292 Radiation Oncology, Biology, Physics Volume 32, Supplement 1

2061 MODIFIED AFTERLOADING HEYMAN TECHNIQUE FOR ENDOMETRIAL CANCER USING Ir-192 SEEDS AND DISPOSABLE CAPSULES

Anu Gupta, Azam Niroomand-Rad, Gregory Gagnon and Michael Kuettel

Department of Radiation Medicine, Georgetown University Medical Center, Washington, D.C.,20007

PURPOSE: Treatment of uterine cancer with intracavitary radium sources was first introduced in 1935 by Heyman.' In 1970, a modified afterloading Heyman applicator was introduced by Simon 2 using miniaturized Cs-137 sources designed to match the classical isodose distribution of Heyman capsules. More recently, disposable Heyman capsules have been made available for use with the aftertoading, miniaturized Cs-137 tube sources. These sources were constructed of stainless steel loaded with thousands of ceramic microspheres labeled with Cs-137 resulting in a consistently uniform dose distribution pattern. Since these miniaturized sources axe no longer manufactured, we have adapted the use of 1r-192 seeds for use with afterloading disposable Heyman capsules_ The physical, clinical and isodose considerations for using this technique are presented.

MATERIALS AND METHODS: The Heyman disposable afterloading applicator set (Best Industries, Springfield, VA) contains ten disposable nylon capsules, a semi-rigid inserting rod and a set of dummy sources. The capsules are 6 mm in diameter and 21 mm long, individually numbered and capped at the loading end Io prevent movement of the Ir-192 source once loaded. Considering the physical dimension of the capsules, strands containing four Ir-192 seeds per strand (4.4 mCi per seed), are ordered. The seeds are mounted without spacing at the tip of a reinforced stainless steel cable with a length the same as the capsule. The actual procedure of placing the catheters has been previously described. ~ Briefly, the uterus is packed with the maximum number of capsules so as to ensure adequate dose distribution. Dummy sources are then placed and localization films taken in the operating room so that any necessary corrections can be made. Ir-192 sources are afterloaded following treatment planning.

RESULTS: The dose distributions of the original 10 mg radium source, 25 mCi cesium source, and a 17.6mCi iridium source show only slight differences among the three sources. The clinical outcomes in the patients we have treated using the iridium source are comparable to those reported for the other sources.

CONCLUSION: The trend in the U.S. is to operate on all patients with early stage endometrial carcinoma with postoperative radiation tailored to the pathologic findings. Some patients, however, are medically inoperable and treated with external beam therapy and intracavitary brachytherapy. In the past five years, we have successfully employed our afterloading technique using disposable Heyman capsules and Ir-192 seeds for these patients. The use of Ir-192 delivers a uniform dose distribution and may be used when either radium or cesium sources are not available.

Heyman, J. The Radiumhemmet method of treatment and results in cancer of the corpus of the uterus J Obst. & Gynec of the British Empire 43:655-666; 1936.

-" Simon. N. Afterloading multiple irradiators tbr the treatment of cancer of the corpus of the uterus. A preliminary report of a new device. J. Mt. Sinai Hospital 36: 443:1969

2O62 COMPUTER-AIDED OPTIMIZATION" FOR STEREOTACT[C RADIOSURGERY

Tim Fox, Ph.D., Patton McGinley, Ph.D., Ken Brooks, Ph D., Ia, Crocker, M D , Lawrence Davis, M.D.

Department of Radiation Oncology. Emory University School of Medicine, Atlanta, Georgia, 30322

Purpose: The goal of radinsurgery treatment planmng is to deliver a large, uniform dose to a small intracranial lesion while keeping the dose to nearby normal structures below a critical dose level. The physician and physicist need to determine the best set of treatment parameters for each clinical treatment case. This is a complex task in radiosurgery treatment planning since the location and size of the tumor volumes and normal structures differ from patient to patient. The purpose of this study is to select a set of gantry angles and weights using a mathematical optimization method to produce a desired target and normal structure dose distributions for a patient rotator used in linear accelerator-based (LINAC) stereotactic radiosurgery.

Methods and Materials: An algorithm was developed using linear programming to determine an optimal treatment plan using points distributed on the surface of a tumor volume and a normal structure. The objective function was to minimize the dose to the surface of a normal structure. Constraints were placed on the dose received by the tumor volume and normal structure. The optimization algorithm determined the beam angles and their associated beam weights appropriate for LINAC-based radiosurgery.

Results: Four different dose constraint sets were used to test the computer aided optimization method for art example patient ease. The example patient case had a normal structure close to the tumor volume A standard treatment plan was compared with the optimized plans using isodose distributions registered with medical images and dose-volume histograms of the anatomical volumes. The standard treatment plan used a set of gantry angles with equal beam weighting and one isocenter resulting in a spherical high-dose region. The optimized treatment plans were all shown to be an improvement over the standard plan.

Conclusions: Proper implementation of the optimization algorithm for the patient rotator method resulted in improved dose distributions for the optimized plans compared to a standard plan This optimization method provides a system for rigorously finding the best beam angles and beam weights which meet a clinically desired goal.