Measurement and calculation of heterogeneity correction factors for an Ir‐192 high dose‐rate brachytherapy source behind tungsten alloy and steel shields

Abstract

Shields made of high atomic number material are commonly used in vaginal applicators with high dose-rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF approximately 0.4, for 20 x 4 mm disk) than for a steel disk with the same dimensions (HCF approximately 0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.

Experimental validation of dose calculation algorithms for the GliaSite™ RTS, a Novel 125I liquid-filled balloon brachytherapy applicator

“…In addition, the precision of RCF response ͑95% confidence interval of Ϯ1%-2% for a single 100 mϫ100 m pixel measurement over the 15-60 Gy dose range͒ is far superior to the TLD dosimetry precision ͑2% for a 3ϫ3ϫ0.9 mm 3 TLD with 3-5 separate readings͒ achieved in our laboratory. 8,9 Due to experimental uncertainties not controlled by our methods, e.g., positional uncertainties, leakage of radioactivity and film damage from trimming, errors significantly larger than 5% occur within 4 mm of physical contact between the balloon surface and the RCF detectors. The 14.5% disagreement between film A and MCPT and 7% for film B ͓Fig.…”

EGSnrc‐based Monte Carlo dosimetry of CSA1 and CSA2 brachytherapy source models

“…The authors just mentioned that photons with energies equal to or higher than 1 MeV exhibited a significant electronic buildup effect over a distance of 1 mm from the point source. In our opinion, the studies by Valicenti et al 11 and Nath et al 12 should not be considered for approximating collision kerma to absorbed dose for the stainless steel encapsulated 137 Cs sources. Similarly, the work by Kirov et al.…”

“…Although the published studies by Valicenti et al, 11 by Kirov et al, 12 and by Nath et al 13 have limitations, these studies are often cited while approximating collision kerma to absorbed dose for the stainless steel encapsulated 137 Cs brachytherapy sources. For example, Pérez-Calatayud et al 7,9,14 referred to the work by Nath et al 13 and Williamson 3 referred to the work by Valicenti et al 11 and by Kirov et al 12 Based on Roesch's work, 15 Valicenti et al 11 evaluated dose to water-kerma ratio D / K for a 400 keV point photon source in water and found that the ratio is less than 1.01 for distances greater than 2 mm from the source. The EGS4-based 16 Monte Carlo study by Kirov et al 12 is devoted to D / K in water for small photon sources ͑including point sources͒ such as 137 Cs, 60 Co, and 24 Na.…”

Anniversary Paper: Past and current issues, and trends in brachytherapy physics

“…Some applicators containing local shielding to spare bladder and rectal tissues can produce dose reductions as large as 50% 103,104 that most treatment planning systems still continue to ignore. The perturbing effects of partial transmission shielding have been shown 73,105,106 to vary rapidly with cross-sectional area and location of shielding materials, especially for sources with energies at or below that of 192 Ir. Only for 137 Cs shielded sources can one-dimensional pathlength algorithms ͑available on some commercial planning systems͒, e.g., generalizations of the Sievert integral model, 107 be used reliably.…”