S26 PRE-CLINICAL VALIDATION OF TRACERS Conclusions: Bremsstrahlung imaging is a satisfying and reliable method in showing the actual macroscopic distribution of therapeutic 90Y-microspheres used for radioembolization of liver tumours. The results further demonstrate the need for a better diagnostic method than the one currently used, with 99mTc-MAA, thereby providing a better pre-dosimetry; hopefully with a better selection of patients, regarding tumour regression and a lower risk of liver failure. 63 poster COMPARISON BETWEEN 18F-CHOLINE UPTAKE AND CHOLINE DETECTION IN MR SPECTROSCOPY IN ONE CASE OF BRAIN TUMOUR R. Sghedoni 1 , A. Gallina 2 , A. Versari 3 , F. Fioronl", A. Nitrosi" , D. Salvo", M. lorl" , G. Borasi! 1 ARCISPEOALE S. MARIANUOVA, Medical Physics, Reggio Emilia, Italy 2 OSPEOALE CIVILE 01 GUASTALLA, Radiology, Guastalla (RE), Italy 3 ARCISPEDALE S. MARIA NUOVA, Nuclear Medicine, Reggio Emilia, Italy Purpose: Magnetic Resonance Spectroscopy (MRS) is widely used to char- acterize brain tumours through metabolite detection and quanli1ation. In par- ticular total Choline (Cho) is considered a marker of membrane turnover and Coo increase is related to tumour growth and histological grade. Besides Coo, differenl metabolites can be considered : N·acetyl-aspartate (NAA) lev- els describe neuronal activity and creatine (Cr) is a marker of cell energy status. 18F-Fluorocholine (FCH) is a new PET radiopharmaceutical which depicts Coo uptakes in brain tumours and overcomes FDG limits in cerebral region. A comparison between the 2 techniques is not deeply discussed in literature. In this report a comparison between FCH uptake and Coo distribu- tion as detected in MRS is presented for one patient, having an high grade glioma. Materials: One patient with a brain tumour underwent to CT/PET FCH ex- amination and MRS in mullivoxel mode during one week. MR examination was carried out on a 1.5 T scanner. Multivoxel MRS was performed in 2 different slices which encompass the tumour and a part of the surrounding edema . A 12 voxel grid was set on both slices, each voxel being 1x1x1.5 ern". CT/PET serie was coregistered with MR anatomical imaging and ROI as in the multivoxel grid were drawn in the corresponding positions. FCH uptake and Cho/NAA and Choler ratio distributions were compared. Results: The correlation between FCH uptake and metabolite ratios were compared separately for the 2 slices. In both slices FCH uptake is better correlated with Cho/NAA ratio (linear regression 1"1=0.84 and 1"1=0.78) than Cho/Cr (1"1=0.63 and 1"1=0.73). All the correlations are calculated to be sig- nificative. Comparison between choline levels detected by MRS and FCH uptake is poorly documented in literature, however these results are coher- ent with Kwee et aI., Journal of Neuroimaging (2004), where the cases of 2 patients are presented. FCH uptake seems to describe choline metabolism according to MRS examination. Conclusions: Choline metabolism in brain tumours can be studied by MRS and FCH·PET. They can contribute to delineate proliferative regions inside the tumour: this can help in the definition of the biological target volume in radiotherapy planning. These techniques rely on related but slightly differ- ent biological mechanisms. However a comparison between them is poorly documented in present literature and would deserve a deeper analysis . This report analyses the case of a patient with a brain tumour, where FCH uptake and choline levels detected in MRS show a good correlation. This should be studied for more patients for different grade or types of brain tumours . Pre-clinical validation of tracers 64 poster CT/MR CONTRAST AGENT FOR LONGITUDINAL LOCALIZATION AND DELINEATION OF TARGET STRUCTURES IN IMAGE-GUIDED RADIOTHERAPY J. Zheng 1 , C. Allen', D. Jaffrayl 1 PRINCESS MARGARET HOSPITAL, Radiation Medicine Program; Depart- ment of Radiation Physics, Toronto, Canada 2 UNIVERSITY OFTOR ONTO, Pharmaceutical Sciences, Toronto, Canada Purpose: To engineer a long-cirCUlating contrast agent and validate its use as a less invasive and less permanenl alternative to current fiducial markers for localization and delineation of target structures during radiation therapy delivery under cone-beam CT (CBCT) and MR guidance. Materials: New Zealand White rabbits bearing VX2 carcinoma in their left lateral quadricep received a single l.v, dose of a recently developed CT/MR contrast agent (0.25 mglkg of iodine in the form of iohexol and 0.02 mglkg of gadolinium in the form of gadoteridol co-encapsulated in liposomes of 80 ± 3 nm in diameter). CBCT (Elekta Synergy) and MR (1.5 Tesla, GE Signa) imaging were performed up to 14 days post-Iiposome contrast administration. The tumors were then excised and evaluated for histological confirmation of disease. Results: The mean vascular circulation for the CT/MR Iiposome con- trast agent in VX2 carcinoma bearing New Zealand rabbits is 63.6 ± 5.8 hours. This corresponds to a 300-fold increase compared to the vascular res- idency time of conventional small molecule CT and MR contrast agents such as iohexol (Omnipaque<!l> , GE Healthcare) and gadoteridol (Prohance<!l> , Bracco). In addition, these Iiposomes have shown to preferentially accu- mulate at tumor sites through a process termed enhanced permeability and retention. Although the peak tumor contrast enhancement occurs at three days post-injection, adequate delineation of the tumor margins in both imag- ing modalities by the liposome agent is maintained up to two weeks post- administration (Figure 1). Ongoing investigation is aimed at correlating the disease margins as marked by the liposome agent in the imaging data sets with those of the resected tumor tissues. (') . " " .. \ - I" . . , . . - ,; .. ' .- " "",_. v...... '-O""d#'- IfdCOl'*_ 0.. .. rI.'''''z.r.o · bM"'\CJ l e4a ,_ -e ... Conclusions: This study demonstrated the feasibility of employing a long- circulating contrast agent to provide persistent contrast enhancement of tar- get sites detectable by CBCT and MR imaging over a two-week period. In addition, previous investigations in mice, rats and rabbits have demonstrated that this CTIMR liposome agent possesses an excellenl toxicity profile. A comparative study with FDG-PET has also suggested that this Iiposome agent may have superior performance in detecting and localizing lesions sit- uated in proximity to FDG-avid normal tissues. Overall, this liposome-based contrast agent takes advantage of currently available image-guidance tech - nologies, and if approved for human use, has the potential to readily improve target localization during radiation therapy delivery.