IMPACT OF OPTIMIZED TREATMENT PLANNING FOR TANDEM AND RING, AND TANDEM AND OVOIDS, USING HIGH-DOSE-RATE BRACHYTHERAPY FOR CERVICAL-CANCER

Purpose: Different treatment techniques are used in high dose rate (HD R) remote afterloading intracavitary brachytherapy for uterine cervica l cancer. We have investigated the differences between ''optimized'' a nd ''nonoptimized'' therapy using both a tandem and ring (T/R) applica tor, and a tandem and ovoids (T/O), applicator. Methods and Materials: HDR afterloading brachytherapy using the Madison System for Stage IB cervical cancer was simulated for 10 different patients using both a T /R applicator and a T/O applicator. A treatment course consists of ext ernal beam irradiation and five insertions of HDR afterloading brachyt herapy. Full dosimetry calculations were performed at the initial inse rtion for both applicators and used as a reference for the following f our insertions of the appropriate applicator. Forty dosimetry calculat ions were performed to determine the dose delivered to Point M (simila r to Point A), Point E (obturator lymph nodes), vaginal surface, bladd er, and rectum. ''Optimized'' doses were specified to Point M and to t he vaginal surface. ''Nonoptimized'' doses were specified to Point M o nly. Using the linear-quadratic equation, calculations have been perfo rmed to convert the delivered dose using HDR to the biologically equiv alent doses at the conventional low dose rate (LDR) at 0.60 Gy/h. Resu lts: Major differences between ''optimized'' and ''nonoptimized'' LDR equivalent doses were found at the vaginal surface, bladder, and rectu m. Overdoses at the vaginal surface, bladder, and rectum were calculat ed to be 208%, nil, and 42%, respectively, for the T/R applicator with ''nonoptimization.'' However, for the T/O applicator, the overdoses w ere smaller, being nil, 32%, and 27%, respectively, with ''nonoptimiza tion.'' Conclusion: Doses given in high dose rate intracavitary brachy therapy border on tissue tolerance. ''Optimization'' of either applica tor decreases the risk of a dose that may have potential for complicat ions. Optimization of a tandem and ovoids best ensures that the doses are not diminished at the treatment sites, and that the potential for overdose is reduced.