SU-FF-T-206: Clinical Investigation of the Effects of Implant Geometry On 3D-Image-Based Intracavitary Brachytherapy of Carcinoma of the Cervix

Abstract

Purpose: To clinically investigate the effects of implant geometry on tumor coverage, bladder and rectal doses for CT‐based intracavitary brachytherapy of carcinoma of the cervix Method and Materials: Eighteen low‐dose‐rate (LDR) gynecological (GYN) implants for ten patients with cervical cancer were undertaken CT‐based planning with defined gross‐tumor volume (cervix + uterus), rectum (upper‐rectum + medial sigmoid), whole bladder, surrounding normal tissue (5‐cm around the applicator), and multiple points of interest. The effects of the implanted geometry on the image‐based plans were evaluated by the target coverage by the fraction of the GTV receiving doses higher than the prescription dose (average A‐point doses), ratios of the maximum bladder or rectum doses to the doses at the points defined in ICRU Report ♯38, and dose drop‐off (or gradient) represented by ratios of total volume within the prescription dose to the total volume in 90% of the prescription dose.Results: The target coverage decreased from ∼50% to ∼30% with the increase of the separation of the two ovoids and the depth from the top‐end of the first tandem source to the mid‐point of the separation due to the enlarged target volumes. The bladder doses in CT‐based plans relating to the conventional bladder doses increased from factor of ∼1.0 to ∼1.5 as increase of the separation and the depth. The rectal doses in CT‐based plans were generally higher than the conventional defined rectal doses by a factor of 1.5. The implant geometry has less effect on the dose gradient at the prescription dose. Significant variations (> 200%) of the rectal and bladder doses occurred among individual cases. Conclusion: Implant geometries do have systematic effects on image‐guided GYN implants which could be used for selection of cases that are desirable for having LDR or HDR GYN intracavitary brachytherapy.