AN ANALYSIS OF THE EFFECT OF OVOID SHIELDS IN A SELECTRON-LDR CERVICAL APPLICATOR ON DOSE DISTRIBUTIONS IN RECTUM AND BLADDER

Purpose: A disadvantage of ovoid shields in a Fletcher-type applicator is that these shields cause artifacts on postimplant CT images. CT im ages, however, make it possible to calculate the dose distribution in the rectum and the bladder. To be able to estimate the possible advant age of having CT information over the use of ovoid shields without hav ing CT information, we investigated the influence of shielding segment s in a Fletcher-type Selectron-LDR applicator on the dose distribution in rectum and bladder. Methods and Materials: Contours of rectum and bladder were delineated on transaxial CT slices of 15 unshielded appli cations. Of the volumes contained within these structures dose-volume histograms (DVHs),were calculated. In a similar way, DVHs of simulated shielded applications were calculated. The reduction, due to shieldin g, of the dose to the 2 cm(3) (D-2) and 5 cm(3) (D-5) volume of the cu mulative DVHs of rectum and bladder, were determined. An isodose patte rn in the sagittal plane through the center of each applicator was plo tted to compare the location of the shielded area with the location of maximum dose in rectum and bladder in the unshielded situation. In tw o cases local dose reductions to the rectal wall were determined by ca lculating the dose in points at IO-mm intervals on the rectal contours . Results: For the rectum, the reduction of D-2 ranged from 0 to 11.1% , with an average of 5.0%; the reduction of D-5 ranged from 2.3 to 12. 1%, with an average of 6.4%. The reduction of D-2 and D-5 for the blad der ranged from 0 to 11.9% and from 0 to 11.6%, with average values of 2.2 and 2.6%, respectively. In 8 out of 15 cases the rectal maximum d ose was located inferior to the shielded area. In all cases except one the bladder maximum dose was located superior to the shielded area. L ocal dose reductions on the rectal wall can be as high as 30% or more in an optimally shielded area. Conclusions: Reductions of D-2 and D-5 to rectum and bladder due to shielding are rather small, because the s hielded area does usually not coincide with the high dose region and e ven if it does, the shielded area is too small to result in large redu ctions of these values. Because local dose reductions vary largely, on e should proceed with caution when calculating the dose in just one re ctal or bladder reference point. Because large overall dose reductions cannot be achieved with shielding, it is safe to use an unshielded ap plicator when post implant CT images are used to realize optimized dos e distributions. (C) 1997 Elsevier Science Inc.