Transperineal permanent seed implantation:
Using the peripheral loading method, embedded 125 Iodine seeds (“Strand”) were implanted.
2-3 weeks prior to TPSI a preplan was created in the same manner as described for the online plan procedure. Main targets of the preplan are:
- Ordering of a defined number of seeds,
- exclusion of pubic arch interference,
- “internal quality control” with comparison of pre- and online-plan data to exclude major differences in these both planning procedure (prostate volume, number of seeds for TPSI).
Using a transurethral catheter for visualization of the urethra the TRUS-online planning was performed under general anaesthesia ( Biplane-probe 5.0, 6.5, 7.5 MHz, Hawk 2102 xdi Extended Diagnostic Imaging ä , Biplane probe 8658 MDI ä , B-K Medical, Medizinische Systeme GmbH).
If necessary, the posterior area of the prostate was elevated with a water bag cover (Brachyballoon ä , Barzell-Whitmore Maroon Bells). The gland was fixed using 2 fixation needles (Cook-localisations needle ä ,Cook Urological) which were inserted in anterior areas of the prostate.
Patients were placed in lithotomy position and, after correct positioning of the transrectal probe, the volumetry began in 5 mm increments with inclusion of one above the first (“Cut above Zero-retraction”) and one below the last plan (“Cut below”). During the online plan procedure the sonography images were directly transferred into the planning computer software VariSeed ä (/6.7/7.0) PSID ä (3.0/3.5).
Seeds with defined activity were placed creating an isodose of 145 Gy surrounding the prostate with a 5 and 3 mm safety margin in the apical and basal areas as well as in central zones of the gland. After achieving the critical dose parameters (Tab. 2) the number of seeds were determined.
V100
(% of prostate volume that receives the prescription dose ) |
>98% |
D90
(Dose that covers 90% of prostate volume ) |
>180 Gy |
D100
(Dose that covers 100% of prostate volume ) |
>140 Gy |
D1/D30 Urethra
(Dose that covers 1% and 30% of the urethra) |
<230/210 Gy |
D100 Rektum
(Dose that covers 100% of of the anterior rectal wall ) |
<200 Gy |
Tab. 2: Indices used to quantify implant quality [12]
Implant quality was defined using the dose volume histograms for the target volume prostate and the organs at risk (urethra, rectum) [20,25]. During the implantation, the position of the needle was controlled by ultrasound and fluoroscopy before implanting the seeds. In all patients, the TPSI procedure was done with interdisciplinary cooperation of urologist, radiation oncologist and medical physicist.
After completion of the procedure a flexible cystoscopy was performed in order to exclude seed dislocation into the urinary bladder which would result in under dosage of the prostate and problems in radiation protection (loss of seeds during urination).
|
