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The feasibility of quantitative ferrous sulfate/agarose gel dosimetry: Comparison with ionization chamber measurements for photons and electrons
Author(s) -
Smajo Mary Ellen
Publication year - 1996
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.597782
Subject(s) - dosimetry , ionization chamber , dosimeter , ionization , materials science , electron , photon , dose profile , irradiation , absorbed dose , nuclear medicine , radiochemistry , optics , chemistry , analytical chemistry (journal) , physics , nuclear physics , ion , medicine , organic chemistry , chromatography
Dosimetry techniques employing a ferrous sulfate‐doped gel read with MRI have shown promise in the measurement of ionizing radiation, particularly as a quality assurance tool for radiotherapy. This work investigated the feasibility of dose quantitation using one such gel. Maps of the change in the spin‐lattice relaxation rate (1/ T 1 ) in large (1.5 liter) phantoms were created by implementing a published T 1 extraction algorithm. Keeping a sufficiently short time between pre‐irradiation imaging, irradiation (typically to 40 Gy at d max ), and post‐irradiation imaging minimized diffusion. Irradiations were performed with 6 and 16 MeV electrons and 1.25 MeV (cobalt‐60), 6 MV, and 18 MV photons. Central axis depth doses and profiles at various depths were calculated and compared with doses based on ionization chamber measurements in water or plastic phantoms. These comparisons showed good agreement for electrons beyond d max , but revealed unexpected discrepancies for photons. These discrepancies are not an artifact of the choice of normalization point and do not appear to be the result of a dose saturation effect. Thus, although the ferrous sulfate/agarose gel dosimeter is well suited to quantitative (relative) measurements of electron dose beyond d max , it is not consistent with ionization‐based dosimetry for photons, implying that care must be taken in the quantitative interpretation of gel‐based dose‐maps. [Copies of the dissertation are available from University Microfilms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106‐1346 (Phone: 1‐800‐521‐3042).]