Does it display the prostate only, or do other organs light up?
With PET PSMA, there are numerous other organs that will demonstrate uptake in addition to the prostate. Probably the most troublesome are the kidneys and the associated urinary excretion because it will actually show up in the ureter and bladder. One of the tricky aspects of PET PSMA is that a peristaltic wave of urine across the pelvic brim can be mistaken as a pelvic lymph node quite easily, so you need to be careful of that. It shows up in the bladder, which can obscure the detection of local recurrences, and is also taken up by the bowel and salivary glands.
Interestingly, it’s been shown that PET PSMA can be a useful tool for kidney cancers as well, but a lot of further work needs to be done in that area before it can be used clinically for that indication.
Do you put in a Foley catheter to decrease the possibility of hydronephrosis or something similar that would throw off the sensitivity of the scan?
Usually we don’t put in a catheter, but we do virtually always give Lasix to get some urinary clearance, then do the scan soon after that.
Our group did a study years ago showing that the CT scan in high-risk disease doesn’t offer much past the bone scan, and this has been confirmed by others. Could you see PET replacing CT?
Very much so. In fact, in Australia that largely has already happened with high-risk disease. We’re waiting with interest the results of a trial being done in Australia, in which I was a co-investigator, called the ProPSMA study, which is looking at conventional imaging versus PET PSMA for staging high-risk prostate cancer. It’s examining the utility of PET PSMA for identifying metastases but also for determining the likelihood of altering treatment. The main question that still arises is whether it makes any difference to long-term patient survival, but there’s no doubt the more information and the more accurate information you have, the better choices you can make for the patient.
One of the big criticisms with PET scanning is that we don’t have histologic confirmation when something lights up. In the ProPSMA study, will we have histologic confirmation?
Patients on trial who have gone on to have a radical prostatectomy and extended node dissection will have histologic confirmation of the positive nodes and how they correlated with the histopathology at the surgery. Hopefully we will get some more information because we’ve made the decision that all the patients in the trial will receive extended node dissections since they’re considered high risk. Even if they’re PET negative, we hope to also identify those patients who are missed because we know that the sensitivity is not perfect with PET PSMA scans.
However, if you look historically at the data when there has been correlation with histopathology, the specificity is excellent—often 95%-plus. Even with the data we have to date, we can be fairly comfortable that if something is positive on a PET PSMA scan, it will be histologically malignant.
For me, the bigger issue is the negative PET PSMA scan in a high-risk patient because we know that 30% to 40% of patients will not have a positive PET PSMA signal even though there might be disease present, which largely has to do with tumor volume. If the patient has lymph node metastasis that’s less than 2 to 3 mm in size, for example, that isn’t going to get picked up on the scan. But if you wait for it to get over 5 mm or so, there’s a much greater chance it will show up. So the problem is that we will end up missing small malignant nodes with a negative PET PSMA scan.