Implementation of 3D-virtual brachytherapy in the management of breast cancer: a description of a new method of interstitial brachytherapy.

PURPOSE: We present the initial description of a new technique of interstitial breast brachytherapy in which a computer-generated image of an implant template is applied virtually to serial-computed tomography (CT) scan images of a patient's breast. Optimal placement of the virtual template around the CT images of the proposed target volume provides the physician with a preplan for improved positioning of implant needles around the actual target volume intraoperatively. METHODS AND MATERIALS: Since March of 1993, 110 patients with early-stage breast cancer were entered onto a protocol of low or high dose rate brachytherapy as the sole radiation modality for part of their breast-conserving therapy. To improve the accuracy and reproducibility of target volume coverage in patients with a closed lumpectomy cavity, 11 of these implants were performed using the virtual brachytherapy technique. The virtual implant procedure was performed by first placing radiopaque skin markers on the breast surface for reference on the CT image and ultimately as intraoperative landmarks for the placement of implant needles. A CT scan of the breast was then performed and the target volume outlined on each CT scan slice by the physician. A virtual image of the brachytherapy template was then positioned around the CT image of the target volume to achieve an idealized implant with optimal coverage. The projected entrance and exit points of all needles on the skin of the breast (from the idealized virtual implant) were then identified (by perspective rendering of multiple 3D views) and hard-copy images taken to the operating room. The implant was then constructed by referencing the virtual implant images (needle entrance and exit points) to the radiopaque skin markers on the breast. After the implant was completed, a CT scan of the breast with the template catheters or needles in position was taken for comparison of the actual target volume coverage with the virtual implant generated preoperatively.
RESULTS: Intraoperative ultrasound was used to check the real-time position of the afterloading needles in reference to the chest wall and posterior border of the target volume. No adjustment of needles was required in any of the 11 patients. Assessment of target volume coverage between the virtual implant and the actual CT image of the implant showed excellent agreement. In each case, all target volume boundaries specified by the physician were adequately covered. The total number of implant planes, intertemplate separation, and template orientation were identical between the virtual and real implant.
CONCLUSION: We conclude that 3D virtual brachytherapy may offer an improved technique for accurately performing interstitial implants of the breast with a closed lumpectomy cavity in selected patients. Although preliminary results show excellent coverage of the desired target volume, additional patients will be required to establish the reproducibility of this technique and its practical limitations.