IMPROVEMENTS IN ABSORBED DOSE MEASUREMENTS FOR EXTERNAL RADIATION-THERAPY USING FERROUS DOSIMETER GEL AND MR-IMAGING (FEMRI)

A ferrous gel, based on ferrous (Fe) sulphate and agarose, was used wi th a clinical magnetic resonance imaging (MRI) scanner to obtain relat ive dose distribution data from therapeutic photon and electron beams. The FeMRI gel was scanned using a new MRI acquisition protocol optimi zed for T1 measurements. Thorough comparisons with silicon semiconduct or detector and ionization chamber measurements, as well as with Monte Carlo calculations, were performed in order to quantify the improveme nts obtained using FeMRI for dose estimations. Most of the relative do ses measured with FeMRI were within 2% of the doses measured with othe r methods. The larger discrepancies (2-4%) found at shallow depths are discussed. The uncertainty in relative dose measurements using FeMRI was significantly improved compared with previously reported results ( 5-10%, one standard deviation, 1 SD), and is today between 1.6% and 3. 3% (depending on dose level, 2 SD). This corresponds to an improvement in the minimum detectable dose (3 SD above background) from approxima tely 2 Gy to better than 0.6 Gy. The results obtained in this study em phasize the importance of obtaining basic FeMRI dose data before the m ethod is extended to complicated treatment regimes.