PO04

Abstract

Purpose The conventional vaginal cylinder design consists of cylindrical segments and a hemi-spherical top, with a diameter matched to the cylinder portion. This design is driven primarily by geometric simplicity, without much consideration for the dose distribution around the brachytherapy source, nor the anatomy of patients. Careful plan optimization is required to make the prescription isodose line conform to the surface of the cylinder. However, the conformity is not optimal due to the discrepancy between the cylinder shape and the intrinsic dose anisotropy of the source perpendicular to the azimuthal plane. We propose an alternative design for an ergonomic vaginal applicator (EVA), which requires minimal optimization, provides a more natural dose conformity, and is more ergonomic for the patient. Materials and Methods In treating with brachytherapy alone for postoperative endometrial cancer, our typical prescription is 6 Gy x 5 fractions to the surface, covering the top 4 cm of the vaginal cuff. To create the proposed applicator, the existing cylinder channel, using the first 10 dwell positions (5mm-interval), are uniformly loaded for 17.33 seconds with nominal 10 Ci Ir-192 HDR source (GammaMedPlusiX and Eclipse). The prescription isodose line then takes the shape of an ovoid, with 3 cm diameter at middle length, paralleling a typical size used for cylinder applicator treatments. Similarly, uniform loading times of 10, 13.6, 21.8, and 26.5 seconds correspond to maximal diameters of 2, 2.5, 3.5, and 4 cm, respectively. The prescription isodose line is converted to a contour that can be exported and used to construct an applicator. The surface of this applicator naturally conforms to the prescription isodose line, which removes the need for any further plan optimization. Dose along the surface and at 5mm distance from the applicator equator to the tip of the dome are reported. Results Using the 3 cm diameter case as an example, the average surface dose (± Std. Dev) is 100% ± 3.2% with the EVA, compared to 100% ± 5.5% with the conventional cylinder, showing improved surface dose homogeneity with EVA, even after extensive optimization of the cylinder plan. The doses at 5mm depth are comparable: 57.3 ± 6.8% and 57.9 ± 6.2%, respectively. In addition, the tapered tip of EVA is likely easier to implant and would cause less discomfort for patients. Another potential benefit of the EVA is that its tapered tip may be less likely to trap air bubbles between the applicator and the vaginal cuff. Conclusions A novel design for a vaginal applicator is achieved that requires minimal planning optimization, with improved surface dose homogeneity, and better ergonomics. These are expected to provide both superior dosimetry and better overall clinical experience. The conventional vaginal cylinder design consists of cylindrical segments and a hemi-spherical top, with a diameter matched to the cylinder portion. This design is driven primarily by geometric simplicity, without much consideration for the dose distribution around the brachytherapy source, nor the anatomy of patients. Careful plan optimization is required to make the prescription isodose line conform to the surface of the cylinder. However, the conformity is not optimal due to the discrepancy between the cylinder shape and the intrinsic dose anisotropy of the source perpendicular to the azimuthal plane. We propose an alternative design for an ergonomic vaginal applicator (EVA), which requires minimal optimization, provides a more natural dose conformity, and is more ergonomic for the patient. In treating with brachytherapy alone for postoperative endometrial cancer, our typical prescription is 6 Gy x 5 fractions to the surface, covering the top 4 cm of the vaginal cuff. To create the proposed applicator, the existing cylinder channel, using the first 10 dwell positions (5mm-interval), are uniformly loaded for 17.33 seconds with nominal 10 Ci Ir-192 HDR source (GammaMedPlusiX and Eclipse). The prescription isodose line then takes the shape of an ovoid, with 3 cm diameter at middle length, paralleling a typical size used for cylinder applicator treatments. Similarly, uniform loading times of 10, 13.6, 21.8, and 26.5 seconds correspond to maximal diameters of 2, 2.5, 3.5, and 4 cm, respectively. The prescription isodose line is converted to a contour that can be exported and used to construct an applicator. The surface of this applicator naturally conforms to the prescription isodose line, which removes the need for any further plan optimization. Dose along the surface and at 5mm distance from the applicator equator to the tip of the dome are reported. Using the 3 cm diameter case as an example, the average surface dose (± Std. Dev) is 100% ± 3.2% with the EVA, compared to 100% ± 5.5% with the conventional cylinder, showing improved surface dose homogeneity with EVA, even after extensive optimization of the cylinder plan. The doses at 5mm depth are comparable: 57.3 ± 6.8% and 57.9 ± 6.2%, respectively. In addition, the tapered tip of EVA is likely easier to implant and would cause less discomfort for patients. Another potential benefit of the EVA is that its tapered tip may be less likely to trap air bubbles between the applicator and the vaginal cuff. A novel design for a vaginal applicator is achieved that requires minimal planning optimization, with improved surface dose homogeneity, and better ergonomics. These are expected to provide both superior dosimetry and better overall clinical experience.