Feasibility Study of Intensity Modulated Intracavitary Brachytherapy for Cervical Cancer

Abstract

To evaluate the dosimetric properties of a novel tandem applicator with a rotatable internal radiation shield developed for intensity modulated intracavitary brachytherapy (IMBT) of cervical cancer. The novel tandem applicator is composed of both external bent tube and internal components, such as a 7 cm long cylindrical tungsten shield with a groove, a rotatable tube driven by a mini servo motor, and a hollow flexible shaft which connects the tungsten shield and the rotatable tube. Outer diameter of the external tube is 6.0 mm in intrauterine part, and a transfer tube can be connected to an end of the rotatable tube in order to transport Ir-192 source. The intensity-modulated dose distribution can be delivered by rotating the internal shield discontinuously and loading Ir-192 source at the optimized source positions during optimized dwelling times. Angular dose distribution was firstly measured by Gafchromic EBT3 film on the surface of an acrylic cylinder (radius=3 cm) where the tandem was inserted. The rotation angles of the tungsten shield were also measured dosimetrically for 90°, 180°, and 270° motor rotation using the acrylic cylinder, and compared with those of the motor. Finally, new tandem was tested on the magnetic resonance images (MRIs) of some patients undergone interstitial brachytherapy. An asymmetric dose distribution was optimized on a para-axial plane by an in-house computer code, and the radiation emission angles and their dwelling times were determined. The dose distribution was delivered to the EBT3 film positioned vertically to the tandem axis in a water phantom, and two-dimensional dosimetric evaluation was performed. Unlike the commercial tandem applicator having a uniform angular dose distribution, our tandem applicator showed a nonuniform angular dose distribution (Table 1). Angular dose was maximum in the radiation emission direction, while it was less than half of the maximum value in the opposite direction. In the rotation angle comparison, there was a retardation of the rotation of the tungsten shield. However, the angle difference between the measurements by dose profile and servo motor was less than 3°. The overlap between the patient MRI and the measured dose distribution showed that the target was covered more conformally, and the critical organs region encompassed by prescription isodose line was reduced dramatically. We evaluated the dosimetric properties of the novel tandem applicator with a rotatable internal radiation shield, and the results indicate the feasibility to realize IMBT for cervical cancer.Abstract 311; Table 1Relative angular dose distribution measured on a cylindrical surface, 3 cm apart from a tandem applicator with a rotatable tungsten shield when the radiation exit window is aligned to 0 degree.Angle (degree)Relative angular dose (%)0100.0-1098.7-3081.2-6071.2-9056.6-12050.1-15046.6-18046.0 Open table in a new tab