An investigation into the use of polymer gel dosimetry in low dose rate brachytherapy.

Abstract

An investigation has been carried out into the properties of the BANG polymer gel and its use in the dosimetry of low dose rate brachytherapy. It was discovered that the response of the gel was reproducible and linear to 10 Gy. The gel was found to be tissue equivalent with a response independent of energy to within experimental accuracy (standard error of measurement +/- 5%). The slope of the calibration curve was found to increase from 0.28 +/- 0.01 s-1 Gy-1 to 0.50 +/- 0.02 s-1 Gy-1 for an increase in monomer concentration from 6 to 9%. Absorbed dose distributions for a straight applicator containing 36 137Cs sources were measured using the gel and the results compared with measurements made with thermoluminescent dosemeters (TLDs) and calculated values. Good agreement was found for the relative measurements. The root mean square residual percentage errors were 3%, 1% and 4% for the gel and the two groups of TLDs, respectively. There were some significant differences in absolute values of absorbed dose in the gel, possibly owing to the effects of oxygen. Measurements of a complex gynaecological insert were also made and compared with isodose curves from a planning system (Helax TMS), and in areas unaffected by oxygen diffusion the isodose levels from 100 to 50% agreed to within less than 0.5 mm.

A comparison of polyacrylamide gels and radiochromic film for source measurements in intravascular brachytherapy

“…The polymer gels employed in this study were prepared in a glove box using the method described by Farajollahi et al [7] with modifications to the phantom filling system developed De Deene et al [17] and Love [18]. The oxygen concentration inside the glove box was monitored, and was always less than 0.2%.…”

“…Using PAG as a dosimeter has the advantages that the distributions can be measured in three dimensions, the gel is both dosimeter and phantom, and the gel is tissue equivalent [7]. To date, in-plane resolution for the dose distributions from gel dosimeters has been reported in the range from 0.1 mm to 1.5 mm for MRI [6,7,[13][14][15]. In principle, the spatial resolution achievable in MRI is determined by the strength of the magnetic field gradient that is applied to achieve spatial discrimination.…”

“…Polyacylamide gel (PAG) dosimetry has been used for measurements of several brachytherapy sources [5][6][7][8][9][10], and relies on radiation-induced cross-linking of the polymer, which alters both the optical density of the gel and the nuclear relaxation behaviour of the gel water. Changes in optical density may be used in optical dosimetry [11,12] but will not be discussed further in this paper.…”

“…In MRI dosimetry, the changes in the nuclear transverse relaxation rate (R 2 ) (the inverse of the relaxation time (T 2 )) with dose are exploited. Using PAG as a dosimeter has the advantages that the distributions can be measured in three dimensions, the gel is both dosimeter and phantom, and the gel is tissue equivalent [7]. To date, in-plane resolution for the dose distributions from gel dosimeters has been reported in the range from 0.1 mm to 1.5 mm for MRI [6,7,[13][14][15].…”

“…Calibration vials were irradiated to doses of 0 Gy, 2 Gy, 4 Gy, 6 Gy and 8 Gy using a 6 MV photon beam with a 150 mm6150 mm field. The response of PAG based on the formulations given by Maryanski et al [5,6] has been reported to be independent of energy for a wide range of photon and electron beams by several authors [6][7][8]. More recently, some energy dependence has been reported [20] for the formulation based on acrylic acid and N,N9-methylene-bis-acrylamide [6].…”