title: Improving calibration accuracy in gel dosimetry.
abstract: A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher accuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately with a 10 x 10 cm2 field end-on in a water bath, such that the characteristic depth-dose curve is recorded in the gel. The calibration is then determined by fitting the depth-dose measured in water, against the measured change in relaxivity with depth in the gel. Increased accuracy is achieved in this simple depth-dose geometry by averaging the relaxivity at each depth. A large number of calibration data points, each with relatively high accuracy, are obtained. Calibration data over the full range of dose (1.6-10 Gy) is obtained by irradiating one test-tube to 10 Gy at dose maximum (Dmax), and the other to 4.5 Gy at Dmax. The new calibration method is compared with a 'standard method' where five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy. The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the new method by a factor of about 4 and 10 respectively, when compared with the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slope of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.
authors.affiliation: Joint Department of Physics, Institute of Cancer Research and the Royal Marsden NHS Trust, Sutton, Surrey, UK.
authors.forename: M
authors.initials: M
authors.lastname: Oldham
authors.forename: M
authors.initials: M
authors.lastname: McJury
authors.forename: I B
authors.initials: IB
authors.lastname: Baustert
authors.forename: S
authors.initials: S
authors.lastname: Webb
authors.forename: M O
authors.initials: MO
authors.lastname: Leach
mesh_terms: D000178:Acrylamides / Q000528:radiation effects; D001918:Brachytherapy; D002138:Calibration; D005780:Gelatin / Q000528:radiation effects; D005782:Gels / Q000528:radiation effects*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D011108:Polymers / Q000528:radiation effects*; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011878:Radiotherapy
chemical_list: D000178:Acrylamides; C106521:BANG polymer gel; D005782:Gels; D011108:Polymers; D005780:Gelatin