Dose-volume analysis for quality assurance of interstitial brachytherapy for breast cancer

The use of brachytherapy in the management of breast cancer has increased significantly over the past several years. Unfortunately, few techniques have been developed to compare dosimetric quality and target volume coverage concurrently. We present a new method of implant evaluation that incorporates computed tomography-based three-dimensional (3D) dose-volume analysis with traditional measures of brachytherapy quality. Analyses performed in this fashion will be needed to ultimately assist in determining the efficacy of breast implants.

Since March of 1993, brachytherapy has been used as the sole radiation modality after lumpectomy in selected protocol patients with early-stage breast cancer treated with breast-conserving therapy. Eight patients treated with high-dose-rate (HDR) brachytherapy who had surgical clips outlining the lumpectomy cavity and underwent computed tomography (CT) scanning after implant placement were selected for this study. For each patient, the postimplant CT dataset was transferred to a 3D treatment planning system. The lumpectomy cavity, target volume (lumpectomy cavity plus a 1-cm margin), and entire breast were outlined on each axial slice. Once all volumes were entered, the programmed HDR brachytherapy source positions and dwell times were imported into the 3D planning system. Using the tools provided by the 3D planning system, the implant dataset was then registered to the visible implant template in the CT dataset. The distribution of the implant dose was analyzed with respect to defined volumes via dose-volume histograms (DVH). Isodose surfaces, the dose homogeneity index, and dosimetric coverage of the defined volumes were calculated and contrasted. All patients received 32 Gy to the entire implanted volume in 8 fractions of 4 Gy over 4 days.

Three-plane implants were used for 7 patients and a two-plane implant for 1 patient. The median number of needles per implant was 16.5 (range 11-18). Despite visual verification by the treating physician that surgical clips (with an appropriate margin) were within the boundaries of the implant needles, the median proportion of the lumpectomy cavity that received the prescribed dose was only 87% (range 73-98%). With respect to the target volume, a median of only 68% (range 56-81%) of this volume received 100% of the prescribed dose. On average, the minimum dose received by at least 90% of the target volume was 22 Gy (range 17.3-26.9), which corresponds to 69% of the prescribed dose.

Preliminary results using our new technique to evaluate implant quality with CT-based 3D dose-volume analysis appear promising. Dosimetric quality and target volume coverage can be concurrently analyzed, allowing the possibility of evaluating implants prospectively. Considering that target volume coverage may be suboptimal even after radiographically verifying accurate implant placement, techniques similar to this need to be developed to ultimately determine the true efficacy of brachytherapy in the management of breast cancer.