Abstract:
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Purpose: Local inflammation may be a consequence of radiotherapy (RT); this is supposed to cause false positive PET scan results, as inflammatory cells, like neoplastic cells, have a high glucose metabolism. Therefore, it has been suggested to carry out PET scan at least 3 months after RT. Aim of our study was to evaluate the incidence of false positive PET scan shortly after radiotherapy.
Methods and Materials: 16 consecutive patients were referred to our centres for lymphoma (7 Hodgkin Disease, 9 Non Hodgkin Lymphoma) with a mediastinal Bulky mass at diagnosis. Each patient underwent chemotherapy and then RT (3600 cGy in case of residual disease at CT, 3060 cGy in case of complete remission, administrated in doses of 180 cGy/day). For clinical reasons, shortly after RT (range 25-50 days; mean 38,7 days) 18F-FDG PET scan was carried out to evaluate the effect of therapy. We intravenously injected 370 MBq of 18F-FDG, and after 60 minutes we recorded images.
Results: Despite a relatively short time after RT, PET scan turned out negative in all cases. There was no pathological tracer uptake in the site of radiotherapy and in any other lymph nodes. We found no signs of possible inflammation (mild to moderate uptake) which could lead to false positive diagnosis.
Conclusion: Lung inflammation is not necessarily a consequence of RT; our data indicates that one month is enough time to avoid PET false positive results after RT. This is different from international guide-lines, which recommend to wait at least three months. In clinically selected cases a PET study may be required at an earlier time: our study shows that FDG PET is accurate even at one month after RT. Further studies are needed to clarify the exact lack of time to obtain an accurate PET result in a time as short as possible.
Castellucci MD, P,
18F-FDG PET Shortly after Radiotherapy in Patients with Malignant Lymphoma: Is There a High False Positive Incidence?. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3101255.html
