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 (HDR) remote afterloading intracavitary brachytherapy for uterine cervical cancer. We have investigated the differences between ''optimized'' and ''nonoptimized'' therapy using both a tandem and ring (T/R) applicator, 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 external beam irradiation and five insertions of HDR afterloading brachytherapy. Full dosimetry calculations were performed at the initial insertion for both applicators and used as a reference for the following four insertions of the appropriate applicator. Forty dosimetry calculations were performed to determine the dose delivered to Point M (similar to Point A), Point E (obturator lymph nodes), vaginal surface, bladder, and rectum. ''Optimized'' doses were specified to Point M and to the vaginal surface. ''Nonoptimized'' doses were specified to Point M only. Using the linear-quadratic equation, calculations have been performed to convert the delivered dose using HDR to the biologically equivalent doses at the conventional low dose rate (LDR) at 0.60 Gy/h. Results: Major differences between ''optimized'' and ''nonoptimized'' LDR equivalent doses were found at the vaginal surface, bladder, and rectum. Overdoses at the vaginal surface, bladder, and rectum were calculated to be 208%, nil, and 42%, respectively, for the T/R applicator with ''nonoptimization.'' However, for the T/O applicator, the overdoses were smaller, being nil, 32%, and 27%, respectively, with ''nonoptimization.'' Conclusion: Doses given in high dose rate intracavitary brachytherapy border on tissue tolerance. ''Optimization'' of either applicator decreases the risk of a dose that may have potential for complications. 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.