A volumetric and dosimetric study of LDR brachytherapy prostate implants based on image fusion of ultrasound and computed tomography

Transrectal ultrasound has been the imaging modality of choice for brachytherapy seed implantation. However, CT scanning has been used by some investigators. Here we first examine the differences in the volume of the prostate as defined based on US and CT imaging studies, where the two image sets were fused together. We also investigated the dosimetric implications of the respective plans. All patients in the study had a CT and a transrectal US exam one month after the iodine seed implantation. A catheter with a balloon was placed in the patient's urethra with contrast. The balloon was pulled firmly against the anterior apex of the bladder. The two image sets were then fused together, using the shape of the urethra as the anatomical landmark for the co-registration. The prostate volumes from the two studies were computed as well as the DVH and dose distributions. An over-estimation of the prostate volume as seen on the US of up to 30% was found with the CT images. Spatially, the relative position of the two volumes is uncorrelated. The DVH and dose distributions showed better than 85% coverage for the US volume but mediocre coverage (as low as 50%) for the CT volume. CT imaging of the prostate tends to over-estimate its size. In post implantation studies where a CT is obtained for the localization of seeds, that volumetric discrepancy needs to be taken into account when assessing the quality of the implant.