Proceedings of the 34th Annual ASTRO Meeting 309

1121 DYNAMIC WEDGE CALIBRATION VIA FILM DOSIMETRY

John P. Clewlow, B.S., William S. Bite, Ph.D., Robert G. Waggener, Ph.D., and John J. Feldmeier, D.O.

Division of Radiation Oncology, University of Texas Health Science Center, San Antonio, Texas, 78284.

Purpose: Develop a more accurate method for calibrating a radiation therapy dynamic wedge using film.

Materials and Methods: Doses were measured at selected points on and off axis for a 6 MV beam of a Varian 2100 C accelerator with an ionization chamber in water phantom for an open field and several dynamic wedge fields. Kodak XV-2 films, sandwiched in a solid phantom, were exposed parallel to the central axis (CAX) of the beam for the same open and dynamic wedge fields. The open field data from a number of films was used to establish optical density (OD) to dose response curves at multiple depths. This set of response curves was applied to the OD of the dynamic wedge films at corresponding depths to generate dose data for isodose curves.

Results: The method of multiple depth OD to dose response curves produced isodose curves that were steeper (which implies a steeper wedge angle) than those obtained from a single depth OD to dose response curve. Differences as large as 3 to 4 percent are seen for off axis points for a field as small as 10 x 10 cm. A comparison of the doses obtained by single depth and multiple depth OD to dose conversion with ion chamber measurements is presented.

Conclusions: The method of the multiple depth OD to dose response curves for the conversion of dynamic wedge film OD to dose provides a clinically measurable improvement in the calibration of a dynamic wedge via film.

1122 FRACl’ICNATED HIGH-DOSE RATE BRA- Y FOR STEPKYl’ACTIC TREATMENT OF BRAIN TUMORS

B. Bemer, W. Grant III, S. Woo, E. B. Butler

Baylor College of Medicine, Houston, TX

Furpose : The use of stereotactic headframes and diagnostic imaging techniques have allowed for identification of the precise location of tlanors in the brain. These areas are treated by radiotherapy using external heam or brachytherapy sources. This project was undertaken to test the efficacy of utilizing the naninal 10 Ci k-192 source of a high-dose-rate (HDR) rmte afterloader to deliver 10 to 20 fractions of highly localized radiation doses over a period. of weeks.

Materials & Methods: The source fraTl our rrrachine travels at a rate of 13 wsec and that created two probl m. The first is that the standard catheters used for low-dose brachytherapy are mde of a siiicon rubber which will not withstand the force created by an HDR source as it enters the catheter. The second question to answer concerned the strength of the impulse created as the source enters the brain.

Resul ts : A catheter has been developed that allows the placanent of a siiicon rubber guide tube by the neurosurgeon. A teflon insert is placed later by the radiation oncologist and permits safe passage of the HDR source wire over my fractions. The iqulse was measured using an acceleralleter and a variety of phantan materials and was determined to be below any worrism level.

Conclusion: A systan has been developed to utilize the benefits of fractionated HDR doses and the spatial accuracy of brachytherapy sources in patient treatments.