Comparison of agents in prostate screening

Dr. Umesh Oza: We’re getting a little into physics. But those are the 2 differences there. Since we’re on the topic of physics (regarding half-life). We know that the F-18 based ones last about 110 minutes, right? Generally gallium-68 is similar to 68. It is about 68 hours; easy to remember that way. That influences how we can use it. Generally, a half-life of one hour is a pretty good indicator. We are not talking about our 10-minute, 2-minute, and 5-minute cyclotron-produced tracers, which decay completely quickly. You just need to be right next to a cyclotron. Good… clinical application of both, in that sense. Do you see any difference? Have you noticed or seen anywhere in the literature any difference, generally speaking, about their sensitivity or specificity, or in certain, say, organs or systems of the body, one has an advantage over the other?

Dr. Jaideep S. Sohi: Just to clarify, gallium-68 has a half-life of 68 minutes, as you mentioned. With respect to biodistribution, for example, both tracers have a similar biodistribution profile with prominent updates in the salivary glands, liver, spleen, intestine, and bladder. The only area where you can see a slight difference, and which may have an impact in the future, especially in patients with metastatic disease, is the liver because F-18 has better capacity and some compatibility. You may see increased background hepatic uptake under normal conditions, in normal biodistribution compared to gallium-68. So, something to keep in mind. Beyond that, they have a similar biodistribution profile. Regarding image quality resolution, I think in theory there is a slight benefit for the F-18, because it has a better profile with respect to image characteristics. However, I think on a practical level it really depends on the type of camera, the imaging equipment you have at your disposal, and the reconstruction parameters you are using to reconstruct the images. I think they play an important role. And what kind of image quality you are acquiring.

Dr. Umesh Oza: Absolutely. I agree. I think there are so many variations. When people talk about sensitivity and specificity, there’s a lot of things you can do between cameras and the way you set up the camera, the pickup time. How many minutes per bed step you are doing, as you mentioned, reconstruction, all of that plays a role. There are many variables that must be taken into account. To your point about liver activity, sometimes I struggle with that, reading these scans I have to, I call it “de-toasting” or de-intensifying the liver so much on the F-18s to see through that activity or that intensity . I have seen it only anecdotally, I have called lesions that are on the CT. Since we are both radiologists, we can look on those CT scans, look for hyperattenuating and low-attenuation lesions on the CT scan and see if there is a correlation on the PET images. I’m sure you’ve noticed that when you de-intensify the liver, suddenly these lesions that you can see on the CT scan come back on the PET image, and that’s when you say, Well, it’s not cold. It is not absent, as would be expected from a cyst or other benign lesions. In fact, it has a low level absorption. And I recommended further evaluation of those lesions with MRI or something else that we can see, and with gallium I did that last. But I’ve certainly seen more of those 18 and have been recommending them, especially when I can’t characterize those lesions as benign lesions. So that’s interesting. Do you have any experience or thoughts on gallium vs F-18 on this uptake in bone lesions? Know that prostate cancer coincides with the nodes and, obviously, the bone is a very common place for metastases. Do you have any ideas, have you read any literature, or have any specific experience regarding bone injuries between the 2 specific tracers?

Dr. Jaideep S. Sohi: Yes, yes, we have done it. In our practice we have noticed it. And I think there is some data from the literature regarding a greater case of nonspecific bone uptake with F-18 compared to gallium 68. Because F-18 has a greater predisposition, predilection for bone… From one point From a practical standpoint and from a patient care perspective, it can be challenging because we know very well that prostate carcinoma tends to metastasize to the bones. If you see areas of increased uptake in the bones, then you need to resolve and determine whether these lesions are benign, nonspecific, or represent true metastatic disease, because that distinction will have real, real implications for patient care. For example, on an anecdotal level, yesterday I was talking with a urologist and the question arose: What is done in a patient recently diagnosed with prostate cancer and disease in the prostate gland, and… some areas of focal uptake in the bones? Now, do we still offer them prostatectomy? Or are they now sent to a different treatment paradigm? Frankly, I would love to see what his experience has been and what he has seen in his practice regarding this particular topic.

Dr. Umesh Oza: Sure. So, having seen more F-18s, I definitely see these. Some people call them unidentified or uncertain bone uptake, UBU, and we are seeing them on scans. I try my best to look at those CT images to see if it’s fibrous dysplasia, if it’s a fracture… something like that? But when we talk about the rib, which can be 4 or 5 mm thick, you try to figure out if it has any ground glass attenuation and you say, Yes, this is definitely fibrous dysplasia. That’s hard. Good? Because, as you said, this is where the urologist wants to know, the patient needs to know to receive care if he has a single lesion in the prostate or if there are multiple lesions in the prostate and there are no nodes. And you have this rib lesion that will be… metastatic disease or nothing with fibrous dysplasia. There really isn’t even an imaging study that I can recommend for some of those findings because even if I’m reading the CT scan or my colleagues are reading the CT scan, they’re not going to hammer and say when they had this hot spot on a PET scan, which Everyone has already accepted that we have the answer to cancer, not cancer. And they’ll say, yeah, this is probably just fibrous dysplasia, cortical fibrous defect, or ossifying and non-ossifying fiber. All these other terms (and) all these other injuries that we usually see. Absolutely, I’ve seen it in gallium studies. I haven’t seen that many, but that’s mostly anecdotal, and I read the same study that you have that shows that, as you mentioned, the F-18 is a sodium fluoride PET bone scan, right? We used to do that all the time, and I don’t think I’ve seen that in a few years. Those false positives certainly exist, and also mention what to do with this increased sensitivity to find these lesions. All of our urologists, and this is kind of getting ahead of the topic here, but urologists know the treatment protocols, the trials, the research, all the data is based on conventional imaging, diagnosing these metastases. We have these patients who were originally in the intermediate stage and do not have any lesions. And suddenly they are in an intermediate stage and we see more injuries, overshadowing them. Should they still qualify for the additional treatments or are we now putting them on a different path? Now, it’s not something you and I can resolve here, and you may have more knowledge about the treatment than I do. I’m not going to go into that, but I think that’s one of the challenges we face with this new tracer and how we’re staging and restaging PET-CT. Would you agree with that? Is that something you’re seeing too?

Dr. Jaideep S. Sohi: Yes absolutely. Just to add to your comment about resolving these areas of nonspecific bone uptake, you mentioned: what do you do next? That’s a real diagnostic dilemma, because if I see something without a defined anatomical correlate, I don’t really have good options regarding a second imaging study, X-ray, CT scan, and MRI. They may or may not show something if I don’t see anything on the CT portion of my PET. The second aspect is the deadline. Getting patients to undergo follow-up imaging is usually not easy. You have to, depending on the modality you’re looking at, for example, MRI, you have to get clearance. There is a time lag between the time you request the study and the time the patient can enter the scanner and follow up. We also have to think about that time frame. Meanwhile, patients suffer genuine and reasonable anxiety and stress about what scanning planning means. We have to think about that aspect too. It is always preferred to use a modality that has a lower incidence of nonspecific activity.

Dr. Umesh Oza: Yes absolutely. I think we can. We sure support it.

The transcript is generated and edited by AI for clarity and readability.

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