Perturbation of TG-43 parameters of the brachytherapy sources under insufficient scattering materials

Evaluation of BEBIG HDR (60)Co system for non-invasive image-guided breast brachytherapy

Purpose

HDR ⁶⁰Co system has recently been developed and utilized for brachytherapy in many countries outside of the U.S. as an alternative to ¹⁹²Ir. In addition, the AccuBoost^® technique has been demonstrated to be a successful non-invasive image-guided breast brachytherapy treatment option. The goal of this project is to evaluate the possibility of utilizing the BEBIG HDR ⁶⁰Co system for AccuBoost treatment. These evaluations are performed with Monte Carlo (MC) simulation technique.

Material and methods

In this project, the MC calculated dose distributions from HDR ⁶⁰Co for various breast sizes have been compared with the simulated data using an HDR ¹⁹²Ir source. These calculations were performed using the MCNP5 code. The initial calculations were made with the same applicator dimensions as the ones used with the HDR ¹⁹²Ir system (referred here after as standard applicator). The activity of the ⁶⁰Co source was selected such that the dose at the center of the breast would be the same as the values from the ¹⁹²Ir source. Then, the applicator wall-thickness for the HDR ⁶⁰Co system was increased to diminish skin dose to levels received when using the HDR ¹⁹²Ir system. With this geometry, dose values to the chest wall and the skin were evaluated. Finally, the impact of a conical attenuator with the modified applicator for the HDR ⁶⁰Co system was analyzed.

Results

These investigations demonstrated that loading the ⁶⁰Co sources inside the thick-walled applicators created similar dose distributions to those of the ¹⁹²Ir source in the standard applicators. However, dose to the chest wall and breast skin with ⁶⁰Co source was reduced using the thick-walled applicators relative to the standard applicators. The applicators with conical attenuator reduced the skin dose for both source types.

Conclusions

The AccuBoost treatment can be performed with the ⁶⁰Co source and thick-wall applicators instead of ¹⁹²Ir with standard applicators.

Brachytherapy treatment planning commissioning: effect of the election of proper bibliography and finite size of TG-43 input data on standard treatments

The aim of this work is to evaluate the performance of a commercial brachytherapy treatment planning system (TPS) with TG‐43 Vendors Input Data (VID), analyze possible discrepancies with respect to a proper reference source and its implications for standard treatments, and judge the effectiveness of certain widespread recommended quality controls to find potential errors related with the interpolations of TG‐43 VID tables. The TPS evaluated was a BrachyVision 8.6 loaded with TG‐43 VID for a VariSource high‐dose‐rate 192Ir source (Vs2000). The reference data chosen were the TG‐43 data published in the literature. In the first step, we compared TG‐43 VID with respect to the chosen reference data. Next, we used percent dose‐rate differences in a point array matrix to compare the outcomes of the TPS on standard treatment setup with respect to an in‐house developed program (MATLAB R2009a‐based) loaded with the chosen full TG‐43 reference data. The cases with major discrepancies were evaluated using the gamma‐index analysis. The comparison with the reference data indicated a lack of sample in the angles between near to the tip (between 165<θ<180) and cable (0<θ<15) of the F(r,θ)VID, which causes a dose underestimation of approximately 17% in the investigated points due to inaccurate interpolations. The differences over 2% encompassed approximately 17% of the surrounding source volume. These results have special relevance in treatment using one applicator with a few dwell steps or in Fletcher treatments where 10% dose underestimates were identified within the tumor or in organs at risk, respectively. Our results suggest that the differences found in the TPS under study are created by a lack of information on the angles in high‐gradient zones in the F(r,θ)VID, which generates important differences in dosimetric results. In contrast, the gamma analysis shows very good results (between 90% and 100% of passed points) in the analyzed treatments (one dwell and Fletcher). Further studies are required to exclude the possibility of finding noticeable effects in the DVH of treatment plans caused by the discrepancies here described. To achieve more strict control over the TPS dose‐rate calculation, we recommend using QA test thinking in a source with nonaxial symmetry, adding a control point on the angles of the high‐dose gradient zones (e.g., between 0° and 15° and between 165° and 180°). More studies are required to achieve full understanding of the clinical implication of such discrepancies.

PACS number: 87.55.Qr