hematoma that takes up FDG.

Okay, how about this? Nice. So 17% had no idea. Alright, so about 50 50. So this is actually a good example of silicosis with progressive massive fibrosis. So there's we see tiny little punctate nodules, there's some calcifications. I'm not showing you a lot of the soft tissue windows, but we see some calcified lymph nodes and bilateral upper lobe masses with intense FDG activity. There's no way to differentiate PMF from cancer unfortunately in this patient. So PET is essentially useless for evaluating that type of patient.

Okay, let's do this. This will be the last case. This is a lung carcinoma. I'll give you that. What I want to know is what's going on here, cancer or not. Okay, 78% got the correct answer. So, and hopefully if nothing else, the MRI obviously gave you the answer, which is there's an enhancing lesion there. But without the MR, I keep in mind that about 20% of metastatic lesions from primary tumors that are intensely FDG avid will be either non FDG avid or relatively non FDG avid in the brain.

Okay, thanks so much.