Fitting and benchmarking of dosimetry data for new brachytherapy sources

Evaluation of interpolation methods for TG-43 dosimetric parameters based on comparison with Monte Carlo data for high-energy brachytherapy sources

Purpose

The aim of this work was to determine dose distributions for high-energy brachytherapy sources at spatial locations not included in the radial dose function g_L(r) and 2D anisotropy function F(r,θ) table entries for radial distance r and polar angle θ. The objectives of this study are as follows: 1) to evaluate interpolation methods in order to accurately derive g_L(r) and F(r,θ) from the reported data; 2) to determine the minimum number of entries in g_L(r) and F(r,θ) that allow reproduction of dose distributions with sufficient accuracy.

Material and methods

Four high-energy photon-emitting brachytherapy sources were studied: ⁶⁰Co model Co0.A86, ¹³⁷Cs model CSM-3, ¹⁹²Ir model Ir2.A85-2, and ¹⁶⁹Yb hypothetical model. The mesh used for r was: 0.25, 0.5, 0.75, 1, 1.5, 2–8 (integer steps) and 10 cm. Four different angular steps were evaluated for F(r,θ): 1°, 2°, 5° and 10°. Linear-linear and logarithmic-linear interpolation was evaluated for g_L(r). Linear-linear interpolation was used to obtain F(r,θ) with resolution of 0.05 cm and 1°. Results were compared with values obtained from the Monte Carlo (MC) calculations for the four sources with the same grid.

Results

Linear interpolation of g_L(r) provided differences ≤ 0.5% compared to MC for all four sources. Bilinear interpolation of F(r,θ) using 1° and 2° angular steps resulted in agreement ≤ 0.5% with MC for ⁶⁰Co, ¹⁹²Ir, and ¹⁶⁹Yb, while ¹³⁷Cs agreement was ≤ 1.5% for θ < 15°.

Conclusions

The radial mesh studied was adequate for interpolating g_L(r) for high-energy brachytherapy sources, and was similar to commonly found examples in the published literature. For F(r,θ) close to the source longitudinal-axis, polar angle step sizes of 1°-2° were sufficient to provide 2% accuracy for all sources.

Bi- and tri-exponential fitting to TG-43 radial dose functions of brachytherapy sources based on a genetic algorithm

PURPOSE

To find the coefficients for bi- and tri-exponential fitting functions to represent the radial dose functions of 16 commercially available brachytherapy sources.

METHODS AND MATERIALS

The search for the coefficients was done using a genetic algorithm. Coefficients were encoded into chromosomes, which were subjected to crossover and mutation. After each operation, chromosomes were evaluated according to their fitness and the better ones were chosen with higher probability for the next generation. The best chromosomes obtained after 2000 operations were used for the coefficients.

RESULTS

For all brachytherapy sources, tri-exponential dose functions agreed with the respective input data within 1.4%. The mean deviation, obtained by averaging absolute deviations of all sources and input data, was <1.0%. For 8 of the 16 sources, the fit offered by bi-exponential functions was virtually identical to that of tri-exponential ones.

CONCLUSION

Tri-exponential functions can accurately represent the radial dose functions of all commercially available brachytherapy sources. For the eight sources where bi-exponential functions provide nearly equally accurate fits, their continued usage is recommended.

Dosimetric characterization of radioactive sources employed in prostate cancer therapy

PURPOSE

Three types of radiation sources are employed currently in the radiation treatment of prostate cancer, namely, external, implant, and high-dose-rate (HDR) sources using an afterloader method. The present article provides a detailed dosimetric characterization of several commercially available implant sources and an HDR source employing the same stochastic code and dataset.

METHODS AND MATERIALS

The radioactive implants considered are (125)I seeds: models 6701, 6702 and 6711, (103)Pd seed: model 200, and a high-dose-rate (192)Ir source: microSelectron-HDR model V7.0x. Detailed modeling of the sources and their associated X-rays and gamma rays has been carried out using the stochastic code MCNP4C. A sensitivity study has been conducted to quantify effects of varying the composition and density of the tissue equivalent material, and a dosimetric comparison is made for different media (tissue equivalent, solid-water, water, and air). Furthermore, a set of measurements using thermoluminescent dosimeters has been done to provide experimental validation of some of the calculational results obtained.

RESULTS

Effectively, high-precision dosimetric values (Monte-Carlo statistical 1-sigma error <1%) are provided in tabulated form over a wide range to enable therapy planning as well as to check numerical values calculated by other methods. A subset of calculated dosimetric values has been experimentally validated by using thermoluminescent dosimeters.

CONCLUSIONS

A detailed comparison of results obtained for the radial dose distribution function, anisotropy factor, and dose rate constant as defined in the TG-43 protocol has indicated reasonable agreement with the values reported in the literature.

Measurements of the dosimetric constants for a new 103Pd brachytherapy source

PURPOSE

This report provides dosimetry information for a new brachytherapy 103Pd source (Model 2335) manufactured by Best Medical International and, through comparisons with data from another published report for the same source, presents a suggested hybrid dataset for clinical applications.

METHODS AND MATERIALS

Dose measurements were made using thermoluminescent dosimeters (TLD), with the 103Pd source in the center of a Virtual Water phantom on a rotating insert, allowing for a number of different possible angles, and 12 TLD cubes (1.0 x 1.0 x 1.0 mm) and 16 TLD rods (1.0 x 1.0 x 3.0 mm) arranged in an outward spiral pattern at distances ranging from 0.5 to 5.0 cm. All measurements are based on the 2000 correction of the National Institute of Standards and Technology 1999 standard.

RESULTS

From these measurements, tables are presented for the radial dose function and the anisotropy function. The dose rate constant=0.71 +/- 0.07 cGy h(-1) U(-1) and the anisotropy constant=0.96 +/- 0.03.

CONCLUSIONS

The dose rate constant for the hybrid=0.70 and the anisotropy constant for the hybrid=0.92, Hybrid tables are presented.