Dose perturbation study in a multichannel breast brachytherapy device

Purpose: A study was conducted to determine the dosimetric effects resulting from air pockets and high atomic number(Z) contrast medium within a multichannel breast brachytherapy device. Material and methods: A 5-6 cm diameter Contura (SenoRx) brachytherapy device was inflated using 37 cm3 of saline.Baseline dose falloff from an HDR Iridium-192 source was measured with the Iridium source centered in the central channeland an anterior off-center channel. Data were collected at distances from 1 to 50mm. Comparison studies were conductedwith identically inflated volume containing varied air pocket volumes (1-4 cm3) and concentrations of contrastsolution (3%, 6%, and 9% by volume). Dose perturbation factors (DPF) were computed and evaluated. Results: Dose perturbations due to air pockets and contrast solutions were observed. As the volume of air increased,the DPF increased by approximately 2.25%/cm3. The effect was consistent for both channels. The contrast effects weremore complex. The 3% contrast media had minimal dose perturbation. The 6% contrast solution caused dose reductionof 1.0% from the central channel but 1.5% dose increase from the anterior channel. The 9% contrast solution caused dosereductions by 4.0% (from central channel) and 3.0% (from anterior channel). The DPF from all contrast solutions mode -rated with increasing distance. Conclusions: Dose perturbations due to air pockets and high-Z contrast solution can be significant. It is importantto control these effects to avoid dose errors.