The influence of geometrical changes on the dose distribution after I-125 seed implantation of the prostate

After prostate implantation, dose calculation is usually based on a single imaging session, assuming no geometrical changes occur during the months of dose accumulation. In this study, the effect of changes in anatomy and implant geometry on the dose distribution was investigated. One day, 1 month and 312 months after seed implantation, a combined TRUS-CT scan was made of 13 patients. Based on these scans changes in dose rate distribution were determined in prostate, urethra and bladder and a ‘geometry corrected’ dose distribution was estimated. When based on the day-1 scan, parameters representing high dose volumes in prostate and urethra were largely underestimated: V150 of the prostate 18 ± 10% and V120 of the urethra 47 ± 32%. The dose to a 2 cm 3 hotspot in the bladder wall (D2cc), however, was overestimated by 31 ± 35%. Parameters based on scans 1 month post-implant or later were all within ±5% of geometry corrected values. Values meant to indicate the adequacy of dose coverage of the prostate, V100 and D90, were not influenced by geometrical changes and were independent of the post-implant scan date. Other parameters representing high dose volumes changed strongly within the first month after implantation.