Purpose: To assess early radiation therapy (RT)-induced variations in total choline (tCho) concentration measured by proton magnetic resonance spectroscopy (H-MRS) and in (18)F-labelled fluoromethylcholine (FCH) uptake measured by PET in a rodent tumour model.
Methods: Nine rats bearing syngenic rhabdomyosarcoma grafts in both thighs were irradiated (13 Gy, one fraction). H-MRS data and FCH-PET were acquired in the same imaging session prior to and 3, 9 and 16 days after external RT. Total choline concentration was expressed in arbitrary units as the area under the curve of the 3.2-ppm peak on H-MR spectra. FCH uptake was expressed as maximum standardized uptake value (SUV(max)) and as the % of injected dose per gram (%ID/g) after precise tumour delineation on hybrid PET-MR images. Pre- and post-RT data were compared using the Student's paired t test, and results were expressed as mean+/-S.D.
Results: Seventeen tumours were available for analysis. A mean drop in choline concentration of 45% was observed 3 days after irradiation (P<.001), whereas a concomitant mean increase in SUV(max) of 41% was observed (P=.006). Choline concentration reincreased on later time points.
Conclusions: Opposite trend between increased FCH uptake and decreased tCho peak was observed at 3 days. Later (9 and 16 days), uptake remained stable and tCho peak reincreased.
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