FDG PET/MR for therapy response assessment in sarcoma.

Abstract

e21518 Background: Magnetic resonance imaging is the preferred modality for local staging of sarcoma. Positron emission tomography (PET) allows for the evaluation of tissue metabolism. Recently, PET and MR can be performed as a combined examination on a single scanner. The aim of this pilot study was to evaluate the agreement of PET and MR regarding therapy response in patients who received PET/MR for sarcoma staging and assess the diagnostic value of PET/MR in contrast to standalone MR. Methods: Thirteen patients with soft tissue sarcoma (7 m, 6 f, mean age 50 y, age range 18-80 y) received FDG PET MR on to occasions to assess therapy response. Average time between the two scans was 47 d (21-98 d). Average time between tracer injection and the beginning of the PET scan was 70 min (58-106 min). 4.5 MBq 18F-FDG /kg body weight were applied before the examination (252-331 MBq 18F-FDG /patient, 289 MBq on average). Image evaluation was performed separately by a radiologist and a nuclear medicine physician, respectively, who were blinded for other imaging procedures or clinical data. On MR images therapy response was determined according to the RECIST criteria. PET data evaluation included the calculation of mean an maximum SUV (=standardized uptake value) and metabolic volume. In this case, an increase or decrease of 30% or more of the base value was considered a progressive disease or positive response, respectively. Agreement of PET and MR regarding response was evaluated using Cohen's kappa. Results: The primary tumor showed increased FDG uptake in all patients. Response assessment based on RECIST and SUVmean differed in six of 13 cases (k=0,25). Assessment based on RECIST and metabolic volume showed slightly better agreement (k=0,35). Assessment based on RECIST and SUVmax differed in x cases (k=0,34). Conclusions: Sarcoma therapy response based on morphologic MR imaging and PET parameters yield differing results in many cases. FDG positive sarcoma may show a decrease of metabolic parameters on PET scans, while they appear largely unchanged on MR scans. Due to additional information about tumor metabolism provided by PET, PET/MR is a promising modality for sarcoma imaging. Further studies are necessary to evaluate the prognostic significance of PET findings in sarcoma patients.