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

Purpose/Objective: The use of brachytherapy in the management of breast can cer has increased significantly over the past several years. Unfortunately, few techniques have been developed to compare dosimetric quality and targe t volume coverage concurrently. We present a new method of implant evaluati on that incorporates computed tomography-based three-dimensional (3D) dose- volume analysis with traditional measures of brachytherapy quality. Analyse s performed in this fashion will be needed to ultimately assist in determin ing the efficacy of breast implants. Methods and Materials: 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. Eigh t patients treated with high-dose-rate (HDR) brachytherapy who had surgical clips outlining the lumpectomy cavity and underwent computed tomography (C T) scanning after implant placement were selected for this study. For each patient, the postimplant CT dataset was transferred to a 3D treatment plann ing system. The lumpectomy cavity, target volume (lumpectomy cavity plus a l-cm margin), and entire breast were outlined on each axial slice. Once all volumes were entered, the programmed HDR brachytherapy source positions an d dwell times were imported into the 3D planning system. Using the tools pr ovided by the 3D planning system, the implant dataset was then registered t o the visible implant template in the CT dataset. The distribution of the i mplant dose was analyzed with respect to defined volumes via dose-volume hi stograms (DVH). Isodose surfaces, the dose homogeneity index, and dosimetri c coverage of the defined volumes were calculated and contrasted. All patie nts received 32 Gy to the entire implanted volume in 8 fractions of 4 Gy ov er 4 days. Results: Three-plane implants were used for 7 patients and a two-plane impl ant for 1 patient. The median number of needles per implant was 16.5 (range 11-18). Despite visual verification by the treating physician that surgica l clips (with an appropriate margin) were within the boundaries of the impl ant needles, the median proportion of the lumpectomy cavity that received t he 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. Conclusion: 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 targ et 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 manageme nt of breast cancer. (C) 1999 Elsevier Science Inc.