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Ann Thorac Surg 2000;70:1159-1160
© 2000 The Society of Thoracic Surgeons
Discussion
DR LARRY R. KAISER (Philadelphia, PA): Can you clarify whether the nuclear medicine physicians had access to the computed tomographic (CT) scans when they read these studies. You know, there are a number of studies that show, and you pointed it out as well, that when you use the positron emission tomographic (PET) scan in conjunction with the CT scan, that the sensitivity and specificity can be quite high. I was impressed that your specificity was as good as it was because often the incidence of false positives might even be higher.
The other thing is, we have had some problem with our nuclear medicine physicians. When you go down and question them, they will change their opinion: "Well, yes, maybe that one is negative." Have you had any problem with bias on the part of the nuclear medicine physicians, because, as you mentioned, the software can vary and the attenuation can vary as well.
DR ROBERTS: Those are two excellent points. In our particular study, the nuclear medicine physicians did have access to the CT scans, and I believe that often times their impression was made using a combination of the anatomic data as well as the physiologic data. Our radiologists and nuclear medicine physicians, like yours, do waffle a bit, and oftentimes they will say something is positive or negative, and when you go back and really press them, they do change their minds. That is one of the attractive properties of standardized uptake value in pinning down a specific quantitative number. Unfortunately, the resolution at the present time for the mediastinum does not seem to be adequate enough to describe a standardized uptake value for each lesion.
DR LESLIE J. KOHMAN (Syracuse, NY): What about the patient who has had induction therapy? Does the PET scan, if they have had it before and then have their induction therapy and then another PET afterward, does the N2 disease revert? Does the mediastinoscopy that you may do early on interfere with the repeat PET scan? Maybe they did not have a PET scan early on and had, either with or without a mediastinoscopy, induction therapy and then had a PET scan before you decide to operate on them. How does this induction therapy or prior mediastinoscopy influence the PET scan?
DR ROBERTS: That seems to be another potential application of it, and, as we know, a CT scan or an anatomic image of a tumor does not always correspond with the response to the chemotherapy. Often, the CT image does not change in size and we find no viable tumor at thoracotomy. The PET scan image does seem to respond to chemotherapy, and we have reimaged patients after induction therapy. The decreased active tumor burden does seem to be reflected by decreased fluorodeoxyglucose uptake.
DR ROBERT J. KEENAN (Pittsburgh, PA): I have a couple of quick questions. One of your patients had a central tumor without mediastinal nodal involvement. I think he had a tumor in the anterior-posterior window, if I recall.
DR ROBERTS: Yes.
DR KEENAN: That would seem to be a patient in whom the PET was actually right and the interpretation was wrong. You know, the PET scan obviously cannot tell what the tissue is, just that there is active tissue. It would seem that the PET actually was correct in that particular instance, and therefore, it may be that some of the newer machines, such as the PET/CT combinations, might provide us with better resolution for that very issue. I do not know if you have any experience with that.
The second question has to do with the uptake values and whether or not there was a quantitative difference in the standardized uptake values between those that were considered to be false positive and those scans in which the PET truly was positive with pathologic nodes and whether you could use that to further define the accuracy.
DR ROBERTS: With regard to the first question, the PET image that was obtained did predict that the tumor was abutting the mediastinum, but in addition to that, they went on to say that they believed that there were additional mediastinal lymph nodes that were positive. When those lymph nodes were removed, they were found to be benign. With regard to the second question, because our study was retrospective and because the standardized uptake values were not recorded in all patients, we were unable to draw specific conclusions about that number. However, there seemed to be a correlation between higher standardized uptake value and the degree of positivity.
DR R. DONALD WOODSON (Port Clinton, OH): Are you aware of any false-positive PET scans from granulomatous disease, berylliosis, asbestosis, or similar disorders?
DR ROBERTS: These processes can give false-positive results in lung parenchyma. We did not have any false-positives from these causes, but in theory, any mediastinal inflammation could cause a false positive.
DR ANDREW L. CARNEY (Oak Park, IL): I would just like to make a comment. Several nuclear medicine physicians in the Chicago area have tried to use the PET scan to separate recurrent malignancy from scar and have not been successful with that at all. Especially in patients who have been operated, they cannot distinguish recurrent tumor from postoperative changes.
DR ROBERTS: I had a little difficulty hearing the question, but I think it was regarding using PET scanning to differentiate between scar and recurrent disease. We have had some experience where patients who had undergone previous thoracotomies and resection were noted to have a change on their follow-up CTs, and in those cases they were positive and then at thoracotomy found to be true positives.
DR CARNEY: How does mediastinoscopy affect the accuracy of PET scan? Do you have a PET scan before or after the mediastinoscopy?
DR ROBERTS: There certainly would be an effect of a surgical operation in the field where you are imaging, and previous studies have shown that in the face of recent operation, false-positives can occur. So getting a PET scan after mediastinoscopy certainly would lead to false positives.
DR MALCOLM M. DECAMP JR (Cleveland, OH): I think one of the previous questions dealing with granulomatous disease is going to be one that is going to vary depending on where you practice. Practicing in Ohio, I can tell you that PET has been a little bit of a problem with the endemic problem of histoplasmosis. So it has been a little bit more difficult for us to incorporate routinely.
DR FRED WEBER (Somers Point, NJ): I think you may have answered it because you get one mediastinoscopy and you get a PET scan and you have induction. So in what order should it be? And if there is a scar after that may show up on the PET scan, then you do the PET first, induce them, and then do the mediastinoscopy second.
DR ROBERTS: Again, our results show that false positives do occur. So I think in the face of a positive scan for the mediastinum, that diagnosis should be confirmed by tissue biopsy of the mediastinum. I do not know how long mediastinoscopy can result in a false positive, but I suspect that follow-up PET scans after induction therapy, weeks after mediastinoscopy may be meaningful.
DR KEENAN: Thank you. The message is do not let your oncologists see the patients before you do.
DR ROBERTS: That’s right.
Related Article
Ann. Thorac. Surg. 2000 70: 1154-1159.
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