Comparison of tumor and normal tissue dose for accelerated partial breast irradiation using an electronic brachytherapy eBx source and an Iridium-192 source

Abstract

The objective of this study has been to compare treatment plans for patients treated with electronic brachytherapy (eBx) using the Axxent System as adjuvant therapy for early stage breast cancer with treatment plans prepared from the same CT image sets using an Ir-192 source. Patients were implanted with an appropriately sized Axxent balloon applicator based on tumor cavity size and shape. A CT image of the implanted balloon was utilized for developing both eBx and Ir-192 brachytherapy treatment plans. The prescription dose was 3.4 Gy per fraction for 10 fractions to be delivered to 1 cm beyond the balloon surface. Iridium plans were provided by the sites on 35 of the 44 patients enrolled in the study. The planning target volume coverage was very similar when comparing sources for each patient as well as between patients. There were no statistical differences in mean %V100. The percent of the planning target volume in the high dose region was increased with eBx as compared with Iridium (p < 0.001). The mean maximum calculated skin and rib doses did not vary greatly between eBx and Iridium. By contrast, the doses to the ipsilateral lung and the heart were significantly lower with eBx as compared with Iridium (p < 0.0001). The total nominal dwell times required for treatment can be predicted by using a combination of the balloon fill volume and planned treatment volume (PTV). This dosimetric comparison of eBx and Iridium sources demonstrates that both forms of balloon-based brachytherapy provide comparable dose to the planning target volume. Electronic brachytherapy is significantly associated with increased dose at the surface of the balloon and decreased dose outside the PTV, resulting in significantly increased tissue sparing in the heart and ipsilateral lung.

Figure 1

Comparison of the percentage of the planned target volume (PTV), breast, lung and heart receiving a given percentage of the prescription dose based on the eBx and Ir‐192 treatment plans.

Figure 2

The total nominal dwell time with balloon fill volume is shown for each balloon size for a prescibed dose of 3.4 Gy per fraction to the PTV. A linear regression trend line for all data points $({\mathrm{R}}^2=0.81)$ is shown (central dark line). The $\pm 7\%$ and $\pm 13\%$ deviation from this linear regression (lighter lines) are shown.

Figure 3

The total nominal dwell time with PTV is shown for each balloon size for a prescibed dose of 3.4 Gy per fraction to the PTV. A linear regression trend line for all data points $({\mathrm{R}}^2=0.76)$ is shown (central dark line). The $\pm 5\%$ and $\pm 13\%$ deviation from this linear regression (lighter lines) are shown.