I-125 seed planning: An alternative method of urethra definition

Background and purpose: To investigate the use of aerated aqueous gel rather than a catheter to define the urethra during permanent I-125 seed implant planning. Materials and methods: Twenty patients were treated between September 2007 and March 2008, each having two sequential volume studies: one visualizing the urethra with a catheter and the other using aerated gel. Two individually optimised plans were produced for each patient: one from the gel and the other from the catheter image set, and the plans were analysed dosimetrically. The plans were also interchanged (putting gel plan onto catheter image set and vice versa), and dose homogeneity within a slice was investigated. Three patients had MRI post-implant to check urethral position and dosimetry. Results: The urethra appears larger when defined with gel rather than when defined with a catheter, with volumes of 0.9 ± 0.3 [range: 0.6-1.6] cc and 0.7 ± 0.2 [range: 0.4-1.0] cc, respectively. Catheter plans appear dosimetrically slightly preferable to gel plans with V100 prostate being 99.7 ± 0.2 [range: 99.3-100.0]% and 99.5 ± 0.5 [range: 98.12-99.9]% for catheter and gel, respectively (p = 0.048). The urethra appears to receive a higher dose when defined with gel with V150 being 0.5 ± 0.7 [range: 0-2.8]% and 0.2 ± 0.2 [range: 0-2.6]% for gel- and catheter-defined urethras. Seed density, COIN and number of seeds were almost equivalent. Statistically, only V100 prostate and D90 prostate are significantly different. Putting the catheter plan on the gel image set shows a significant increase in urethral dose with V150 urethra increasing significantly from 0.2 ± 0.2 [range: 0-0.6]% from the catheter plan to 15.3 ± 11.9 [range: 0.8-47.0]% for the catheter plan on gel image set (p < 0.001). D90 for the inner core of gel and catheter plans was 193.9 ± 6.7 [range: 180.3-202.3] Gy and 198.7 ± 4.7 [range: 190.7-211.5] Gy (p = 0.023). Gel plans are cooler centrally and less homogeneous, which could be counteracted by increasing seed activity and/or prescription dose, though the clinical significance of this should be investigated before implementation. Conclusions: This investigation has shown that the urethral position is distorted by the presence of a urinary catheter. The dosimetry may be different from that planned if this is not taken into account.