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 () and 2D anisotropy function (,) table entries for radial distance and polar angle . The objectives of this study are as follows: 1) to evaluate interpolation methods in order to accurately derive () and (,) from the reported data; 2) to determine the minimum number of entries in () and (,) that allow reproduction of dose distributions with sufficient accuracy.

MATERIAL AND METHODS

Four high-energy photon-emitting brachytherapy sources were studied: Co model Co0.A86, Cs model CSM-3, Ir model Ir2.A85-2, and Yb hypothetical model. The mesh used for was: 0.25, 0.5, 0.75, 1, 1.5, 2-8 (integer steps) and 10 cm. Four different angular steps were evaluated for (,): 1°, 2°, 5° and 10°. Linear-linear and logarithmic-linear interpolation was evaluated for (). Linear-linear interpolation was used to obtain (,) 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 () provided differences ≤ 0.5% compared to MC for all four sources. Bilinear interpolation of (,) using 1° and 2° angular steps resulted in agreement ≤ 0.5% with MC for Co, Ir, and Yb, while Cs agreement was ≤ 1.5% for < 15°.

CONCLUSIONS

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