RADIOGRAPHIC VISUALIZATION OF VAGINAL CYLINDERS IN GYNECOLOGIC HIGH-DOSE RATE BRACHYTHERAPY

Purpose: To develop a marker system allowing an accurate determination of vaginal applicator dimensions and geometry from a radiograph. Methods and Materials: The markers consist of two sets of gold seeds embedded into each cylinder identifying the cylinder diameter, and a thin stainless steel disk interposed between adjacent cylinders identifying their interface. An evaluation of the dosimetric properties of the markers was undertaken. An applicator was assembled using four cylinders (4 cm diameter) surrounding a stainless steel uterine tandem with a stainless steel disk 0.05 mm thick and 3.6 cm in diameter interposed between each consecutive pair of cylinders. The assembly was placed on a film and an Ir-192 high dose rate source was programmed to a single dwell position within the applicator. The markers were removed and a second film was exposed with the same dwell position and time. This procedure was repeated with various dwell positions along the applicator. A scanning densitometer was used to measure the density profiles and isodensity distributions of each film. Results: The optical density profiles and isodensity distributions with and without the markers in place were identical for all source dwell positions except when the source was centered in the plane of one of the stainless steel disks, where a maximum decrease of less than 2% in the dose rate was measured. The disks had no effect on the profiles measured along axes more than 2 cm from the projection of the applicator central axis on the film. Conclusion: The markers provide geometrical information about the position of the applicator relative to the anatomy necessary for optimized treatment planning. Slight dose perturbations resulting from the markers do occur, but only for dwell positions that center the source in the plane of a disk, and even then only at points very close to the disk. The markers can therefore be ignored from a dosimetric point of view.