Prostate Seed Implant
Cheng B Saw, PhD,
University of Pittsburgh Cancer Institute, Pittsburgh, PA 15232
Various treatment options are available in the management of localized early prostate cancer including radical prostatectomy, modified radical prostatectomy, external beam radiation therapy, brachytherapy, cryotherapy, hormonal therapy, and careful observation. Radical prostatectomy provides excellent local control but with significant morbidity such as impotence, urinary incontinence, and urethral strictures. A nerve-sparing operation by Walsh results in a much lower rate of impotence. External beam therapy has been given daily over 7-8 weeks with success for stages A-C prostate cancer. Three-dimensional conformal therapy (3DCRT) or Intensity Modulated Radiation Therapy (IMRT) technique is now routinely used to deliver high doses to the prostate. High doses of radiation concentrated in a small volume with sparing of surrounding normal tissues can also be achieved by brachytherapy. The practice of low dose rate brachytherapy for prostate cancer will be reviewed in this presentation.
Radioactive sources of iodine-125 with a half-life of 60 days and palladium-103 with a half-life of 17 days, and mostly recently cesium-131 with a half-life of 9.7 days are being used for low dose rate prostate implant. There are a number of vendors producing iodine-125 seeds with different seed designs. Iodine-125 decays via electron capture by emitting 35.5 keV gamma and characteristics x-rays in the range of 27-35 keV. Palladium-103 seed model 200 consists of a laser welded titanium tube containing two graphite pellets plated with the palladium-103. The source decays via electron capture with the emission of characteristics x-rays in the range of 20-23 keV. Cesium-131 decays via electron capture and emits characteristics x-rays in the range of 29-35 keV.
The dose distribution of the radioactive seed exhibits anisotropic behavior. Hence analytical method is not practical and complex because of the source construction, filtration, and attenuation of low energy photon. Instead, the AAPM Task Group 43 recommendation circumvents this issue by parameterizing the dose distribution around the specific type of seed. The dose at a point in water is computed based on the air kerma strength, the dose rate constant, geometric factor, radial dose function, and the anisotropy function.
Current prostate implant procedure involves transperineal ultrasound-guided permanent brachytherapy technique. Initially, a volumetric study is performed to determine the volume of the prostate for implantation. The patient is placed in the dorsal lithotomy position and the transrectal ultrasound probe is securely anchored. The probe is moved precisely in a stepping fashion to obtain the transverse images of the prostate gland from base to apex at 5 mm intervals. A grid is superimposed onto the transverse image to provide spatial coordinates. The prostate gland is visualized on each image and the implantation region is drawn. The stacking of these images forms a 3D prostate for volume determination.
Based on the implantation volume, a pre-plan dosimetry is carried out to determine the activity per seed. The minimal peripheral dose is 145 Gy for iodine-125 implant, 125 Gy for palladium-103 implant, and 115 Gy for cesium-131 implant for monotherapy. The dosimetry follows the AAPM TG-43 formalism as described above.
The implantation procedure is performed on an outpatient basis under general or spinal anesthesia with the patient in the lithotomy position. The stabilization and guidance apparatus are securely fastened to ensure stability. The patient is prepared and a Foley catheter may be inserted into the bladder and inflated with dilute hypaque solution for easy identification of the urethra. The stepping unit of the ultrasound probe is attached to the stabilization apparatus. The probe is then attached to the stepping unit and the prostate is scanned at 5 mm intervals. The probe is then set at the most superior plan of the implant. The template is then secured to the ultrasound probe 2-3 cm from the perineum. The dots seen on the electronically grid superimposed onto the ultrasound image correspond to the template hole, thereby guiding the needle placement. The ultrasound probe is aligned such that the first horizontal row on the grid is parallel to the posterior margin of the prostate through all transverse images. If pre-loaded needles are used, the needles are inserted into the template holes corresponding to the needle positions defined in the matrix pattern constructed at pre-planning stage. Instead of pre-loaded needles, Mick gun may be used to inject seeds into the prostate. The procedure will be discussed in the presentation.