Dose rate determination for ¹²⁵I seeds

Abstract

Dose rates in water have been determined for the two types of 125I seed currently used in brachytherapy. The need for such determinations became evident when water/air ratios measured with a silicon diode were found to be lower than expected. Extensive measurements using lithium fluoride thermoluminescent dosimeters (TLD's) have been performed in a solid water phantom, at distances from 0.1 to 10 cm from the seed center and at angular increments of 10 degrees, 15 degrees, or 30 degrees within a plane through the seed axis. Dose calibration of the TLD's was accomplished by irradiation in air with 125I seeds of the same type and of strengths traceable to a calibration at the National Institute of Standards and Technology (NIST). Relative calibration of TLD's was monitored by irradiation, in an oven-type x-ray machine, of control dosimeters simultaneously and all dosimeters intercurrently with the 125I irradiations. Values obtained for the dose rate constant, i.e., dose rate per unit air-kerma strength at 1 cm on the transverse axis, were 0.853 and 0.932 cGy h-1 U-1 (1.08 and 1.18 cGy h-1 mCi-1) for the 6711 and 6702 seeds, respectively. Measured data were supplemented with Monte Carlo-calculated relative dose rate data generated using the MORSE code. These calculations used 100 energy groups from 10 to 35.4 keV and involved energy collection bins ranging from 0.025 to 1.2 cm on an edge. Normalized at 1 cm, transverse axis calculated data are not significantly different from measured data (ours or cited literature) at distances either less than 2.5 or greater than 8 cm. Normalized at different distances along the transverse axis, our off-axis calculated and measured distributions agree closely at all angles but differ from literature measured distributions at small (less than or equal to 1 cm) distances and, for small angles, increasingly at larger distances (greater than or equal to 5 cm).

Anisotropy functions for low energy interstitial brachytherapy sources: an EGS4 Monte Carlo study

“…For a better understanding of the discrepancies observed in the different data sets we represent in figure 5 the anisotropy as a function of the angle at three different distances. We also included values obtained from the matrix fit to diode measurements in liquid water by Schell et al (1987) and Monte Carlo calculations with MORSE code performed by Chiu-Tsao et al (1990), who modelled the ion-exchange resin beads in the titanium capsule by three spheres symmetrically located with their centres 0.125 cm apart on the seed axis and the titanium capsule as a hollow right circular cylinder. There is an excellent agreement of our results with those of Chiu-Tsao, showing that differences caused by assuming a model of cylindrical resin beads are negligible.…”

“…Several investigators have reported on the two-dimensional dosimetry of 125 I (Dale 1982, Arcovito et al 1985, Ling et al 1983, Schell et al 1987, Burns and Raeside 1987, Williamson and Quintero 1988, Piermattei et al 1988, Chiu-Tsao et al 1990, Scarbrough et al 1990, Nath et al 1993, Wang and Slovoda 1996, Weaver et al 1989, Weaver 1998) and 103 Pd seeds (Meigooni et al 1990, Chiu-Tsao and Anderson 1991, Nath et al 1995. The AAPM Task Group No 43 (Nath et al 1995) recommends a dosimetry protocol based on measured (or measurable) quantities allowing for two-dimensional dose calculations around cylindrically symmetric sources.…”

Multidimensional dosimetry of 106Ru eye plaques using EBT3 films and its impact on treatment planning

“…They were able to reduce the uncertainty down to ±15% as compared to the 95% confidence interval of ±25% specified by the manufacturer. 11 Other studies investigated the use of radiochromic films, 10,[12][13][14][15][16][17] TLDs, 15,16,[18][19][20][21][22][23] extrapolation ionization chambers, 24 and diodes 25 which are considered to be a standard for 125 I eye plaque dosimetry. Other dosimetric techniques which are less widely used, such as plastic scintillators, 11,26,27 threedimensional liquid scintillators, 28 and polymer gel dosimetry 29 were explored as well.…”

Brachytherapy technology and physics practice since 1950: a half-century of progress

“…Using TLD-100 chips and powder capsules embedded in machined solid-water phantoms, the ICWG developed procedures, for calibrating TLD detectors and correcting for higher TLD response to low-energy photons, for quantitatively estimating absolute dose rates in water. Each of the three ICWG investigator groups independently measured (Chiu-Tsao et al 1990, Weaver et al 1989 transverse-axis dose distributions for the 125 I and 192 Ir then available to validate their TLD measurement methodology. This was followed by more complete 2D dose distributions about the extant 125 I, 192 Ir and 103 Pd brachytherapy sources (Chiu-Tsao and Anderson 1991, Nath et al 1993.…”