Dosimetry and dose specification for a new gynecological brachytherapy applicator

Abstract

A new afterloadable gynecological intracavitary applicator has been designed and is now in use for the treatment of a wide range of vaginal, cervical, and endometrial cancers in a single application, with a dose distribution that more closely matches the prescribed treatment than previous methods. The plexiglass applicator consists of a bulbous section that is inserted up to the vaginal apex and loaded with right and left ovoid sources. The distal portion is cylindrical with a central channel for tandem sources. A straight tandem is used when the target volume extends to the vaginal apex, but an intrauterine tandem can also be used. Extensive dosimetric and radiographic evaluations were performed to guide design refinements and to validate the surface dose predictions of the brachytherapy treatment planning system. The final applicator design delivers 110–120 cGy/hr to the vaginal apex'surface and 95–100 cGy/hr to the distal vaginal surfaces when loaded with a 144.6 U (20 mgRaEq) cesium tube'in each ovoid channel and 72.3, 72.3, and 144.6 U (10, 10, and 20 mgRaEq) cesium tubes in the vaginal cylinder channel. A system for treatment dose specification has been established that includes dose tables for manually calculating surface doses to reference points. A dose distribution comparison with a sequential colpostat-vaginal cylinder treatment demonstrates that dose delivery is more precise and uniform with this new applicator.