Measurements of radiation dose distributions for shielded cervical applicators

Cervical applicators with shielded ovoids are employed to reduce dose to the rectum and bladder. Because of asymmetries introduced by the shields, dose distribution calculations for individual patients will require extensive computer reference data for the ovoid sources. Requisite 3-D dose distributions were measured for an unshielded and a shielded ovoid containing a Cs-137 source, using a computerized system employing a diode in a water phantom. The probe stops at each measurement point and accumulates dose for several seconds. The system automates horizontal positioning of the detector and angular motion of the ovoid to obtain dose in one plane. The detector is moved manually to other planes for a complete three dimensional set of measurements. In order to suppress the energy and directional dependence of the diode, final dose distributions are calculated from ratios of shielded to unshielded data in conjunction with independently measured TLD data for unshielded sources.