Deidre Batchelar

Skin dose in breast brachytherapy: Defining a robust metric

PURPOSE To define a simple, robust, and relevant metric for measuring skin dose in breast brachytherapy. METHODS AND MATERIALS Postoperative treatment plans (Day 0) for 15 permanent breast seed implant (PBSI) and 10 multicatheter high-dose-rate (MC-HDR) brachytherapy patients were included. Retrospectively, three skin structures were contoured: 2 mm external from the body; and subsurface layers 2 mm and 4 mm thick. Maximum point dose (Dmax), doses to small volumes (e.g., D0.2cc), and the volumes receiving a percentage of the prescription dose (V%, e.g., V66) were calculated. D0.2cc was investigated as a surrogate to the dose given to 1 cm(2) of skin (D1cm(2)). Pearson product-moment correlation (R(2)) was computed between metrics. RESULTS Observed trends were consistent across brachytherapy technique. V% did not correlate well with any other metrics: median (range) R(2), 0.63 (0.43, 0.77) and 0.69 (0.3, 0.89) for PBSI and MC-HDR, respectively. Dmax was inconsistently correlated across contours and not well correlated with doses to small volumes: median (range) R(2), 0.85 (0.76, 0.93) and 0.88 (0.83, 0.93) for PBSI and MC-HDR, respectively. In contrast, doses to small volumes were consistently well correlated, even across skin layers: D0.1cc vs. D0.2cc median (range) R(2), 0.98 (0.97, 0.99) and 0.97 (0.94, 0.99) for PBSI and MC-HDR, respectively. CONCLUSIONS Doses to small volumes are robust measures of breast skin dose and given skins strong area effect, D0.2cc for a 2 mm thick skin layer, a simple surrogate of D1cm(2), is recommended for recording skin dose in any breast brachytherapy. Dmax is not robust and should be avoided.

3D ultrasound guidance system for permanent breast seed implantation: integrated system performance and phantom procedure.

Permanent breast seed implantation (PBSI) is a single visit accelerated partial breast irradiation method that uses needles inserted via a template to distribute Pd-103 radioactive seeds with two-dimensional (2D) ultrasound (US) guidance. This guidance approach is limited by its dependence on the operator and average seed placement errors greater than benchmark values established by dosimetric studies. We propose the use of a three-dimensional (3D) US imaging approach for needle guidance with integrated template tracking. We previously described the preliminary development and validation of the 3D US mechatronic system. The present work demonstrates the accuracy of the integrated system by quantifying agreement between tracking and imaging sub-systems and its use guiding a phantom procedure. Tracking error was measured by inserting a needle a known distance through the template and comparing expected tip position from tracking to observed tip position from imaging. Mean ± standard deviation differences in needle tip position and angle were 2.90 ± 0.76 mm and 1.77 ± 0.98°, respectively, validating the needle tracking accuracy of the developed system. The system was used to guide 15 needles into a patient-specific phantom according to the accompanying treatment plan and micro-CT images taken before and after to evaluate placement accuracy. Seed positions were modelled using needle positions and the resulting dosimetry compared to a procedure specific benchmark. The mean tip difference was 2.08 mm while the mean angular difference was 2.6°, resulting in acceptable dosimetric coverage. These results demonstrate 3D US as a potentially feasible technique for PBSI guidance.