MAXIMUM AND MEAN BLADDER DOSE DEFINED FROM ULTRASONOGRAPHY - COMPARISON WITH THE ICRU REFERENCE IN GYNECOLOGICAL BRACHYTHERAPY

The reference bladder dose for gynaecological intracavitary brachyther apy (BT), as defined by the ICRU 38 Report, is often criticised as it is seldom representative of the highest bladder dose nor it gives an i dea of the area exposed to a significant dose. Since November 1990, ul trasound measurements are routinely made in order to determine the act ual dose delivered to the bladder of each patient. The technique was a s follows. (1) the bladder is filled up with 150-200 cm(3) of sterile isotonic saline. (2) The intrauterine position of the tube is checked. (3) The bladder anatomy is controlled. (4) Points of measurements are identified: ICRU bladder reference, minimum distance between bladder mucosa, uterine tube and other similar measurements taken every 15 mm along the radio-active line. Maximum and mean doses are calculated at the sagittal plane. Measurements are performed by moving the transduce r along the skin of the patient and included in the calculation of dos e distribution. Doses delivered to each relevant point are compared. T his enables determination to be made of the differences between the IC RU and the doses actually observed at the bladder wall with aid of ult rasonography. BT applications were checked in 58 patients (69 measurem ents). The method was feasible in all cases. The comparison between IC RU dose from orthogonal films and the ICRU dose from ultrasonography r esulted in a 90% accuracy. The maximum and mean doses for utero-vagina l BT are higher than the ICRU dose in 75% of cases (range, 2-8). Measu rements are now abandoned for vaginal applications as the ICRU dose on ly could be measured. A single G3 complication was observed after 20 m onths in a patient in whom a high bladder dose was planned intentional ly. The conclusion is that it is expected that this new approach will be of help to decrease the rate of severe complications by pointing ou t those patients in which there is a large discrepancy between the ICR U dose and doses determined with aid of ultrasonography.