Imaging experts discuss the challenges with traditional imaging modalities for prostate cancer, commenting on the bone scan and CT scan.
Umesh D. Oza, MD: So, thinking about those traditional imaging models you mentioned, Dr Sohi, bone scan and the CT scan. What are some of the limitations or challenges associated with these traditional imaging studies for prostate cancer? You know, as far as quality interpretation, sensitivity, specificity, if you can comment. I’m not looking for perfect numbers on that, but just generally. What have you seen [as] the challenges of those types of modalities?
Jaideep S. Sohi, MD: I think this is a very timely and important question. As we look at the landscape, prostate cancer is one of the leading causes of mortality and morbidity in men. And until recently, some of the imaging modalities that we’ve had at our fingertips have been CT and bone scan; we talked about that…. And although they are wonderful in their own right, they also come with some inherent challenges. For example, a bone scan starts to become useful once the patient’s PSA is elevated; for example, a PSA of 20 or higher. Unfortunately, at that PSA level, oftentimes the disease has spread outside of the pelvis, and at that time the treatment options may be limited. [It’s] the same thing with CT scan. CT imaging is the workhorse of radiology. We use CT for many, many indications in prostate cancer. I might use it as a first-line modality. Some challenges with CT include [that] oftentimes it’s ordered as CT abdomen and pelvis, which is statistically where you’ll see most of the prostate cancer involvement. But you can see evidence of metastatic disease outside of the abdominal pelvic cavity, in the chest, head, and neck, and so forth. So you’re missing some key areas right there.
And the second equally important challenge with CT imaging is that we look at lymph nodes with a size criteria. So [if] we have short axis dimensions of 1 cm or greater, [that] is when we start calling these lymph nodes abnormal. The challenge with that thought process is that, again, we know that in prostate cancer, up to two-thirds of the time, the lymph nodes that are involved with prostate cancer are not enlarged for CT criteria. You can see a dilemma; you have a CT scan with multiple lymph nodes that are not enlarged. What do they mean? We don’t know. So we have to look at an additional imaging modality or biopsy to resolve those lymph nodes. Those are some of the challenges that we see with the conventional traditional imaging modalities.
Umesh D. Oza, MD: I absolutely agree. And I think that’s what we’ve learned with these PET scans [is that] size really doesn’t matter anymore. You’re really looking for the metabolic activity or the avidity of these lesions. So it’s really changing and upstaging some of our patients. One thing I would also add on the bone scan, you’re absolutely right, the PSA matters in that situation, but I liken this bone scan sometimes to a set rate for the clinicians. It’s going to be positive, right? It’s positive in everything. We just have to sort out what it is. So we’re trying to find those early lesions, yet the bone scan’s not going to be very specific about what we’re seeing. We may see a little dot here and there, and it may be a tendon insertion, a benign osteophyte. Maybe it’s a little degenerative change. We’re still left with having that bone scan and maybe getting another follow-up, as I’m sure you’d recommend, just like I do recommend a CT correlate. Sometimes plain films are OK, but at the end of the day, a CT scan will probably answer the question, and we can close the book on that unknown lesion. And, of course, we talked [about MRI] earlier. The limitation is obvious. It’s the field of view, right? As you mentioned, they order that in the pelvis for the CT. And we’re missing out on CT chest and neck, where some of the PET imaging, what we’re about to talk about, overcomes that limitation of the field of view, and it’s not just being limited to the pelvis—even it doesn’t even go past the chest, right? So if you have a lesion by the iliac vein, [where] we’ve seen a lot metastatic disease, I agree with you [that] we’re not going to pick those up.
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