The Dosimetric Impact of Supplementing Pre-Planned Prostate Implants With Discretionary 125I Seeds

Purpose:TheBritishColumbiaCancerAgency (BCCA) Provincial Prostate Brachytherapy program was established in 1998. Over 3000 implants have been done to date. Implants are performed using a pre-planned, real time ultrasound-guided transperineal technique, with stranded seeds and modified peripheral loading to deliver an mPD of 144 Gy to the prostate plus margins. For each implant, 5 extra seeds (2 stranded and 3 loose) are provided to be used at the discretion of the physician. The aim of this research was to investigate the dosimetric impact of these extra seeds, and the circumstances under which they are most commonly used. Materials and Methods: Post-implant questionnaires were completed by 5 experienced physicians to prospectively collect information on 70 consecutive implants performed over a 4 month period. After each implant, the location and rationale for using any extra seeds was recorded by each physician. All study patients underwent day-0 post implant dosimetry. A previously developed plan reconstruction algorithm was used to distinguish the extra seeds from those which were planned. The dose distributions with and without the extra seeds were compared for the whole prostate, anterior-superior (ASQ), anterior-inferior (AIQ), posteriorsuperior (PSQ), and posterior-inferior (PIQ) prostate quadrants, urethra and rectum. The Conformity Index (CI) and External Index (EI) were computed to assess collateral dose outside the target. Results:Extra seeds were used in 83%of the cohort with amedian of 5 extra seeds/implant. The majority of the extra seeds were deposited in the ASQ (64%) and less frequently in other quadrants; PSQ (24%), AIQ (7%) and PIQ (5%). The most commonly reported reasons for the use of extra seeds was to improve coverage of the anterior base (42% of responses), and target regions of biopsy confirmed cancer (26%). The use of extra seeds resulted in a mean increase in whole prostate V100, V150 and V200 of 3.7% (mean V100 90.5% to 93.8%), 13% (mean V150 45% to 50.6%) and 19% (mean V200 13.5% to 16%), respectively. Mean whole prostate D90 increased from 147Gy to 156Gy. The use of extra seeds increased V100 over the 90% threshold in 15 (26%) patients. Five patients (9%) who received dose supplementation would otherwise have been classified as having suboptimal coverage (mean V100 of 83.2% improved to 91.1% after using extra seeds). Quadrant analysis demonstrated that extra seeds had the most impact on ASQ coverage, in line with the stated goals of their use, with a mean V100 increase of 13.6% (mean V100 74% to 83%) and a mean D90 increase of 9.1% (mean D90 123Gy to 134Gy). Extra seeds increased the rectal dose (VR100) by a mean of 5.9% (mean VR100 0.543 cc to 0.579cc). The mean urethral dose increased from 134.8Gy to 140.6Gy with a mean increase of 4.5% (range 1.16Gy to 5.8 Gy). The CI increased from a mean of 1.89 to 1.92 while the mean EI increased from 0.809 to 0.869. Conclusions: Bolstering coverage of the anterior prostate base is the main reason extra seeds were used in this cohort, and this aim was consistently achieved with only minor impact on the urethral, rectal and extraprostatic tissues. However, there was a considerable increase in prostate V150 and V200. The accuracy with which dose was boosted in the biopsy positive regions is under investigation.