Evaluation of hypothetical (153)Gd source for use in brachytherapy

Determination of dosimetric parameters for shielded 153Gd source in prostate cancer brachytherapy

Background

Interstitial rotating shield brachytherapy (I-RSBT) is a recently developed method for treatment of prostate cancer. In the present study TG-43 dosimetric parameters of a ¹⁵³Gd source were obtained for use in I-RSBT.

Materials and methods

A ¹⁵³Gd source located inside a needle including a Pt shield and an aluminum window was simulated using MCNPX Monte Carlo code. Dosimetric parameters of this source model, including air kerma strength, dose rate constant, radial dose function and 2D anisotropy function, with and without the shields were calculated according to the TG-43 report.

Results

The air kerma strength was found to be 6.71 U for the non-shielded source with 1 GBq activity. This value was found to be 0.04 U and 6.19 U for the Pt shield and Al window cases, respectively. Dose rate constant for the non-shielded source was found to be 1.20 cGy/(hU). However, for a shielded source with Pt and aluminum window, dose rate constants were found to be 0.07 cGy/(hU) and 0.96 cGy/(hU), on the shielded and window sides, respectively. The values of radial dose function and anisotropy function were tabulated for these sources. Additionally, isodose curves were drawn for sources with and without shield, in order to evaluate the effect of shield on dose distribution.

Conclusions

Existence of the Pt shield may greatly reduce the dose to organs at risk and normal tissues which are located toward the shielded side. The calculated air kerma strength, dose rate constant, radial dose function and 2D anisotropy function data for the ¹⁵³Gd source for the non-shielded and the shielded sources can be used in the treatment planning system (TPS).

Effects of Siemens TT-D carbon fiber table top on beam attenuation, and build up region of 6 MV photon beam

AIM

This study deals with Monte Carlo simulations of the effects which the 550 TXT carbon fiber couch can have on the relevant parameters of a 6 MV clinical photon beam in three field sizes.

BACKGROUND

According to the reports issued by the International Commission on Radiation Units and Measurements (ICRU), the calculated dose across a high gradient distribution should be within 2% of the relative dose, or within 0.2 cm of the isodose curve position in the target volume. Nowadays, the use of posterior oblique beam has become a common practice. It is clear that, in radiotherapy, the presence of the couch affects the beam intensity and, as a result, the skin dose.

MATERIALS AND METHODS

Firstly, Siemens linear accelerator validation for 6 MV photon beam was performed, and satisfactory agreement between Monte Carlo and experimental data for various field sizes was observed. Secondly, the couch transmission factor for the reference field size and depth was computed, and the skin dose enhancement by the couch was assessed.

RESULTS

The largest impact of the carbon fiber couch effect was observed for the 5 × 5 cm2 field size. Such evaluation has not been reported for this couch before.

CONCLUSION

Despite providing minimal attenuation for the primary radiation, the assumption that carbon fiber couches are radiotranslucent is not valid, and the effects of couches of this type on the transmission factor, and on the skin dose should be carefully investigated for each field size and depth.