Proceedings of the 28th Annual ASTRO Meeting 161

134 A CT-BASED BACK PROJECTION METHOD FOR PRE-PLANNING INTERSTITIAL BRACHYTHERAPY OF THE BRAIN.

Andrew Wu, Ph.D., R.G. Zamenhof, Ph.D., and E.S. Stemick, Ph.D.

Medical Physics Division, Department of Therapeutic Radiology, Tufts-New England Medical Center, Boston, Massachusetts 02111.

Recently, the high accuracy of CT guided stereotactic systems has made interstitial brachytherapy an attractive and practical method of therapy for malignant brain tumors. In order to deliver a homogeneous dose distribution to the tumor volume, a simple CT back projection method has been developed for pre-implant planning. During the CT scan prior to the implant procedure, burr hole location is selected and marked with radiopaque paste. From the pixel coordinates of the target on the CT scan where the reference catheter is to intersect the tumor, a linear equation is derived for the three-dimensional trajectory of the implanted reference catheter. This equation also gives the total depth of insertion and the active length of the radio- active-seed array. The trajectories of subsequent catheters parallel to the first one and at pre-determined separations are similarly computed and back projected to the skull. This method enables us accurately to determine the positions of each of the parallel catheters and their active seed loading to give the desired dose uniformity throughout the tumor volume. A computer iso-dose calculation and mapping is then performed to verify adequate coverage, and seed activity is determined from the chosen dose rate.

135

A NEW METHOD OF DETERMINING TREATMENT DEPTH FOR ELECTRON BREAST BOOSTS

J. Chu, Ph.D.' , L.J. Solin, M.D.' , R. Larsen, Ph.D.*, B. Fowble, M.D.' , R.L. Goodman, M.D.' 1 Department of Radiation Therapy, University of Pennsylvania School of Medicine and the Fox Chase Cancer

Center, Philadelphia, PA; *Department of Radiation Therapy, JFK Hospital , Atlantis, FL

For patients undergoing definitive irradiation of early stage breast cancer, the boost is usxlly delivered with electrons or iridim implants. The choice of energy for the electrons is determined by the depth to the tumor bed, bt measurement of the depth has been problemtical. Although such measurements can be obtained directly by CT scanning, dedicated CT scanners for radiotherapy planning are not widely available for this purpose. In order to address these issues, a new technique has been developed to determine the depth for the electron boost treatments.

After the clinician has determined the site of the boost, a series of link chains are placed in parallel on the breast contour at 1 cm spacing. These chains are flexible and m&e continuous contact with the breast contour. Additionally, each chain is uniquely identifiable on radiographs by the length of the chain and by the link characteristics. Under fluoroscopic control, the clinician identifies I;ldiographically the target (i.e. of the breast biopsy) beneath the chains.

radiopaque surgical clips placed at the time A pair of orthogonal films and/or rotational stereo shift

films are obtained using. a 100 cm SAD isccentric technique with the isocenter placed at the proximity of the surgi&l clips. The target depth can &en be measured directly from orthogorval films or calculated from stereo shift films between each individual surgical clin and the corresponding chain on skin.

In order to test the accuracy of this system in a clinical setting, we compared this roethod with CT scan data in 17 patients undergoing definitive irradiation for early stage breast cancer. When the depth from skin surface to the deepest clip was compared, the depth determined from orthogonal radiographs was within 5 mn of the depth measured on CT scans in 16/16 patients. The depth measured using the rotational stereo shift method, was within 6 mn of the Cl? result in 17/17 patients. When the clinically selected C!J, E/17 patients were within 5' and 17 7

ntry angle for the electron boost field was compared to 17 patients were within 10'.

From a practical perspective, this method is extremely efficient, and the entire procedure generally takes about 20-30 minutes. Also, this system can be used for other anatomic sites (e.g. for soft tissue sarcomas of the extremities).

In sunmary, a new technique is described to calculate the target depth for electron breast boosts in patients receiving definitive irradiation for early stage breast cancer. This method is accurate, efficient, and has broad clinical applicability.