Isometric wedge shielding for pelvic boost external beam therapy after Curietron treatments

In the treatment of carcinoma of the cervix uteri radiotherapy patients in Perth undergo a three course treatment. The treatment involves a course of external beam radiotherapy followed by a caesium--137 implant using the Curietron 2 afterloading device and finally an external beam boost dose with a block shielding the cervix. The aim of this project was to use the boost dose to compensate for the falling gradient in the summed dose profile introduced by the caesium implant. The required boost dose was calculated using sealed source dosimetry software and the measured attenuation coefficient for low melting point alloy was then used to determine the required block profile. The clinical requirement was for a dose of 85Gy to the Manchester system A points from a combination of caesium insertion and external irradiation. Due to the geometry of the caesium implant the pelvic gland regions will receive slightly more than 85Gy. Use of the block gives rise to total doses between 85.0Gy and 89.5Gy across the field at the level of the A points. At positions 25mm above and below the A point level doses between 87.5Gy and 89.0Gy are achieved.